chapter 9 breathing systems - exocorriges.comexocorriges.com/doc/41209.doc · web viewfysisk...

Medisinsk-teknisk avdeling, RikshospitaletFysisk institutt, UiO

Chapter 9

GAS INSTRUMENTATION

av

Sverre Grimnes

Chapter 9 20 May 2023 1 Sverre Grimnes

2008


INTRODUCTIONIn each cell the complex mechanisms of life are based upon the simple use of two gases: oxygen supplied - carbon dioxide produced. In the lungs these two gases are separated by the gas/blood membrane but transported as gas on the ventilation side and as liquid (dissolved blood gases) on the blood side. Most tissues of the body do not contain gas in the gas phase; gas bubbles in the small blood vessels are dangerous because they act as emboli hindering blood flow. The guts and the lungs are the only organs where gases are to be found normally. Gas instrumentation in medicine serves first and foremost the lungs, and both for diagnosis (gas analysers); therapy (aerosol nebulizers) and support (ventilators, anaesthesia workstations). We can not be without lung ventilation for many minutes; therefore support instrumentation is critical equipment with respect to technical malfunction and wrong use. .

Table 1 Content of air

Volume %, equal to kPa if the barometric pressure is 100 kPa.dry saturated 37oC

nitrogen 78.1 73.4oxygen 20.9 19.6argon 0.9 0.8carbon dioxide 0.04 0.04water vapor 0 6.3

Table 1 shows the content of air. Notice the influence of water vapour. Oxygen and carbon dioxide are called blood gases, together with nitrogen these gases are dissolved in the blood and therefore are also in the liquid phase. Nitrogen is not used by the body, so there is no net nitrogen transport across the lung membrane. In blood most of the oxygen transport is performed by oxygen chemically bound to haemoglobin (bluish) forming oxyhaemoglobin (reddish).

If anaesthetic drugs in gas or vapour phase (e.g. N2O or sevoflurane) they are supplied through the ventilation. Scavenging systems remove such gases before they reach the operating room ambiance.

AIRWAY AND LUNG ANATOMYThe lower airways below the throat comprise the trachea and the bronchi. The trachea is split into the two main bronchi, at the distal part they end in the alveoli (Fig.1). Here the air and the blood meet but separated by the very thin membrane of the air sac (alveolus). On the tissue side blood capillaries envelop an alveolus.


Oxygen is transported as O2 gas molecules in the trachea down to the alveoli, as dissolved gas and chemically bound to haemoglobin in the blood, and in the end diffuses the last tenths of a millimetre from blood capillaries through the extracellular liquids up to the living cells. Oxygen supply is from outside of the body, and it is therefore a concentration gradient with falling values from the mouth to the cells. Carbon dioxide is produced in the cells, diffuses to the blood capillaries and is then transported by blood to the lung

capillaries, diffuses through the lung membrane and is expelled from the body as CO2-gas through the airways. The CO2 gradient is therefore with the highest values in the cells and lowest in the mouth.

The gas exchange takes place in the alveoli. Most textbooks present alveoli as a bunch of grapes, but pulmonary alveoli are prismatic or polygonal in shape, i.e. their walls are flat. There are about 600 millions of them in our two lungs. The membrane surface in an adult healthy person is about 160 m2 and this assures a very effective gas exchange between the air and the blood. The exchange is as a gradient driven diffusion process through the membranes, tissue and the walls of the blood capillaries.

Lung volumes, lung capacitanceThe total volume of both lungs of an adult

healthy person at maximum inspiration is about 6L, fig.2. The residual minimum volume at maximum expiration is about 1L: it is impossible to empty the lungs completely all the way to collapse. The difference (5L) is the vital capacity. The tidal volume is the normal inspiration or expiration volume under quiet breathing, for instance 0.5L.

Lung compliance, pneumothoraxEach lung is enclosed in a gas-tight pleural volume by the double-walled lung sac membrane. The outer membrane is fixed to the thorax cage, the inner to the lungs. Because of the surface tension of the liquid films a lung tends to contract and reduce its volume. Therefore the intermembrane volume has a negative pressure of about -4 cmH2O with respect to atmospheric pressure. During inspiration the diaphragm pulls the lower surfaces of the pleural volume down increasing the lung volume and thereby increasing the negative pressure in the alveoli. A puncture of the lungs destroying the negative pressure is critical for the patient. The lungs will collapse and the patient will not be able to breath (pneumothorax). Normally a pressure change of as little as -1cmH2O (+1 cmH2O during expiration) in the alveoli is sufficient for a quiet respiration. When the patient is breathing spontaneously the inhalation is caused by the work of the lung


Figure 1 Airways with larynx, trachea and bronchi

muscles resulting in the alveolar negative pressure. During expiration little muscle work is done, it is the relaxation process of the stretched tissue which brings the air out. During forceful ventilation also the rib rise increases the pleural volume and increases the negative pressure. Then also special muscle groups actively compress the pleural volume during expiration.

The lungs may be soft and easy to fill, meaning that a relatively large inspiration volume is obtained with only a small negative pleural pressure change. Compliance is a much used parameter to describe the expansibility of the lungs, compliance C is defined as:

Equation 1 Compliance C = ΔV / ΔP [L/Pa, L/cmH2O]

The compliance of the normal lungs and thorax is about 0,13 [L/cmH2O]. Reduced compliance makes the patient more difficult to ventilate. A therapy is the use of surfactants, substances which lowers the surface tension at the inside alveoli surfaces. A near ideal zero compliance closed volume is a gas supply bottle, a near ideal maximum compliance volume is the closed volume spirometer.

Flow resistance, gas viscosityThe trachea is equipped with cartilage rings so that it will not collapse at negative pressure. The basic model for flow resistance in tubes is based upon the law of


Figure 2 Lung volume parameters

Figure 3 Parabolic velocity profile in accordance with the Poiseuille model.

Poiseuille1, describing the resistance R to flow through a tube of radius r and length L under the influence of gas viscosity [Pa s]:

Equation 2 Poiseuille [Pa/m3/s = pressure / flow rate]

Validity 1) Laminar flow in a straight tube geometry2) Gases and liquids (fluids), but better model for gases than for liquids3) Flow rate in [m3/s], not [mol/s]4) Gas viscosity is increasing with temperature (in contrast to liquids). It is pressure

independent, and R is therefore independent on the mean pressure level in the tube

Thus, the resistance is not dependent on friction between the fluid and the walls, only on the internal friction in the fluid. At the walls the velocity is zero, increasing to maximum at the centre of the tube. Fig.3 illustrates the Poiseuille ideal flow model in a tube, the flow profile is parabolic. The frictional forces between layers of the fluid are forces parallel to the flow, they are shear forces. Ohms law for electrical parameters is V=RI where V is voltage difference [V] and I current flow [A] through R. As a parallel to Ohms law P=RQ where P is pressure difference [Pa] at the wall and Q is mean flow [m3/s]. In spite of the variable velocity illustrated in Fig.3, R is therefore related to the mean velocity. By measuring P we have a gas mean velocity sensor, see subchapter on gas sensors.

The extreme dependence on the tube radius shown in Eq.2 has very important consequences e.g. with catheters and syringes for the injection or aspiration of fluids. It is also important in obstructed airways (airway resistance work, asthma). It is a very

1 Jean-Louis Poiseuille (1799-1869), French physicist


effective regulating mechanism in the body when the arterial blood vessel walls are equipped with muscles to contract and reduce the radius of the vessel.

TurbulenceWhen the velocity of a fluid is increased beyond a threshold value, the flow modus

changes from laminar to turbulent. The resistance to flow is increased and Poiseuilles law no longer describes the process correctly. The flow resistance is not determined so much by the fluid viscosity as by fluid density.

Turbulence is important in many parts of the body, both in the airways and in the blood stream, in particular at

bifurcations and around heart valves. The laminar model is useful, necessary and important, but its validity range must always be kept in mind. .

GAS PHYSICSWater has a very low compressibility because of the strong polar bonds between the molecules. The molecular bonds in oil are somewhat weaker, and oil is therefore slightly compressible. Air at room temperature and 1 bar has a density of about 1,25 kg/m3, about thousand times lower than that of water, and the distance between the molecules is accordingly roughly 10 times larger than in water. A gas is very compressible, but if a gas is at a temperature higher than its critical temperature, it is impossible to press the molecules together into a liquid phase.

The amount of gas substance in a closed compartment can be characterised according to two traditions: either by volume and pressure or by the number of mol. Flow rate can accordingly be given by mass: kg/s; or volume: m3/s; or substance: mol/s. The metabolism of the body is based upon the chemical reactions between molecules, so the number of molecules (mol) is perhaps the most basic unit for medical gases used by the body.

Dynamic gas model and the universal gas lawGas molecules (or atoms or small particles) at room temperature are not at rest. It was the British botanist Robert Brown who in 1827 discovered in his microscope that small grains of pollen in water moved and collided, he thought it was a life process. The average effect of gas molecule collisions with the walls constitutes the pressure of the gas. It was Avogadros2 great discovery that the pressure is proportional to the number, not the mass, of the particles. With reference to Avogadro the number of particles therefore has got its own numbering system: the mol which is ≈6·1023 particles and 2 Avogadro (1776-1856), italian autodidact chemist and physicist


Figure 4 Flow lines with local hindrance and a back eddy (non-laminar zone)

corresponds to the molecular weight [gram] of a gas, e.g. 1 mol of oxygen is 32 g oxygen. The type of particles should be specified, but often we mean both atoms (e.g. Ar) and molecules (e.g. O2). The reason for the number dependence is that smaller particles move faster so that their contribution to pressure statistically is the same. Accordingly, usually for a certain amount of substance the pressure will be dominated by the smallest particles because they are usually more numerous (the mass of spheres is proportional to the radius r3, so with equal mass it is 106 as many 0,1 μm spheres as 10 μm spheres).

The thermal movement of gas molecules has an important consequence: in a mixture of particles such as air, the particles do not necessarily sediment in layers according to their weight. It is the mass of a particle at a certain temperature which determines whether the molecule settle under gravitational forces or thermal movements are forceful enough to overcome gravity. At room temperature light particles such as N2 (molecular weight 28), O2 (32), CO2 (44), Ar (40) and H2O (18) will not settle with the heaviest molecules at the lowest levels. Larger molecules like sevoflurane (200) however do settle to the floor to a noticeable degree. According to the gas law a certain gas pressure is obtained by fewer molecules the higher the temperature. Accordingly, warm gas is lighter and ascends into the air (warm air balloons).

The universal gas law is based upon Avogadros discoveries. It is given in a variety of forms; here by the Boyle&Mariotte / Gay-Lussac3 version using the amount of substance n [mol] as a parameter:

Equation 3 Universal gas law PV = nRT

n is the number of all particles in the enclosed gas volume, that was Avogadro's great discovery, The particles may be atoms (noble gas), molecules (e.g. O2 or CO2) or any small particle from electrons to droplets. The contribution to pressure is from each particle, it is the number that contributes and not the size. P is the pressure [Pascal], V is the volume [m3], R the universal gas constant (8,3 [Joule/ºKmol]) and T the temperature [ºK].

Validity1) Closed volume, all n contained in V2) Ideal gas (= far from the condensation point) 3) Static conditions

Other versions P1V1 = P2V2 (constant number of particles and constant temperature) P = (n/V) RT (n/V is number density) Closed volume but soft walls so that V is a function of P, i.e. compliance C > 0, see

chapter on lung compliance

3 Boyle (1627-1691) Irish physicist; Mariotte (≈1620-1684) French physicist; Gay-Lussac (1778-1850) French physicist.


There are two different pressure scales in common use: absolute pressure (AP) or relative to atmospheric pressure (RP). Often it is not defined but determined by convention. Most pressures are relative pressures. Bottle filling is RP, an oxygen bottle is “empty” when it has atmospheric pressure, and negative pressure is not good as ambient gas may be aspired into the bottle. Absolute negative pressures do not exist, so if negative pressure is given it must be RP. Also with suction devices relative (negative) pressures are often used. Higher negative pressure then means higher vacuum and lower absolute pressure.

PV diagrams, non-ideal gases, condensation Fig.5 shows a pressure-volume (PV) diagram of a gas in a closed volume. At high temperatures the gas is more ideal following the Boyle-Mariottes law, Eq.3. In this region the gas can not be compressed into a liquid, irrespectively of how high the pressure is. At lower temperatures the curves loose their hyperbolic form, and at the critical temperature Tc a point is reached where it is possible to compress the gas into a liquid. At temperatures below Tc there is a constant pressure range where the substance is more or less liquefied. A liquid is not very compressible and when completely liquefied the pressure rise is very rapid at a further lowering of volume. Tissue (except lungs and guts) does not normally contain gas, so tissue is like a liquid, highly incompressible, but may easily change its form.

Figure 5 PV-diagram for a closed volume

Above the critical temperature it is not possible to compress a gas into the liquid state, the vibrational thermal energy is high enough to break the tight bonds between the molecules. Below the critical temperature the gas may be compressed to a liquid, the gas


in this range is called a vapor. In daily talk we do not make the precise division, for room temperatures we should say water vapor, nitrous oxide vapor and oxygen gas. This would clarify the important fact that a bottle of carbon dioxide at room temperature may contain gas in the liquid phase. Then the bottle filling must be determined by weighing and not by pressure measurement. The oxygen bottle can not contain liquefied oxygen at room temperature, and the degree of filling can therefore directly be determined from reading the manometer pressure.

Examples: a) Calculate how many liter of gas you have left in a 10L oxygen bottle at 120 bar. Solution: Tc for oxygen is -119 oC, so the oxygen must be in the gas phase. We use the Boyle-Mariotte gas law in the form P1V1=P2V2 so that 120.10=1.V2 and V2=1200 [L]. b) Calculate how many liter of gas you have left in a bottle of N 2O having weighed it and subtracted the empty bottle weight (tared) and found the N2O content to be 2.2 kg. Solution: Tc for N2O is 36.5 oC and the N2O may be in the liquid phase. We need not know the ratio of liquid to gas, our weighing takes care of all the N2O molecules. The molecular weight of N2O is 44 and 1 mol is therefore 44 gram. The amount of substance in the bottle is 2200/44=50 [mol]. We use Eq.3: PV=nRT and put P=100kPa and T=300 K, then V=1245.3 [L].

Table 2 Critical temperature Tc, pressure Pc ; boiling point (1 bar) Tb.

Transport and storage of liquefied gas is practical because of the reduced volume, a large hospital is a big consumer of e.g. oxygen and nitrogen. According to Table 2 the storage of liquid oxygen must be done in large thermos bottles at temperatures lower than the

Tc. Oxygen can e.g. stored at -119 oC and 50 bar, or -160 oC and 6 bar. To reduce the storage pressure to atmospheric pressure the temperature must be lowered, for oxygen in an open bottle (1 bar) down to the boiling point -183 oC. Nitrogen is used for cooling in the laboratories, for tissue long term storage and for cryospray. It is often practical to transport small volumes of liquefied gases in open thermos bottles at 1 bar and boiling temperature.

Water is an important substance in medicine. Air can absorb water in the gas phase, such water vapour is invisible to the human eye, just as oxygen and nitrogen. The warmer the air, the more water molecules can be absorbed. Air saturated with water vapour corresponds to a relative humidity (RH) of 100%. At 37oC the partial pressure of the H2O molecules is then 6kPa. If the gas is saturated with water vapour and the temperature is lowered, water condensates in small droplets. This is mist or fog and is visible to the human eye, it is particles (droplets) and not single molecules.


Tc [oC] Pc [bar] Tb [oC]Helium (He) -268 2,4 -269Nitrogen (N2) -147 33,6 -196Argon (Ar) -122 49 -186Oxygen (O2) -119 50,3 -183Carbon dioxide (CO2) 31 73 *Nitrous oxide (N2O) 36,5 72 *Water (H2O) 374 218 100

*can not exist in liquid phase at 1 bar (fig.34).

Laplace law In a blood vessel the blood pressure exercises a force against the walls which is counteracted by three different force components in the wall: 1) elastic tissue tension, 2) surface tension, and 3) active muscle tension (tonus). Tension T is measured as force pr meter length perpendicular to the force [N/m], Fig.6. Laplace4 found the following formula for the pressure P [Pa] in a cylinder of radius r [m] and the total wall tension T [N/m]:

Equation 4 Laplace P = T/r (cylinder)

Some peculiarities of this equation are linked with the fact that it does not contain pi, that pressure increases beyond limits as r 0, and that the pressure and the tension components are orthogonal, ref. Problem 10. T may itself be a function of r, so that the increase in pressure with small values of r may be modified. A tube has one dimension of curvature, and since the sphere has two such dimensions, the pressure for a sphere is doubled: P = 2T/r .

These equations are applicable in many medical situations also with active muscles in the walls, e.g. blood vessel, spherical pathological enlargement of blood vessels (aneurisms), pressure in the ventricles of the heart during systole, pressure in the urine bladder, pressure in the airways of the lungs. In the lungs the alveoli are prismatic or polygonal in shape, i.e., their walls are flat, and the Laplace law applies only to curved regions. Alveoli do not readily collapse into one another because they are suspended in a matrix of connective tissue "cables" and share common, often perforated walls, so there can be no pressure difference across them. Surfactants have important functions along planar surfaces of the alveolar wall and in mitigating the forces that tend to close the small airways. Laplace’s law as it applies to cylinders is an important feature of the mechanics of airway collapse, but the law as it applies to spheres is not relevant to the individual alveoli.

Solubility, partial pressureGases do dissolve in liquids like oil, water and blood. This process is called physical solubility if there are no chemical reactions between the gas and the liquid. The gas

4 Pierre Simon Laplace (1749-1827), French astronomer, mathematician and physicist


Figure 6 Laplace cylinder model

molecules find their positions between the liquid molecules, and if the gas molecules fit well in the space between them the solubility is high. The gas becomes a part of the liquid phase, and is not to be regarded as small gas bubbles. There is a transport of gas across a gas/liquid interphase as long as there is a concentration difference. The concept of partial pressure is essential in this respect as a practical measure of gas concentration in a liquid. Henry’s law states that the amount of gas dissolved in a liquid is proportional to the partial pressure of the gas in equilibrium with the liquid. Partial pressure in a liquid may be measured with a sensor covered by a membrane permeable to the gas but not to the liquid.

Figure 7 Left: dry gas mixture, right: after insertion of a water filled dish

On Fig.7 a closed chamber at 37 oC is filled with dry nitrogen gas up to a pressure of 80 kPa and then with dry oxygen gas so that the total pressure is 100 kPa (1 bar). At 37 oC no chemical reactions between the two gases occur, we have a mechanical mixture. According to Daltons law the partial pressure of each gas contributes to the total pressure as if it were alone. The partial pressure of oxygen (Fig.7) is therefore 20 kPa. On Fig. 7 (right) we then introduce a small dish of water into the chamber. The water has already been in prolonged contact with room air so that the water is in equilibrium with the oxygen and nitrogen of the air. With the dish inside no net transport of oxygen and nitrogen occurs across the gas/water interphase, but as the gas was dry a transport of water molecules into the gas starts (evaporation). This goes on until the chamber gas is saturated with water vapour. The relative humidity (RH) is then 100%, at 37oC corresponding to a water vapour partial pressure of about 6 kPa. The total pressure in the chamber has increased to 106 kPa. The slightest temperature fall somewhere in the chamber will then start water vapour condensation.

Table 3 shows the solubility coefficient of different gases in blood and oil. It is practical to give the amount of a dissolved gas as shown: litre gas per litre liquid [L/L=1], the Ostwald solubility coefficient which is temperature, but not pressure dependent. If the gas pressure is doubled the amount of dissolved gas is also doubled according to Henry’s law. But that is as amount of substance [mol], not as volume [L], because the doubled pressure has also halved the volume. The number of dissolved molecules pr litre liquid [mol/L] may therefore be more physiological relevant, but less practical because it is pressure dependent.


Table 3

Solubility of gases in blood and oil at 37 oCBlood gases are transported in the

circulation, and Table 3 shows that the solubility of oxygen is low and the transport of dissolved oxygen is therefore not sufficient to supply the metabolism of the body. To increase the transport capacity blood is therefore equipped with haemoglobin also binding oxygen chemically. The carbon dioxide solubility is much higher so that the transport is not so dependent on chemical binding for the exhalation of CO2.

If nitrogen is to be replaced by nitrous oxide in an anaesthesia, the transport of nitrous oxide in

the blood stream is more rapid than the wash out of nitrogen (why?), therefore gas volumes filled with nitrogen e.g. in the guts can increase dramatically but transiently when large amounts of nitrous oxide suddenly arrives.

Oil may seem to be a curious choice of liquid, but it equals fat sufficiently and body fat may store large amounts of gas and so influence gas kinetics strongly. After a prolonged anaesthesia it may take a long time to wash out the anaesthetic gases from the fat during wakening. Oil data is also used because oil is more stable and give more reproducible data than fat.

BREATHING SYSTEMS (SUPPORT) In the operating room or intensive care unit the breathing system connects the patient to the ventilator or anaesthesia machine. For resuscitation smaller and simpler portable units are used with a patient mask. For longer periods of time the patient is intubated. A tube is positioned in the patients airways with the help of a laryngoscope, see Fig.8. The patient must be unconscious. The laryngoscope contains battery and light source to ease correct positioning. With the tube correctly positioned in the trachea a cuff is inflated so that a tight coupling with the lungs is obtained.


gas/blood [L/L]

gas/oil [L/L]

Nitrous oxide 0.5 1.4Halotane 2.3 224Enflurane 1.8 96Isoflurane 1.4 91Desflurane 0.4 19Sevoflurane 0.6 53Ether 12 65Oxygen 0.02Carbon dioxide 0.8Nitrogen 0.015

Figure 8 Laryngoscope and tube insertion.

One-way breathing systemIn such systems all the expired gas leaves the system and nothing is recirculated back to the patient. One-way systems are used with ventilators when anaesthetic gases / vapours are not used. Fig.9 shows an example, a simple resuscitation model used for supplying oxygen-rich gas to the patient via a mask, more effectively than by the mouth-to-mouth method. Fresh gas from an oxygen bottle continuously flows into the system; the flow control is outside the illustration as a part of the bottle with manometer, flowmeter and pressure reducing valve. Flow direction valves are an essential part of such a system. Squeezing the bag starts the inspiration cycle, the gas inlet valve flap A is pushed to the left, and the fresh gas is flowing into the patient. The operator feels the patient lung pressure in her hand and the resistance to gas flow, and perhaps also the rise and fall of the thorax surface. When the operator considers the lungs to be adequately filled the bag squeezing is stopped. The pressure drops and the expiration cycle starts driven by the lung pressure. The valve flap A is pressed to the right and the fresh gas starts to fill the bag. The expiration airway is free out to ambient air. When the operator considers the lung empty, the bag is again squeezed and a new inspiration cycle starts.

The pressure in the bag will not raise proportional to its filling. According to Laplace law the bag pressure P = 2T/r, cf. the subchapter on Gas Physics. However, as r increases also T increases and usually roughly proportional to the circumference of the bag and therefore r. The result is that the pressure in the bag is not very dependent on bag filling. The tubes are often of a spiralled type so that they do not collapse at low pressures (vacuum).

Figure 9 One-way small portable resuscitation system


For short term use a special humidifier is not necessary even if the fresh gas is dry. Usually the parts are single-use low price items, so disinfection or sterilization by the users is not necessary.

Circle system, rebreathingIn these systems a part of the expired gas is returned to the patient. The advantages are: water is returned to the patient airways, heat is returned, use of costly inhalation drugs is reduced. An example of such a system is shown in Fig. 10. The expired gas is partly returned to the circle, partly leaving the system via the pop-off valve. The carbon dioxide is absorbed in the CO2 canister, this also develops heat. Fresh gas flows continuously into the system from an anaesthesia machine, and the breathing rhythm is determined by the bag squeezing. The patient is intubated with the tube coupled to the Y-piece. The uni-directional valves secures the correct flow direction.

The inspiration starts when the bag is squeezed. The one-way valve 2 is closed, and the content of the bag, humidity included, flows through the absorber and valve 1 and is mixed with the dry fresh gas. The gas mixture enters the lung via the Y-piece. The gas will not continue in the circle because valve 2 is closed. With one hand on the bag the operator palpitates lung filling and lung pressure. When the lungs are adequately filled the operator stops squeezing, valve 2 opens and valve 1 closes. The spring pressure on the pop-off (also called the automatic pressure limiter (ALP) valve plate determines at which pressure the valve opens, and thereby how large part of the gas is expelled. The opening phase of the valve corresponds to the high pressure phase at the end of inspiration.

The surplus gas enters a reservoir open to the ambient air, from the same reservoir a suction system aspirates at a flow rate high enough to secure that no gas escapes into the room. The open reservoir is an important safety measure so that the suction tube can not bring negative pressures to the circle and the patient lungs. A Y-piece is used as near to the patient as possible in order to separate inspired and expired gas. Dead space is the problem; the first gas inhaled to the lungs is the newly expired gas content of the patient airways. The dead space is the part of a breathing system common to inspiration and expiration. The volume of the trachea is a natural dead space. One-directional valves must be included in such systems to clearly define the inspiration and expiration tubes.

Fig.10 shows the main components of a circle system, however there are many ways of putting the components together. The one-directional valves may for instance be put in the inspiration and expiration tubes, or the bag tube may be connected to the right of the lower one-directional valve. The function in normal mode is perhaps not so different, but if something goes wrong the difference and consequences for the patient may be very dependent on the exact configuration.


The bag may be put into a tight bottle, and the outer volume may be cyclically pressurized by a ventilator.

Such a bag-in-bottle system is a part of the ventilator.

What happens if the bag is not squeezed? The patient is not ventilated, the fresh gas flows continuously directly to the pop-off valve.

Risk considerations Breathing systems: The uni-directional valves have important safety functions. If they are not functioning correctly (e.g. open all the time), tube connections swapped, pop-off (scavenging) valve closed, manometer or pressure relief valve at the Y-piece, water condensation in the expiration tubes, too high fresh gas flow. The wheel of a trolley pressing tube to closing.


Figure 10 Rebreathing circle with uni-directional valves 1 and 2

Humidifiers and nebulizers (therapy)

Passive humidifiers The humidifier adds water vapour to the breathing system. A simple way is to insert a HME filter (Heat and Moisture Exchanger) at the Y-piece, cf. Fig.12. Expired gas saturated with water is cooled when leaving the patient and the condensed water is absorbed in a sponge with a hygroscopic material. At the same time the latent heat of the condensation process contributes to reduce the patient temperature loss at the next inhalation.

Active humidifiersActive humidifiers are usually positioned on the inspiratory side of the breathing system. Often the condensation in some part of the system is so strong that water traps must be installed. The problem is that a breathing system is not isothermal. If the gas is water saturated in the warmer parts, the colder parts will cause water condensation. This can be avoided by using an electric heating wire inside the tube.

Hot water humidifierThe inspiration gas is lead over an electrically heated water bath with a sufficiently large contact area between water and gas. Added advantage of heat supply to the patient.

Ultrasonic vibrator humidifierUltrasonic vibrating plate near the water surface or water drops falling on the plate creates a water mist.

Gasdriven jet Bernoulli humidifieris a suction device (see last chapter) aspiring water into the jet stream and thus generating droplets. They may too large to be able to penetrate down to the bronchia.

Nebulizer (aerosols)The nebulizer is a therapeutic device for the inhalation of pharmaceuticals in aerosol form. Aerosols are particles (powder or droplets) suspended in gases, the therapeutically useful size spectrum is the diameter range 0,5 – 10 μm. The largest particles carry the main amount of substance (volume of a sphere is proportional to r3). For the larger particles sedimentation is an important deposition process. For the smaller particles diffusion is the most important deposition process (collisions with the walls). The smaller the particle, the deeper it penetrates into the lungs towards the alveoli. However, in the upper airways the smaller particles tend to evaporate in the air, the larger to agglomerate.

Two important nebulizer types are based upon jet generation and ultrasonic generation.

Risk considerations Nebulizers: Condensed water forms traps impeding intended gas flow. Jet humidifier may introduce high pressure in the breathing system. Growth of micro organisms in humid atmospheres.


GAS MEASUREMENTSImportant variables characterising a gas mixture are pressure, flow (volume) and concentration. Each desired variable has a separate transducer being able to selectively measure the variable. Probe is perhaps the broadest concept; sensor is a little more specific comprising the transducer and its protective housing perhaps with a sampling part bringing the transducer in correct position with respect to the gas to be measured. Sensor considerations include biocompatibility/disinfection/sterility. Transducer is the part which converts the energy correlated to the variable into (usually) electric form.

Sensor response time A response time better than 0.1 – 0.2 s is needed in order to obtain in-vivo undistorted real time curves during patient respiration.

Sensor selectivityA measuring instrument is constructed to be maximum sensitive to the intended (desired) variable(s). By selectivity we mean the degree of reduced sensitivity to other variables. Important unintended variables interfering with the measurement may be temperature, ambient pressure, water vapour, alcohol etc. Medical gas measurements are usually done in multigas systems, and interfering variables may then be all other possible gases than the intended. Example: The sensitivity of a paramagnetic oxygen analyzer to nitrous oxide (NO) (unintended) in an oxygen (intended) gas mixture. Selectivity is dependent on the measuring principle and whether the sensor is directly sensitive to the intended variable or the measured variable is recalculated to the desired variable. Example: Oxygen sensor sensitive to partial pressure [kPa], result to be given as oxygen saturation [%]. A special case is water, as vapor or condensed water droplets. There are two problems: the sensor may be sensible to water vapor in an unintended way interfering with he results. Or the sensor function is disturbed by being covered by liquid water. Some instruments dry the sampled gas before it is measured. The concentration of the intended gas may then be too high relative to what it is in the patient airway. In many cases the sensor is heated to cancel water condensation.

Sensor calibrationSingle point calibration, for instance a zero point calibration with an oxygen sensor placed in pure nitrogen. Two-point calibration with the oxygen sensor placed in pure nitrogen and then in pure oxygen; three-point calibration (checking linearity) adding measurement in air. NB! The measurements may be disturbed by interfering variables such as ambient pressure, electromagnetic radiation, relative humidity, mechanical position etc. Calibration intervals are dependent on sensor stability and needed accuracy.


Gas sampling Usually we are interested in the variables (flow, pressure, concentration) as near (proximal) to the patient as possible. The choice of sampling position is of interest, from proximal somewhere near the Y-piece to distal inside the ventilator. In the ventilator a sensor is well protected, at the Y-piece it must stand rough handling and the cables are a source of annoyance. These factors are well illustrated in the sidestream and mainstream sampling systems.

Sidestream sampling

A constant gas flow is aspired through a thin tube from the breathing system as shown in Fig.11. The choice of internal diameter and sampling (aspiration) flow rate must be carefully considered and be based on a compromise (See problem 12). The sampling flow rate [mL/min] should of course be small in comparison with the respiration flow rate. Even with a small sampling flow rate the gas velocity should be high so that the delay between the sampling and display instants is small. The gas concentration is a function of time at the sampling position on the respiration tube, but a function of position along the length of the sampling tube. The sampling flow is continuous, and along the tube length there will be gradients according to the concentration variation. A longitudinal diffusion process will occur, smearing the peaks out, but leaving the area under the curve unchanged. The curve will be softened, and the high frequency components reduced (low


Figure 11 Sidestream sampling to a multigas analyzer

pass (LP) filtering).To reduce this LP filter effect the gas velocity in the sampling tube should be as high as possible.

A HME (Heat & Moisture Exchange) filter is often used to reduce patient water and heat loss. Such a filter also reduces mucus (slime) in the distal part of the breathing system, so the sampling position should be at the distal part of the HME filter. The minimum internal sampling tube diameter is related to the trouble with tube obstruction. Sampling directly from the inspiration tube will be much easier, but expiration data is not obtained.

At the entrance of the instrument the sampled gas must pass a trap to take away mucus and water. The measuring chambers are accordingly spared for contamination. If the sampling point is chosen to be on the right (patient) side of the HME filter the risk of sampling tube and instrument contamination is larger.

Due to the suction pump there will be an increasing negative pressure in the sampling and measuring system from the Y-tube along the sampling tube, the trap, and the measuring chambers to the pump. (see problem 12.). The aspired gas can be brought back to the breathing system, or sent to a scavenging system. Paramagnetic oxygen instruments mixes the unknown gas with a reference gas (room air), and the returned gas to the breathing tube is therefore not the same as the aspired gas. By sampling just a small gas volume pr min the unknown gas is not very disturbed.

Some characteristic properties of sidestream sampling:Different measuring chambers may be mounted in series to form a multigas analyzer. Sampling gas is aspirated from the breathing system, this poses problems in paediatric anaesthesia particularly.Sample gas flow rate must be small and the tube thin to obtain sufficient high gas velocity so that concentration gradients along the sampling tubes are not smeared out. Measuring results not in real time but delayed e.g. 0,2s.Thin sampling tube can easily be obstructed. Humidity and mucus must be filtered out in a special trap before the gas can be allowed into the measuring chambers. The transducers are well protected inside the instrument.

Mainstream sampling The sensor is situated in the mainstream as shown on Fig.12. Even if there are no thin sampling tube there may also here be problems with humidity, poor transparency and mucus build up. The sensor must be heated to avoid water condensation. Characteristic properties of a mainstream sampling system are:

Measurement in real time. Difficult to realize multigas analysis in one sensor head. No thin tube which can be obstructed. Sensor head optics must be cleaned frequently. Sensor head increases dead space. Sensor head heavy, fragile and warm (anti condensation precaution).


Sampling inside the ventilatorSidestream and mainstream sampling are parts of the breathing system proximal to the patient. However, the sampling position may be moved to inside the ventilator/ anaesthesia machine avoiding extra cables/tubing outside the box. But then the results do not necessarily reflect true patient data. In a test procedure before use on the patient the Y-tube may be connected to a special connector on the machine so that the machine can apply gas a short moment and measure volumes and compliances of the breathing system. In this way the machine to a certain extent can calculate the true patient data continuously during use. In such a system the tubing must not be changed during use.

Gas concentration measurements Three different measuring principles in widespread use are shown in Table 4.

Table 4 Three measuring principlesMeasuring principle medi

umvariables time

constcomments

1a Spectrophotometric gas CO2, H2O, agent vapors

0.1s capnography included

1b Spectrophotometric blood O2 1-10s also in-vitro cuvette-


Figure 12 Mainstream sampling

pulsoximetry oximetry and in blood gas analyzers

2a Paramagnetic, contin. gas O2 10s sample gas unchanged 2b Paramagnetisk, pulsed gas O2 0.2s sample gas changed 3a El.chem. fuel cell,

membrane coveredgas or liquid

O2 30s limited lifetime, drifts and frequent calibration, single use

3b El.chem. polarographic membrane covered (Clark)

gas or liquid

O2 0.1-20s

membrane & el.lyte change and reuse, used in blood gas machine

3c El.chem. membrane covered (Severinghaus)

gas or liquid

CO2 30s used in blood gas machine

Table 4 shows different in-vivo and in-vitro principles for blood gases. This is of interest for quality control, but also raises questions with respect to which result is the most correct one. There are e.g. often discrepancies for the same patient between the oxygen results obtained with pulsoximeter (in-vivo), the sidestream paramagnetic analyzer (in-vivo) and blood samples analyzed on a stationary blood gas analyzer (in vitro). There are many reasons for these differences: the handling of the in vitro samples from the patient to the measuring instrument perhaps in a remote laboratory, different calibration, recalculation of data obtained with different measuring principles. In order to assess such problems it is important to know the different measuring principles and their characteristic properties. In this chapter a survey is therefore given of the different measuring principles and a more detailed description of gas analyzers. The non-gas instrumentation is more detailed described under clinical chemistry and intensive care.

Gas spectrophotometryIt is well known that he colour of oxygen-rich blood is reddish, of oxygen-poor blood bluish. When the photon absorption is within the visible spectrum such colour changes illustrates the spectrophotometric principle based upon the selective absorbance of light. Spectrophotometry is measurement of colour (colorimetry) and it may be used both in liquids and gases. Many gases are transparent and colourless in the visible spectrum (e.g. nitrogen, oxygen, water, argon) meaning that there is no photon absorption in that range. In the infrared (IR) spectrum however many of the gases of interest do absorb. Each gas absorbes in a characteristic way (Fig. 13), so that selective measurements are possible.

The following gases show selective absorption in the IR spectrum: CO2, N2O, water, anesthetic agent vapours. IR gas monitors measure the absorption at several wavelengths in the 3.3 or 8–12 µm areas and then solve a series of simultaneous equations to calculate the concentration. Multiple wavelengths are required in order to identify the anesthetic


Figure 13 IR absorption spectra for some anaesthetic agent vapours.

Datex Ohmeda Division, Instrumentarium Corporation

gases used, and the 8–12 µm range is preferred as this represents the area of the infrared spectrum where anesthetic gases show maximum absorbance (Fig.13). Automated anesthetic agent identification is then possible. But often the instrument must be told which gas is the intended; it is only the concentration which is unknown. The absorption may follow the Lambert-Beers law:

Equation 5 I = I0 e-Lμ or ln(I0/I)=Lμ

if the molecules are much smaller than the wavelength. Referring to Eq.5, I is the measured photon flux, I0 the input flux to the sampled gas, L absorption length in the gas and μ the linear attenuation coefficient (the product Lμ is the absorbance and must be dimensionless). If there are larger particles in the sample other attenuation mechanisms related to scattering will take place, not necessarily obeying Eq.5.The linear attenuation coefficient μ is dependent on the gas, wavelength and concentration [mol/L]. Concentration is the measured variable, but the displayed variable may be percentage [%] of the total volume. If the total pressure in the measuring chamber changes because of the suction sampling system or the barometric pressure, the concentration or partial pressure is proportional to the total pressure, but the percentage is independent.

Oxygen and nitrogen gas can not be measured spectrophotometrically because these gases do not have characteristic absorption bands in the optical spectrum.

Fig.14 shows how multiple wavelength measurements are possible. The sampled gas is aspired into the measuring chamber, where some photons from a filtered IR source are absorbed by the gas and others reach the IR detector on the other side. A rotating filter wheel inserts 6 different filters corresponding to different gases in rapid succession. The IR detector must be fast enough to discriminate between each filter. Each rotation also


http://bja.oxfordjournals.org/cgi/content/full/92/6/865#AEH154F3%23AEH154F3

http://bja.oxfordjournals.org/content/vol92/issue6/images/large/aeh154f3.jpeg

involves a reference filter and a stop filter. Partly overlaying spectra can be separated by using multivariat analysis on the different wavelengths measured data.

Figure 14 Multigas spectrophotometric gas analyzer with rotating filter wheel

The better the optical systems with filters and lenses, the better the selectivity. Filters in the 10 μm range do not look very transparent to the human eye!

Paramagnetic oxygen gas analyzerOxygen is one of the few gases which are paramagnetic. Paramagnetism and diamagnetism are the weak magnetic forces in contrast to ferromagnetism. Most substances are diamagnetic, meaning that the substance is repelled by the magnetic poles. Oxygen however is paramagnetic and will be attracted to the magnet poles. Around the poles of a permanent magnet the oxygen concentration is therefore higher than elsewhere


in the room. Unpaired electrons in the outer shell give the atom magnetic properties; this is the case for oxygen. Table 5 gives the magnetic susceptibility (intensity of magnetization) of some respiratory gases and shows that the selectivity for oxygen gas is due not to the gas being paramagnetic (+ sign), but that it has a more than 100 times higher magnetic susceptibility than many (not all!) of the other gases. As Eq.6 shows, the force will be proportional to the magnetic moment, which will be proportional to the oxygen concentration [mol/L] or partial pressure [kPa], and therefore also to the chamber total pressure.

Table 5 Magnetic molar susceptibility m of respiratory gases. SI unit: [m3/mol], but according to customary practice, cgs units are used and given here as m/10-6cm3mol-1

(CRC Handbook of Chemistry and Physics). gas m m relative

oxygen O2 +3449 +100nitrogen N2 -12 -0.35nitric oxide NO +1461 +42nitrous oxide N2O -18.9 -0.55nitrogen dioxide NO2 +150 +4.3water vapour H2O -13.1 -0.38carbon dioxide CO2 -21 -0,61argon -19.3 -0.56

The measuring principle is shown in Fig.15, it was invented by Nobel laureate Linius Pauling in 1946; The Beckman Oxygen Analyser. A diamagnetic gas (e.g. nitrogen) is enclosed in two spheres fixed to the end of an arm which is fixed to a suspended metal wire so that the arm can rotate. The rotation is read by a light beam reflected from a mirror fixed to the arm. The magnetic field is from permanent magnets.

Increased oxygen concentration disturbs the magnetic balance and the spheres are driven out of the magnetic field. The time response is slow, e.g. 5-15s, because of the large chamber volume and the mass of the dumbbell. A somewhat quicker version is made by fixing a magnetic coil to the arm. The coil is supplied with an electric current from a servo system so that the bell positions are virtually unchanged under different oxygen concentrations. The measurement result is read from the coil current.


The force F [newton] on a magnetic moment m [Am2] in a magnetic field of flux density B [tesla=weber/m2]:

Equation 6 F = grad (m B)

Accordingly, if the magnetic field B is strong but constant there is no force on m.

Figure 15 Paramagnetic oxygen analyzer. The construction is enclosed in a tight box with inlet and outlet for the gas to be examined, the reference gas is enclosed in the two spheres.

Fig.16 shows a more rapid system using a pulsed magnetic field. It is a differential measuring principle, using a differential pressure transducer (microphone) to detect the difference in magnetic action on the unknown gas and a known reference gas, usually room air. Increased concentration of oxygen leads to increased suction of the oxygen molecules into the magnetic gradient field zone, and therefore reduced pressure outside the zone. In order to have a response time < 0.1s, the magnetic field is switched at a frequency of a few hundred hertz. The differential measuring principle implies that the gas output is not the same as the aspired unknown gas.

An important advantage with the paramagnetic measuring principle is long term stability and minimal need of maintenance (if the measuring chamber is kept clean). The differential principle of the pulsed type may also be advantageous. However, the measuring chamber is heavy (the magnet) and must therefore be mounted in a sidestream sampling system.


Figure 16 Paramagnetic oxygen analyzer using pulsed magnetic field. Gray lines are tubes.

Multigas analyzersFig.11 showed a multigas analyzer in a sidestream sampling system. The gas first arrives to the spesctrophotometer where CO2, N2O, water and anesthetic agent vapors are measured. Then the gas sample is mixed with the reference gas and oxygen concentration is measured.

Problems with such instrumentation are the risk of contamination of the measuring chambers. Water condensation in the chambers must be avoided, by warming the chamber and/or filtering the aspired gas before it enters the chambers. This filtering system is an important part of the construction and determines to a large extent the robustness of the system. If mucus and other contaminations still reach the measuring chambers, special rinsing liquids plus days of dry gas flushing may bring the instrument alive again.

Other gas measuring principles Membrane covered electrochemical electrodes for oxygen and carbon dioxide are described in chapter 10.2. They can be used for measuring partial pressure both in liquids and gases.

Mass spectrometers (MS) are large and expensive instruments which therefore must use sidestream sampling. A MS consists of a vacuum chamber where the gas is ionized. The ions are accelerated and focused in an electric field by suitable electrodes and then enters a magnetic field zone where they are deflected according to their mass. The selectivity is


very good, but molecules with identical mass numbers will interfere with each other, for instance N2O and CO2 which both have mass number 44. It can measure several different gases fast enough to measure respiration in real time. The instrumentation maintenance is expensive, and the instrument is best adapted to research or in a central installation serving several patients simultaneously.

More on measured and calculated variablesIf the total pressure of a gasmixture e.g. air is increased (Fig.17), the partial pressure of oxygen (pO2) increases, but the oxygen volume % is constant. Sometimes we are interested in the volume %, sometimes in the pO2. Let us take the example that oxygen is measured by a partial pressure (pO2) sensitive sensor, but the result is recalculated to volume %. The volume % will display false values if the total pressure varies. This illustrates that the transducer working principle should always be known.

Figure 17 Closed variable volume

Another example is the one shown in Fig.7, where a water dish is inserted into a dry gas chamber. Then the % oxygen will fall, but not the pO2. In medicine the inspired gas will always have a lower water content than the expired gas.

Gas pressure sensorsPressure is force per area: P = F/A. The classical measuring device is a liquid filled tube measuring level difference. Often it is formed as an U, if closed in one end it measures absolute pressure, if open it measures relative (gauge) pressure. If it is filled with water cmH2O may be the preferred unit, if filled with mercury mmHg may be preferred. A mechanical pressure measuring instrument is called a manometer. A pressure sensor alone is not a manometer.

Fig.18 shows two sensors both based upon a precision moulded thin membrane as a part of the sensor house, cf. Fig.2.4 in chapter 2 Webster. The membrane thickness and material properties determine the deflection pr change in pressure level. The thinner the membrane the more sensitive the sensor, but also the more fragile the membrane. The deflection is in principle not a linear function of pressure. The deflection also determines the compliance C of the sensor: C = ΔV /ΔP [mm3/kPa]. ΔV is a volume which implies a


transport of inert substance (gas molecules in this case) to and from the membrane according to the variable pressure. Compliance is therefore important for the dynamic response, not for static or slowly varying pressures. A high quality sensor shall have a stiff membrane and low compliance. The membrane deflection may be measured by a piezoelectric beam (left) or an optical reflection system (right). The sensor principles can be operated with the interior volume closed. Then absolute pressure is measured, but if the closed chamber is gas filled this introduces a temperature dependence according to Eq.3. With the interior open to the surrounding air relative (gauge) pressures are measured. With a tube connected to the interior we have a differential transducer which e.g. may be used for flow measurements, see under gas flow sensors. A dome closing the volume above the membrane may be connected to a second tube to adapt it better for differential measurements. The dome can be made of plastic with a soft membrane interfacing the sensor membrane. The complete dome may be sterilized so that it can by used in invasive applications. The whole sensor house may also be sterilized for use as a tissue implant.

Figure 18 Pressure sensors. Left: piezoelectric transducer with an optional dome to be positioned so as to form a closed volume above the membrane. Right: optical transducer

Important specifications for a pressure sensor are: sensitivity, compliance, linearity (membrane property), hysteresis, membrane absolute maximum pressure, size, temperature zero drift, temperature sensitivity drift, temperature range, long term stability, negative pressure properties, sterilizability, biocompatibility.

Gas flow sensorsRotameter and turbine flow meters, mainstream

A rotameter consists of a slightly conic vertical glass tube with a bobbin at the bottom. With gas flow through the tube the bobbin lifts and starts to rotate. The scale is engraved on the tube external surface so the flow rate can be read. The rotameter is the classical instrument for measuring flow, however it is non-linear, gas viscosity dependent, critically dependent on the conic boring and the bobbin size, and it is a unidirectional device. The calibration is valid only for one gas, and it must be used in a vertical position.


The free bobbin may be replaced by a propeller or turbine with a fixed axis. Such a device is bidirectional, and the rotations pr minute may easily be read by an optical system so that an electronic flow rate signal is available.

Hot wire flow sensor, mainstream (also Fig.8.13 in chapter 8)

Figure 19 Hot wire flow meter with two termistors, cross section shown to the right

Two temperature dependent resistors (thermistors) are used in a tube, one of them is used for measuring gas inlet temperature (Fig.19). The other is heated by an electric current at the same time as its resistance can be measured (how?). The flowing gas with a lower and known temperature cools the heated thermistor according to the gas velocity and the temperature is followed by monitoring its electrical resistance. It is a spot sensor, and the thermistor transducers can be made very small with thin wires, with just a small disturbance of the flow profile. The construction is robust and lightweight, but the position of the thermistors is critical. In its simplest form as shown it is a unidirectional device. The sensitivity is dependent on the thermal properties, heat capacity in particular, of the gas. It is sensitive to gas pressure.

Vane deflection flow sensor, mainstream

Figure 20 Vane flow sensor in a tube, cross section shown to the right


Vane deflection is dependent on gas velocity and density as well as gas pressure. Non-linearity dependent on the back eddy behind the vane, less pronounced if the vane is soft and deflects. Direction sensitive. Vane disturbs the parabolic flow profile, and it is more a cross sectional area sensor than a spot sensor. Transducer may be of a piezoelectric or straingauge type.

Pitot flow sensor (also with remote transducer)

Figure 21 Pitot flow sensor in a tube, cross section shown to the right

The Pitot tube flow sensor is based upon Eq.8 (Bernoulli) and the so called kinetic part of it: ½ v2. The sensitivity is accordingly dependent on gas density (and therefore gas pressure) and the calibration factor is dependent on gas type and the gas mixture. The output is proportional to the square of gas velocity, and Fig.21 shows two Pitot tubes with the tube opening with and against the velocity direction. The sensitivity is doubled with a kinetic factor v2. Actually it is a gas velocity spot sensor, and a velocity profile has to be assumed to obtain flow. The disturbance of the velocity profile, turbulence included is considerable as small diameter Pitot tubes destroy the dynamic properties of the sensor. If the tubes and transducer are low compliance components mainstream sampling with a remote differential transducer is possible. The sensor is direction sensitive. The pressure differential transducer may be based upon light reflection from the sensing membrane, discuss other possible technologies.

Poiseuille flow sensor (also with remote transducer)This sensor measures differential pressure ΔP across a well defined flow resistance, it is also called a pneumotachometer. Because it is based upon the law of Poiseuille (Eq.2) the sensitivity is gas viscosity dependent. The dimension of R in Eq.2 is [Pa/m3/s]. If the mean pressure in the tube doubles the molar flow [mol/s] doubles, but R is constant because gas viscosity has a surprisingly low pressure dependence. Volume flow [m3/s] is therefore the intended variable of this sensor, not molar flow. The viscosity dependence is a problem if it is a mixture of different gases; each gas will have its own calibration factor.


The pressure difference is proportional to gas flow if the flow pattern is laminar. The pressure difference is measured at the wall where the gas velocity is zero, and the pressure difference represents the whole cross sectional area. Flow direction can be determined. If the sampling tubes and transducer have low compliance the transducer may be remote as a part of a main stream sampling system, it may be then be realised as a very robust system.

Figure 22 Poiseuille gas flow sensor (pneumotachometer) The flow resistance can be obtained with or without a narrow passage zone as shown on Fig.22. A narrow zone increases sensitivity ΔP/Q (Poiseuille). A so called Fleisch tube is a special resistance with many small gas channels (capillaries) in parallel. Thin capillaries will be vulnerable to accumulation of secretions or other contaminants and from the condensation of water vapor. Therefore the tube must be heated if used on expired gas.

Doppler flow meter, mainstreamA usual ultrasound Doppler flowmeter for blood is not useful for ordinary gas measurements. Blood flow can be measured because the blood cells are sufficiently large (about 5-10 μm), small molecules do not give sufficient reflected signal strength. However, in a gas the flow can be measured in transmission instead of reflective mode. The velocity of sound will be higher if sound direction is in the gas flow direction. The problem is that such transmission mode requires separate transmitting and receiving probes on each side of the organ.

VENTILATORS (SUPPORT AND THERAPY)When the patients are unable to breathe themselves they must have artificial (assisted) respiration. As this can go on for many hours and days (intensive care units) it must be taken care of by a machine, the ventilator5. We are talking about lightening the bagging

5 Respirator and ventilator are usually considered to be synonyms, perhaps with a certain tradition that respirators are simpler devices, e.g. not electrically driven.


burden for medical personnel, giving them a “third hand”. In addition to this support function the ventilator is used therapeutically by controlling pressure and flow to the optimum condition for lung healing.

As we have seen earlier in this chapter the natural lungs initiate inspiration by lowering the diaphragm thus increasing the lung sack volume and creating a negative pressure in the lung alveoli so that external gas is inhaled. The ventilator functions the opposite way, during inspiration the ventilator creates a higher pressure pushing fresh gas into the lungs. It is only the “iron lung”6 with the whole patient (except head) enclosed in a chamber that reproduce natural physiological conditions: chamber reduced pressure initiates inspiration.

An advanced ventilator must, like a pacemaker, have a demand modus. If there are no efforts from the patient, the ventilator should be in control. But as the patient recovers or wakens, the patient starts to breathe spontaneously, and the ventilator can let the patient gradually take over. The respiration cycle time parameters are important: inspiration time, pause, expiration time. An advanced respirator may be set either to volume or pressure controlled mode. In the volume controlled mode the inspired volume is measured continuously, and when it reaches a preset level the inspiration phase is terminated. Pressure is also measured continuously, and the user has chosen values that shall not be exceeded or shall result in an alarm. For instance a selected PEEP (Peak End Expiratory Pressure) modus decides when to leave the expiration phase so that the lung pressure does not drop below a critical point where the lungs might collapse. In the pressure controlled mode it is the pressure reaching a preset value which triggers the ventilator to end the inspiration phase. The choice is made from what is considered to be best for the patient. And remember that the flow and volumes which we just have referred to are different in different parts of the airways inside and outside the patient, the sampling position is important as already stated in the subchapter on gas sampling.

Technology: piston, bag-in-bottle, servoSmall ventilators for use outside hospitals may be purely gas driven, but all advanced ventilators are electrically driven. The main components are:

1. A pressure generating and controlling device (gas under pressure or electric pump)

2. A cycling device with timer, changing the modus between inspiration, pause and expiration

3. Sensing elements and displays

The most direct version is to put the bag of a manually operated breathing system (Fig.10) system into a bottle, and control the pressure in the bottle from an external ventilator supply. Another way is to let a motordriven piston supply the pressure cycle. Fig.23 illustrates a servocontrolled system, shown during the inspiration cycle.

6 Used for patients with poliomyelitis in the large epidemic outbreak in the 1950ies.


Figure 23 Servocontrolled ventilator shown in the inspiration cycle

The fresh gas enters a pressure-controlled static reservoir. The pressure setting there determines the maximum pressure which can be supplied to the patient, and is therefore also a safety feature. The gas then enters the inspiration servo-controlled part. The flow in this example is measured by the deflection of a vane and compared with the set-point coming from the electronic control box. The minute volume is a basic parameter set by the operator and used in the control system. Deviations result in a servo correction signal which is sent to the control valve. The set point can represent different wave forms, not just a square wave on-off function. The inspiration pressure is measured and the information sent to the control box. Dependent on the mode chosen by physician the pressure can control the servo e.g. during CPAP operation (Continuous Positive Airway Pressure). During long-term patient treatment in an intensive care unit the gas must be humidified. In series with the humidifier a vaporizer for anaesthetic volatile drugs may be inserted for use in the operating room. When the control box so determines the inspiration phase is ended, the inspiration valve closes and the expiration valve opens. Also here the expiration flow is measured continuously and the control box can set up any flow curve preferred. The expiration pressure can also control the valve if the ventilator is set in such a modus (e.g. PEEP Positive End-Expiratory Pressure). Both pressure measuring systems may be coupled to alarm circuits if a preset pressure level is exceeded.

The ventilator shown on Fig.23 has no rebreathing system.


Compression losses

Figure 24 Compression loss model

The pressure P in the patient system increases during the inspiration cycle when a piston is pushed in as shown on Fig.24. The lungs are gradually filled until the machine ends the inspiration cycle. Let us simplify and only consider the inspiration and expiration tubings and with zero compliance. With a certain ventilation volume ΔVresp, the pressure build up is dependent on the volume of the two tubes V1 = Vi + Vex. According to the universal gas law P2=P1V1/V2 in a closed system. If V1= 1L, P1=100kPa and ΔVresp is 0.2L, then P2=1·100/0.8 = 125 kPa. If the lung compliance CL = ΔVL/ΔP is low and the airway resistance R is high, the rise in pressure ΔP = 25 kPa may result in a very low lung ventilation ΔVL. The ventilation of the lungs may be much smaller than the ventilator setting. Therefore: the larger the tube volume with respect to the ventilation volume, the less part of the ventilator gas enters the lungs of the patient. This is called ventilation loss, and must be taken into consideration especially with small and stiff lungs (children) or long tubes.

Risk considerations Ventilators Difference between inspiratory system and expiratory system with respect to humidity, mucus (slime) and need of disinfection and sterility, especially in one-way breathing systems. Transducer breakdown. Gas leakage, wrong tube connection. System control before a new patient is connected to the ventilator. Stops, electric power failure, gas delivery failure. Accidental change of settings.


ANAESTHESIA MACHINES (SUPPORT AND THERAPY)Anaesthesia may be done by the injection of an agent into the blood and/or by inhaling a substance like nitrous oxide or a volatile vapour like sevoflurane. In addition to gas anaesthesia, anaesthetics are also given directly into the blood stream. Here we will describe the anaesthesia gas / vapour delivery system as a separate machine, although it may be an integral part of a ventilator and/or a patient monitoring system. Its main purpose is to deliver fresh gas to the patient via a breathing system and perhaps a ventilator. It may comprise also the gas scavenging system and the suction device. The

suction described later in this chapter is an important additional device, and the breathing system may be directly coupled to the machine or also to a ventilator supplied by the machine. The multigas analyzer may also be on the same trolley.

Fig.25 shows the main components: the supply of medical grade air, N2O and O2 gases, the gas mixer with flow monitors, the vaporizer and some safety devices.

The gas supply may either be from local gas bottles filled to high pressure e.g. 60 bar (N2O) or 150 bar (O2), and equipped with manometers, pressure reduction valves and simple gas flowmeters. Or it may be taken from the hospital installed gas pipeline system at medium high pressure (3-7 bar).

The gas mixer has individual gas flow sensors [L/min] for each gas,

measured before mixing. Flow setting is adjusted with individual spindle valves. The gas mixer is connected to the vaporizer, where anaesthetic volatile vapors may be added such as: Halothane, Enflurane, Isoflurane, Sevoflurane, Desflurane and ether.

Often the machine comprises a gas scavenging system and suction for clearing airways. The surgeon may have their own suction for use in the wound.

Risk considerations Anaesthesia machineLoss of oxygen is of course critical, and a special oxygen flush can supply large direct oxygen flow (NB! lung pressure). Functioning of the suction may be critical. Anaesthetic dose is important, and concentration measurements in the breathing system near the patient are very useful.


Figure 25. Anaesthesia machine

SPIROMETERS (DIAGNOSIS) A spirometer is an instrument for measuring lung volumes. There are basically two different types: the water sealed and the pneumotachometer models. This is an instrumentation which usually need not be sterile, disinfection procedures are sufficient.

Water sealed modelsFig.26 is an outline of the system. The patient is connected to the respiration tube via a tight mask. With a few breaths the patient respires into a closed volume dominated by the gas drum. The drum rises and sinks with minimal friction following the respiration. The drum is balanced by a counterweight, and the vertical movement is registered by a pen

Figure 26 Spirometer, watersealed

fixed to the connecting wire. The scale is graduated in litre. The respirogram is then drawn on the paper passing under the pen. Because of the inertia of the system the instrument is best adapted to static or slowly changing volumes. Precautions must be taken to avoid problems with temperature changes and humidity with condensation. An important advantage of the instrument is stability and ease of calibration; it is well suited to be a standard reference instrument in a laboratory. The spirometer represents a closed


volume of maximum compliance. In the form shown on Fig.26 the spirometer can not determine the residual lung volume. By using a dilution technique with a tracer gas7

absolute volumes can be determined.

Risk considerations The breathing system in its simplest form as shown is closed and without CO2 absorber, the measuring time is accordingly very limited. It may have a bothersome cleaning and disinfection procedure between each patient.

Electronic modelsThe electronic models may be very small, wireless and convenient, and they are also well adapted to dynamic measurements e.g. of FEV (Forced Expiratory Volume, often during the first second, FEV1). The model may be just a mouthpiece with a differential pressure transducer coupled to Pitot tubes or a flow resistance in the form of a narrowing tube or Fleisch flow resistor, se earlier in this chapter. In this form they are also known as pneumotachometers. The signal from the transducer may be transferred wirelessly to a computer system, where signal processing may produce e.g. volumes by integration of flow, peak flow values etc.

Risk considerations Spirometers The tubes in contact with the mouth need not necessarily be sterile, but disinfected or acquired for single patient use.

WHOLE BODY PLETHYSMOGRAPHS (DIAGNOSIS)Determination of absolute lung volumes and airway resistance can be performed in a whole body plethysmograph, Fig.27. The patient is closed into a chamber and breaths into a mouthpiece with a pressure sensor and a flow outlet where the flow can be stopped by a closing actuator. Chamber volume Vc is known, and the chamber pressure Pc and mouthpiece pressure Plu are measured. At the end of a normal expiration the gas flow is closed, and the patient performs forced ventilation against the closed mouthpiece. The 7 tracer means a substance not harmful to the patient and the concentration of which can be conveniently determined.


variations in chamber pressure ΔPc and airway pressure ΔPlu are measured as static values (sufficient time at inspiration and expiration effort). In this way it is no flow at the sampling moment so that the mouthpiece pressure is equal to the lung pressure. By considering the whole body but the lungs incompressible the forced respiration pressure diminishes the lung volume and increases the chamber volume by the same amount, and reduces the chamber pressure Pc. By using Boyle-Mariottes law it is possible to show that the lung volume (residual volume included) Vlu is given by:

Equation 7 Vlu = - Vc ΔPc /ΔPlu

Risk considerations Some patients have a problem with being in a closed narrow chamber (claustrophobia). Mouthpiece and airway must be disinfected or be of single-use types.

Figure 27 Whole body plethysmograph


HYPERBARIC OXYGEN THERAPY

Figure 28 Hyperbar chambers

Hyperbaric oxygen therapy is performed with the patient in a closed chamber and by slowly supplying pure oxygen washing out the nitrogen of the air in the chamber and in the patient. Slowly the oxygen pressure is increased up to e.g. 2.5 bar absolute pressure. At the end of treatment the pressure is slowly taken down to atmospheric pressure and the pure oxygen replaced by air. The chamber may be a complete room where patients can walk around (like the models used for divers). In hospital smaller units (Fig.28) are used for each patient and several patients can be treated simultaneously in one department.

Oxygen itself does not burn or explode, but it increases the burning rate of combustible materials. A paper strip burns 30% faster at 25% than 20% oxygen. NASA allows 25.9% in its space shuttles. Oxygen rich environment is defined as >25% or 27.5 kPa partial pressure by IEC (IEC 2005). 100% oxygen implies explosive combustibility. In hospitals oxygen rich environment is used in infant incubators (chapter 13.7). It is well known from such use that too much oxygen is dangerous for the infant, in particular the eyes. The toxic property of oxygen is used in hyperbar therapy e.g. for enhanced healing of selected problem wounds.

Risk considerations Hyperbaric chamber Hyperbar oxygen therapy can only be used if all the necessary precautions have been taken. These precautions must have been taken before the closing of the chamber for oxygen pressure build-up. Pressure chamber certified for the necessary pressure, safety margin included. Most materials change from normal inflammable to explosive in pure oxygen. Prohibited to use inflammable liquids/vapours e.g. for disinfection just before or

under therapy. Use of open flame absolutely forbidden. Antistatic precautions inside the chamber so that no spark can ignite an explosion. Special precautions for patient monitoring equipment.


VENTURI SUCTION SYSTEM (SUPPORT)Suction may be very important during surgery and intensive care. The operating field must be kept free from blood and liquids hindering visual control. During intensive care it is critical to keep the airways free from mucus and other obstacles. The suction is a vital part for respiratory systems.

Fig 29 shows the main components of a Venturi suction system, it is gas (air) driven. Other models may use an electric pump to create the vacuum. The operator holds the suction tube handle in the airways or as a sterile instrument in the wound. The aspired debris is assembled in the bottle. The bottle is kept at low pressure via a second tube connected to the Venturi. For the operator it is important that suction is available when needed, and that the degree of vacuum and flow can be chosen. The suction must not be too strong so that tissue is destroyed, but strong enough to ensure safe removal of debris. The tubing must be reinforced so that it does not collapse at high vacuum. It must be sterile for many of the procedures.


Figure 29 Venturi suction system

The suction device can be characterised by static and transient parameters. The static parameters are max. vacuum (min. absolute pressure) and flow capacity as a function of drive gas pressure. These parameters are determined by the Venturi construction and the driving gas flow rate. The dynamic parameter is the transient flow for instance in the starting phase if the operator has closed the suction inlet for vacuum build up. Flow and volume is then also dependent on the bottle volume which again is dependent on the degree of bottle filling.

Venturi / Bernoulli principleThe Venturi principle is interesting and somewhat contra intuitive: Is it possible to create vacuum pressure from high pressure? The clue is to make the gas molecules pass a nozzle (cone) where they are accelerated. Venturi8 developed this practical method, Bernoulli9 had already explained it from his discoveries in kinetic gas theory and mathematical modelling, see Eq.8.

As Fig.29 shows, a high pressure gas supply is coupled to the Venturi, where the gas must pass the cone. The increased kinetic energy during molecule acceleration is taken from the gas pressure. The local pressure is reduced (Bernoulli text box) and picked up by the perpendicular tube there. Notice that the Venturi outlet contains both the driving gas and the aspirated gas from the bottle.

8 Giovanni Baptista Venturi, 1746-1822, Italian physicist, studied hydraulics, sound and colours.9 Daniel Bernoulli, 1700-1782, part of an important Swiss scientist family studying astronomy, mathematics, medicine, hydrodynamics.


Figure 30 Vacuum pressure as a function of static suction flow

Equation 8 Bernoulli

Ps + ½ v2 + gh = constant

Ps = static (v=0) pressure [Pa]. = density [kg/m3]. v = velocity [m/s]. g = acceleration due to gravity [m/s2]. h = height difference [m].

Validity range: Laminar flow, any geometry, valid at any point along a line of flow, gases or liquids, no frictional (viscous) losses.

Actually the Bernoulli equation is about the conservation of energy along a flow line, but it is usually given as here in terms not of energy, but pressure.

Fig.30 shows the pressure/flow curve for a Venturi driven by a gas pressure of 5 bar at 38 L/min. The bottle between the Venturi and the patient will act as a capacitor, so the flow at the suction handle may be different from the Venturi suction flow. If flow is stopped at the suction handle the Venturi will start emptying the suction system. The vacuum pressure will increase gradually until the Venturi no longer can draw any more gas molecules from the system. The system can be characterised with a time constant, the larger the bottle volume and the less the Venturi flow, the longer the time constant. At zero suction flow a static (maximum vacuum) pressure level is reached. With open flow suction handle maximum flow will occur, and the vacuum pressure in the bottle will be small. Taking the time constant into consideration the operator can chose at which vacuum level suction shall start. The time constant will be dependent on the degree of liquid filling of the bottle. The static vacuum pressure may be too high for certain procedures; the sudden start from closed to open suction head may be too violent for the tissue concerned. A false air leakage device between the pump outlet and the room air can be inserted so that max. vacuum is reduced, without influencing the suction capacity at lower pressures.

Dynamic performance analysisThe suction system is a simple medical device which lends itself well for the use of simple models to understand its function better. A usual practise is to use electronic equivalent circuit models, Fig.31, as knowledge of electric network theory is widespread.


Suction and aspiration are synonyms.

Figure 31 Equivalent electrical circuit for a dynamic suction system

Ohms law is ΔV=RI , and the equivalent pressure formula is ΔP=RQ. A voltage source is therefore the equivalent of a suction pressure pump. The source voltage is designated as V, but V is not in [volt], but in pressure [Pa]. The output is negative, so that direction of flow Q (current I) is suction. With a series internal resistor Ri the voltage source becomes non-ideal. The pump vacuum pressure will then be dependent on flow, in agreement with Fig 30. The resistance of the tubings is not in [ohm], but [Pa s/m3] and can be calculated with the Poiseuilles formula. The bottle is modelled as a capacitor, and its capacitance is not [farad], but molecules/pressure, that is [mol/Pa] or [V/RT] where R is the universal gas constant (≈8 [J/molK]). The output is suction flow [m3/s or L/min].

Electric formulas such as Ohm’s law or the relaxation time constant =RC can now be applied. The formulas are electric, but the quantities mechanical. From Fig.31 it can for instance be seen that the time constant for attaining vacumm in the bottle is smaller with closed handle ( = Rp||Rs C) than with open handle ( =Rp C).

Example: Find the tube-bottle time constant . The tube is 1m long with inner diameter 20 mm and coupled to a closed bottle of volume 10L. R is found from Poiseuille (viscosity 10-3) to be 0.3 106 [Pas/m3]. C is V/RT equal to 4 10-6

[mol/Pa]. =RC is equal to 1.2 seconds.

For consideration: The maximum suction flow in [L/min], is it greatest in air or in water?


Adiabatic gas lawThe thermal effect of a gas volume expansion is described by the adiabatic gas equation:

Equation 9 T1 ·V1k = T2 ·V2

k

whereT = temperature [o Kelvin]V = volume [m3]K = constant dependent on gas, e.g. 0,4 for O2

Validity rangeClosed volume thermally isolated (adiabatic condition)Ideal gas (far from condensation)

Risk considerations VenturiGas dependent, not electricity dependent Obstacles in the Venturi exit filter reversing vacuum suction pressure to positive blowing pressure. Using a tube of a diameter so that all debris may pass.Room air pollution, central vacuum installation ?Sterility, disinfections

CRYOTHERAPY Cryo- is a word of Greek origin meaning cold and is the antonym to thermo- (thermostat – cryostat). The cryo technique is used in general surgery and by ophthalmologists and dermatologists to destroy tissue. It is often an ablation method, meaning that the dead (necrotic) tissue is not cut out and taken away, but left in-situ. But it may also be a technique where frozen tissue can be taken out as an alternative to scalpel surgery.

Once the cells are destroyed, components of the immune system - primarily the white blood

cells - clear out the dead tissue. A killing mechanism is ice formation only outside a cell that causes the cell to shrink as it gives up water by osmosis to replace the water that has


Figure 32 Cryo principle according to a Joule-Thomson capillary model.

turned into ice. As the area thaws, water rushes into the shrunken cell and causes it to burst. For this reason, cry therapy often consists of a series of steps in which the tissue is repeatedly frozen and thawed. At lower temperatures intracellular ice formation is important, below approximately -40°C intracellular ice crystals begin to form that destroys the cells completely. Tumour cells also die when their blood supply is choked off by ice formation within small tumour vessels.

The cryo source is a gas under pressure at room temperature. The effect is based upon the temperature drop in an expanding gas, the Joule-Thomson effect. An expanding gas performs work, and the energy is taken from the internal heat energy of the gas, and the gas temperature drops. The basic model is described by the adiabatic gas equation 9 (see text box). In the original Joule-Thomson experiment gas was flowing through an insulated pipe with an obstacle in the middle, in the form of a porous disk or a silk handkerchief. The temperature and pressure were measured on each side of the disc. The usual practical construction is to replace the disk by a capillary, Fig.32. The high pressure room temperature gas is brought into the capillary so that one end is at e.g. 67 bar and the outlet at 1 bar. The pressure drop along the capillary is due to the viscous losses as illustrated by the law of Poiseuille. These losses generate heat reducing the cryo effect. Capillary length is determined to give the correct resistance and therefore a suitable flow rate.

Due to the gradual fall in pressure the gas expands, and because the same number of molecules (mol) must pass a cross-sectional area per second, the gas velocity increases gradually. Due to the gas expansion the temperature drops if the adiabatic cryo effect is larger than the heat effect from the viscous forces. Coming out of the capillary the gas is


Figure 33. Principle components of cryo

equipment according to Joule-Thomson

at its minimum temperature, and is sent right to a wall constituting the internal part of the active cryo probe surface to be brought in contact with the tissue.

The basic components of a cryo instrument is shown on Fig.33. The freezing cycle is started by pressing the footswitch. In approx. 5 seconds a freezing temperature of approx. -80°C (-176° F) is reached. After release of the footswitch the freezing cycle stops and the defrost cycle starts automatically as the pressure in the probe increases and the gas is compressed and heat is liberated. In this way the cryoprobe is defrosted within 5 seconds without use of electric heating. The tip is equipped with a cryometer (cf. thermometer). The same equipment may handle either carbon dioxide or nitrous dioxide without any modification. Special more performing systems use both argon and helium gas.

It is important that the supply gas is very dry, since water will freeze and tighten the capillary nozzle. The gas passes the pressure regulator and then through a flexible but highly armoured tube out to the capillary. In the defrost cycle the high pressure will propagate also to the outlet tube, which accordingly also must be able to withstand maximum pressure. Argon, CO2 and nitrogen are not polluting, while N2O should be taken care of by a scavenging system.


Figure 34 CO2 phase diagram

Risk considerations capillary cryo1. Sudden loss of cryo effect (very thin capillary, tiny gas obstructions)2. Room gas pollution

Cryo sprayThe phase diagram for CO2 on Fig.34 shows that at room temperature the pressure in a filled10 CO2-bottle is about 67 bar, and a part of the substance is in the liquid phase. If the bottle is in the vertical position and the gas (vapour) is abruptly let out in the room the pressure suddenly drops to 1 bar. Because of the gas expansion the temperature quickly drops, for a short time down to -78 oC or lower. The CO2 gas is transformed to the solid phase (“dry ice”) without passing the liquid phase. Because of the non-adiabatic conditions the snow soon reverts to the gas phase. The liquid phase is an impossible state for CO2 if the pressure is below 5.1 bar. The point in the phase diagram where all three phases meet is called the triple point (5.1 bar, -56 oC).

A cryo source may also be cold, a liquid gas kept in a thermos bottle. Liquid nitrogen (LN2) may be kept in a thermos bottle at -196 oC and 1 bar, or at higher pressures and temperatures. Liquid nitrogen applied with a spray or probe (temperature of -196º C) is much colder than liquid nitrogen applied with a swab (-20º C), than cryogens that come in disposable spray cans (-55º C and -70º C), and than nitrous oxide (-75º C). Table 2 shows the boiling point of some usual gases.

In its most simple form for patients a cryo liquid or vapour is applied directly on the tissue, e.g. the skin (cryospray). In dermatology or surgery liquid nitrogen (LN2) is often used because of its powerful cryogenic effect. Cryosurgical freeze times vary according to lesion type, size, depth, and location. The handling of liquid nitrogen is of course cumbersome, and therefore Joule-Thomson based cryo equipment is much used.

10 CO2 in liquid form is incompressible. Therefore a CO2 bottle completely filled with liquid is dangerous. The supplier is not cheating when a “filled” bottle has a vapour pocket at delivery to allow for only a moderate pressure rise at rising ambient temperatures.


PROBLEMS1. Find the compliance of the lungs when the ventilator has been set to a ventilation

volume of 3L and the pressure varies during inspiration from 10 to 15 cmH2O. 2. In a breathing system water droplets condensate on the sensor causing false

readings. How can the problem be solved?3. How would you calculate how much [L] gas you have left on a bottle of N2O?

And a bottle of compressed air? Both bottles are equipped with a manometer. 4. What is sensor sensitivity? And selectivity?5. Referring to Fig.9: Discuss fresh gas flow rate adjustment and patient lung

pressure safety (hint: bag as a pressure stabilising device).6. What happens on Fig.9 if the bag is not squeezed? And at Fig.10?7. An oxygen sensor has partial pressure as primary variable pO2 , and the results are

given in [kPa]. Does a varying barometric pressure influence on the measurements if all other factors are constant? Explain.

8. What is the difference between a spirometer and a pneumotachometer?9. Is the paramagnetic oxygen analyser sensitive also to diamagnetic gases? Discuss.10. Derive the law of Laplace for a tube of radius r and wall tension T. (Hint:

consider half of the tube and put the tension contribution equal to the pressure contribution. The pressure contribution has a radial direction and only the vertical component is selected and integrated around the half tube circumference.)

11. Find the resistance for a 1 meter long suction tube with internal diameter 2 and 10 mm. Air viscosity 18.6 10-6 [Pa s]. Calculate the pressure drop at a flow rate of 30 L/min.

12. The aspiration flow to a multigas analyzer is 0.2 L/min through a sampling tube of length 2 m and internal radius 0.4 mm. Calculate the signal time delay between patient gas flow and analyzer. Calculate the necessary pump negative pressure to assure the sampling gas flow under the assumption that pressure drop in the analyzer itself is negligible. Gas viscosity 20 10-6 [Pa s].

13. Referring to the Venturi suction system equivalent circuit on Fig.31: Calculate the time constant with 5L air in the bottle, tube internal diameter 8mm, suction tube length 1 [m], patient tube length 2 [m]. Is the suction flow dependent on whether suction is performed in air or in water. Discuss the effect of a parallel leakage resistance from the suction pump outlet to the surrounding air.

14. For the paramagnetic oxygen analyzer, discuss the linearity problem on the basis of eq.6.


chapter 9 breathing systems - exocorriges.comexocorriges.com/doc/41209.doc · web viewfysisk...

Documents