vaporizers!
TRANSCRIPT
Prof. Mridul M. Panditrao
Professor, Head & In-Charge of ICU
Dean of Academic Affairs
Department of Anaesthesiology and Intensive Care
Adesh Institute of Medical Sciences and Research (AIMSR)
Adesh University
Bathinda, Punjab, India
PreviouslyConsultant
Department of Anesthesiology and Intensive Care
Rand Memorial Hospital
Freeport
Commonwealth of Bahamas
Declaration
Author declares his gratitude for being able to use some of the
graphics from various web based resources, inclusive of
Howequipmentworks
Anesthesiavaporization2000
Vaporizers
Wikipedia
Google search
Google Images etc………
INTRODUCTION
An essential & integral part of the anesthesia work station
Physics, construction, working principles and classification
A very complex/a tedious affair
The novices/younger generation of anesthesiologists
tend to relegate this aspect to realm of ‘mystery’ and avoidance
This is an attempt to demystify very important/neglected topic
Many a monographs have been written about them,
Still there is scope for describing them in an understandable manner.
Effort to simplify
“scientific principles and theoretical aspects”
To make concepts become much easier to understand.
INTRODUCTION:
Definitions
Colloquially, a vaporizer or vaporiser is a device
Used to vaporize the active ingredients of plant material, commonly cannabis,
tobacco, or other herbs or blends for the purpose of inhalation.
However, they can be used with pure chemicals when mixed with plant material
(e.g. tobacco-free nicotine)
INTRODUCTION: Definitions:
All the inhalational anesthetic agents
In a liquid state at standard room temperature & pressure.
Need to be transformed in to a form, can be inhaled : Vapor
The process of transforming the liquid in to a vapor is : Vaporization
Pure vapors of the modern anesthetic agents are extremely potent
Can have deleterious effects if administered in high concentration
INTRODUCTION: Definitions:
Accordingly, the vaporizer can also be defined as:
A vaporizer is a device that changes a liquid anesthetic
agent in to its vapor/ allows vaporization and adds a
controlled and predictable amount of that anesthetic
vapor to the fresh gas flow/ Carrier gas or the breathing
system for delivery of the subsequent admixture to the
patient
Basic design
Fresh Gas Flow (Carrier Gas) enters vaporizer
Splitting of Gas Flow
Some amount is allowed to enter the vaporizing chamber
Majority is allowed to bypass
Gas Saturated with vapor exits the chamber
Mixing with the bypassed gas takes place
Diluted Gas + Vapor mixture at the outlet
Finally delivered to the Patient
Historical aspects:
‘Inhalational Anesthesia’ tried by early ‘clinicians’ from time immemorial
Historical records ; use of “Soporific sponges” soaked in ‘medicinal elixirs’
Idea of using vaporization for sleep & pain relief well accepted
Actual use of easily ‘vaporizable substances’ came much later, in 18th
century
16th October 1846 to be exact
WTG Morton used his ‘ Letheon’ inhaler - Ether inhaler first time
to achieve surgical anesthesia, as a public demonstration, in the history of mankind.
Thus ether then chloroform again back to ether,
Led to evolution of various devices used for vaporization
of these liquids
Historical aspects:
The main ‘reviver’ of ether was Kurt Schimmelbusch and his ‘mask’
Contraption made with wires and layer of gauze pieces/used along with
‘open ether - drop by drop method’ for administration of ether.
“Yankauer’s mask” in 1904, Flagg’s can/ KEM Bottle,
More sophistication: Epstein Macintosh Oxford (EMO) vaporizer with Oxford
inflating bellows (OIB)
“Anesthesia Machine” was invented
Glass- “Boyle Bottles” for ether, chloroform and trichloroethylene,
Ethyl Chloride
Historical aspects:
With deeper insights into physical principles, properties and laws
Advances for development of more sophisticated devices
As a result Oxford Miniature Vaporizer (OMV), Copper Kettle
halogenated compounds like halothane/halogenated ethers
has produced the Tec series of vaporizers.
Presently available modern vaporizers
advanced in their construction
capable of delivering precise, predictable and calculated/ constant concentration of
the Volatile anesthetic agent.
Thus the humble beginning has evolved in to a
precision perfect and an analytical science.
Physical Principles:
Process of vaporization:
Constantly happening even in the atmosphere
Formation of water vapor.
Surface molecules of the liquid, coming in contact with air / any
gas
Carried off along with these gases, because of their higher kinetic
energy.
Gas carrying these :‘Carrier gas’.
Surface molecules of the liquid now existing in gaseous form :
‘vapor’
Physical principles:
Vapor pressure
They now behave more like gases, rather than original liquid
form
In this vapor state, they exert a certain, specific pressure on the
things surrounding them.
This pressure is called ‘Vapor pressure (VP)’
Pressure exerted by the molecules of vapors on the wall of container
VP increases with temperature/physical characteristics of
volatile anesthetic agent
Not affected by the ambient pressure
Physical principles:
Saturated vapor pressure
If vaporization is happening in a closed container or a chamber,
Then some of these molecules will bounce back till the equilibrium is
reached.
When the molecules leaving the liquid and reentering it become equal,
at that temperature and specific to the agent, this pressure is called
“Saturated Vapor Pressure” (SVP).
The most important factor governing vaporizer design is the saturated
vapor pressure (SVP) of the anesthetic.
SVP is a measure of the volatility of the liquid anesthetic in the carrier gas:
Anesthetics with a high SVP will require
a smaller proportion of the total gas flowing through the vaporizer
To pass through the vaporizing chamber to produce a given concentration
Surface area of the liquid
larger the surface area of the liquid coming in contact with the carrier gas
directly proportionately larger the number of molecules will come in contact
more rapidly they will escape from the liquid
Increasing the surface area for the carrier gas by the use of
Wicks
Series of Baffles
Bubbling the gas through the liquid volatile anesthetic
Volatility/Volatile nature of the liquid
directly proportional the relationship.
Physical principles:Factors involved in the process of vaporization
Physical principles:
Temperature:
Temperature plays multiple roles in the process of vaporization.
As liquid is heated up, more and more kinetic energy is generated
More and more chances of its surface molecules getting lost by the process
of vaporization.
As the heating up continues, the temperature in the liquid will increase
proportionately, till a certain point of temperature
Where the saturated vapor pressure will become equal to atmospheric
pressure.
So now not only the surface of the liquid but the bubbling through the entire
quantity of liquid is involved.
Bubbling is indicative of boiling of the liquid and the point of temperature is
called the boiling point of the liquid.
Boiling point
Defined as
The temperature point, where the equalization of the SVP is achieved with that of the atmospheric pressure
Halothane Enflurane Isoflurane Sevoflurane Desflurane
SVP at20c 243 175 240 168 664
MAC at 20c 0.75 1.68 1.15 1.7 approx. 6
Boiling point
at 20C
50.2 56.5 48.5 58.5 22.8
Physical principles:
Temperature: Heat of Vaporization:
The Number of calories required to vaporize 1 ml. of the liquid
Latent heat of vaporization
The Number of calories needed to convert 1 gram of liquid to vapor without atemperature change
Temperature of remaining liquid falls and may decrease rate of vaporization
Specific heat : The quantity of heat energy required to increase the temperature of a 1 gm.
of a substance/1 ml. of a liquid by 10 Celsius is called the Specific Heat of thesubstance/ liquid
Thermal conductivity
Measure of speed with which heat flows through a substance.
Are made of materials having high specific heat & high thermal conductivity.
It helps maintain a uniform temperature.
Heat of Vaporization & Specific heat
As the vaporization with carrier gases, continues, temperature
within the liquid gradually falls
As a result, slowly the heat from the surrounding is taken up to
continue the process of vaporization.
As the gradient between the heat being supplied Vs. the heat
being lost due to vaporization is gradually neutralized, an
equilibrium is reached.
At this temperature point no vaporization is possible any further,
until and unless, external heat is supplied.
How much of this heat energy is required will depend upon the
specific heat of the liquid.
.
Clinical importance :
Heat of Vaporization & Specific heat
Plus container in which vaporization is going on is also very
important.
More the specific heat of the material of the container :
Gradual and slower will be the change of temperature fall and
deceleration of the vaporization,
better the control in the hands of the clinician.
Copper being the most ideal metal with very high specific
heat,
is most commonly employed, somewhere or the other,
during the construction of the most of the modern vaporizers
Clinical importance :
Thermocompensation
Temperature compensation is achieved by
Heat transferred from surrounding metal (vaporizer)
&
With the help of valves which are controlled by
Bimetallic strip
single metal strip
Bellows
Here the aperture of the valve increases or decreases depending upon the temperature
Thereby regulating the flow of gas through the vaporizing chamber
Final Output:
Flow of the carrier gas:
As is true with surface area of the container, same
principle also applies here
Larger the flow, more and more gas will come in
contact with the surface of the liquid
more will be rate of vaporization.
Final Output: Concentration
Correlation between the
Pressure of the gas mixture and its final output
volume/ concentration:
The gas mixture (gas + vapor), in any container exerts
certain pressure on the container walls.
This pressure is the sum total of the individual partial
pressures of all the constituents of the mixture.
So the volume concentration in percentage of the vapor
can be calculated by
dividing the partial pressure of the vapor by total pressure
inside the container, Multiplied by 100
With employment of
Intermittent Positive Pressure
ventilation (IPPV)
Given manually or by
automatic ventilation,
Certain back pressure that
is produced
As the pressure in the
breathing system
increases,
it is transmitted back to
the vaporizer
compressing the gas
inside the chamber,
especially at the bypass.
During the conduct of general anesthesia
“Pumping effect”.
The outlet gas which was already saturated with the vapor along with expanding
additional gas has entered the vaporizing chamber, carries more vapor.
Both of these gases leave the outlet leading to, with more than the set
concentration being delivered to the patient.
This is called as “Pumping effect”.
Automatic ventilator with low flows and low dial setting,
especially when the lower quantity of the agent is available in the vaporizer.
With manual ventilation when the pressure changes are not even (Too frequent
variations, swinging between high and low pressures).
Corrective measures:
Logically decreasing the volume of vaporizing chamber.
Increasing the size of variable bypass
Adding an increased resistance at both the bypass as well as at
vaporizing chamber.
Adding a long spiral tube before gas can reach the vaporizing chamber,
leading to bypass thus preventing the gas from reaching back
Applying One way valve
“Pressurizing effect
As opposed to this, again during the automatic ventilation,
In some vaporizers, output decreases than what has been set
This is called as “Pressurizing effect”.
The factors here are opposite of that in pumping effect,
High flows, but lower dial settings and frequent pressure changes.
As the extra pressure at the outlet is applied, the pressure inside the vaporizing chamber is increased.
However the increased carrier gas pressure will not carry an additional vapor molecules
because the final concentration is mainly dependent upon the SVP of the anesthetic rather than, total pressure inside the chamber.
As a result dilution takes place leading to decreased final output concentration.
Level of liquid anesthetic.
In both pumping as well as pressurizing effects, lower levels of anesthetic is
one of the causes.
If too much is filled in the chamber, then area available for the contact will
be decreased.
Spilling of the liquid in to the bypass can actually increase the
output to dangerously high levels.
Stabilizers like Thymol known to decrease the available surface
area and also interfere in the intricate mechanism of the
vaporizer.
Mounting of the vaporizer. If vaporizer is tilted or not exactly
straight, can increase the output.
Other factors affect the performance/ final output
Classification:
Extensive Variations in terms of their
Construction
structure and
mechanism of their functionality,
Placement
Each of the vaporizers can fit into multiple parameters of classifications.
Classification:
The concentration achieved (Output Concentration)
Variable Bypass
Measured flow
The Method of vaporization employed
Flow-over
Bubble through
The specificity of the anesthetic agent used
Agent specific
Multiple agents
If the thermo-compensation is possible or not
The Location, in relation with the breathing system.
Outside the circuit
Inside the circuit.
Draw over/Bubble through Variable bypass
Variable bypass with dial control
Bubble through/measured flow
Another Classification
Another nomenclature for older vaporizers like
EMO, EMOTril, OMV, Copper Kettle and even in TEC series TEC 2, 3 and 4
Draw-over
The carrier gas was drawn over the liquid directly inside the vaporizer.
Sub atmospheric pressure is developed in gas stream distal to vaporizers either by patient or
mechanical means(EMO, Oxford Miniature Vaporizers)
Low in resistance to gas flow
Nowadays obsolete.
Plenum
Fresh gas is forced in to the vaporizing chamber (Plenum System)
Positive pressure is developed upstream of vaporizers,
All Tec type vaporizer and kettle type
High resistance to gas flow
Simple glass bottles: inside the circuit e.g. Goldman vaporizer.
Characteristics of ideal VAPORIZER
Performance not affected by changes in
FGF,
Volume of liquid agent
Ambient temperature & pressure
Decrease in temperature & pressure
Low resistance to flow
Light weight with small liquid requirement
Economical and safe to use
Corrosion and solvent-resistant
Features of modern vaporizer
Variable bypass Fresh gas splits into bypass gas and carrier gas
Flow overCarrier gas flows over the surface of the liquid volatile agent in the vaporizing
chamber
Temperature compensated Equipped with automatic devices that ensure steady vaporizer output over a
wide range of ambient temperatures
Agent-specificOnly calibrated for a single gas, usually with keyed fillers
Out of circuit
Operating principles of variable bypass vaporizers
Variable bypass vaporizers can be of the plenum or draw-over type
Splitting ratio
Between carrier and bypass depends on concentration control dial
Temperature compensated valve
Effect of flow rate- Variation is notable at extremes of flow rates
Output is constant over the range of flow from 250ml-15L/min.
Effect of ambient temp: Output is linear (uniform increase) from 20-350c
Automatic temp compensation devices
Wicks in contact with the walls of vaporizer replace heat used for vaporization
Metals of high specific heat and thermal conductivity
Effect of intermittent back pressure/ Pumping effect.
Long inlet tube to vaporizing chamber.
One way check valve at common gas outlet.
Equal vol. of bypass and vaporization chamber
Most vaporizers in current use: variable-bypass type and
concentration-calibrated
• Include the Ohmeda Tec series (except 6) the Drager Vapor 19.1
• The total gas from the anesthesia machine flow meters is split with some gas flowing into
the vaporizing chamber picking up anesthetic agent molecules,
• While a larger gas Flow bypasses the chamber completely
• Vaporizer outflow is based on the re- mixing of the two streams• Results in administration to the patient of the anesthetic concentration indicated on the
dial.
Safety features
Important safety features include:
Color specific (for each agent)
Keyed fillers bottles
Low filling port
Secured vaporizers Interlocks
less ability to move them about minimizes tipping
Only one vaporizer is turned on
Gas enters only the one which is on
Trace vapor output is minimized when the vaporizer is off
Vaporizers are locked into the gas circuit, thus ensuring they are seated correctly
Concentration dial increases output in all when rotated counterclockwise
(as seen from above
Hazards
Tipping
If tipped >45 degrees-liquid can obstruct the outlet valves
Treatment: Flush for 20-30 min at high flow rates with dial set at high
concentration
Overfilling May result in high output
Fill only up to max filling line
Fill only when the vaporizer is off
Leaks
Relatively common due to malposition or loose filler cap.
Not detected with standard checklist perform negative pressure check
Hazards
Misfilling
Vaporizers not equipped with keyed filling lead to misfilling
Contamination
It occurs by filling a vaporizer with contaminated anesthetic bottle.
Underfilling
Leads to decreased vaporizer output.
Simultaneous Inhaled Anesthetic Administration
Happened in old machines with no interlock system
Individual Vaporizers Monologues have been written about the individual
vaporizers, which is beyond
the scope of present
discussion, because of the
paucity of space However we shall be
discussing, salient features of
only the Tec 5, 6 and 7 and
Drager 19.3 and Aladin
cassette
for example: while describing TEC 5 vaporizer
Variable Bypass,
Flow over,
agent specific,
thermo-compensated
outside the circuit.
Comparative properties
The commonly used vaporizers are enumerated in the Table I.
Property TEC 4, Vapor 19n,
2000, Aladin
TEC 5 TEC 7 Vapor 19n Vapor 2000 D Vapor
TEC 6 Des.
Principle of
vaporization
Flow over, Flow over Flow over Flow over Flow over Gas-vapor blender
Carrier gas flow Variable bypass Variable bypass Variable bypass Variable bypass Variable bypass Dual circuit
Capacity mls.
With dry wicks
With wet wicks
135
100
300
225 225200
140
360
280
D-vapor 300
TEC 425
Thermo-
compensation
Automatic Automatic Automatic Automatic Automatic Thermostatically controlled
at
39 0C.
Position Out of circuit Out of circuit Out of circuit Out of circuit Out of circuit Out of circuit
specificity Agent-specific Agent-specific Agent-specific Agent-specific Agent-specific Agent-specific
Low flow suitability Not very good Good Very Good Good Very Good Very Good
Problems of Desflurane
Desflurane is much more volatile than all the other inhalationals.
Its boiling point is low -- only 22.80 C, so most of it gets evaporated at normal room temperatures
Vapor pressure of desflurane at 200 C is 664 mm Hg.
While that of enflurane, isoflurane, halothane are 172, 240, 244 MM Hg. respectively
At 1 atmosphere and 200 C , 100mL/min flow passing through vaporizing chamber would carry
735 mL/min. of desflurane
versus
29, 46 and 47 mL/min of enflurane, Isoflurane and halothane respectively.
Under these conditions to produce 1% of desflurane,
we need 73 L/min Fresh Gas Flow as
compared
to 5 L/min for other anesthetics, to pass through vaporizer
• In the above figure, note different vapor pressure-temperature relationships
between common volatile agents
• Recognize that desflurane falls outside the grouping
• Not surprisingly, special vaporizer is required for desflurane
Specifically designed to deliver desflurane
Described as a gas/vapor blender than as a vaporizer.
It is heated electrically to 350 C
Pressurized Device with a pressure of 1550 mmHg (2 atm)
Electronic monitors of vaporizer function
FGF does not enter vaporization chamber, instead
Desflurane vapor enters the path of FGF
Percentage control dial regulates flow of Desflurane into FGF
Dial calibration is from 1% to 18%
Provided with back up 9 volt battery
Datex-Ohmeda Tec 6 Vaporizers for Desflurane
Is an electrically heated,
thermostatically controlled,
constant-temperature,
pressurized,
electromechanically coupled,
dual-circuit,
gas-vapor blender.
The pressure in the vapor circuit is electronically regulated to equal the pressure in the
fresh gas circuit.
At a constant fresh gas flow rate, the operator regulates vapor flow by use of a
conventional concentration control dial.
When the fresh gas flow rate increases, the working pressure increases proportionally.
At a specific dial setting, at different fresh gas flow rates, vaporizer output is constant
because the amount of flow through each circuit is proportional.
The latest model of the TEC series
It delivers Isoflurane, Sevoflurane, Enflurane, and Halothane
efficiently
Easy-fil* is designed to simplify agent filling and help minimize agent leaks while filling
The Tec 7 Vaporizer is also available with Quik-Fil* (Sevoflurane only)
accommodates 225 mL of anesthetic agent.
Anesthesia delivery systems equipped with the Selectatec*
Non-spill system limits movement of liquid agent
if the vaporizer is tilted or inverted
helping to protect internal components.
TEC
7
Clinical Performance
Designed to provide consistent output
Throughout the clinical flow range from 200 mL/min to 15 L/min.
Large diameter control dial incorporating
fine graduations of 0.2% between 0 and 1%,
0.5% from 1% to 8%.
The dial for Sevoflurane is marked
in steps of 0.2% up to 1% v/v,
in steps of 1% between 1% and 8%.
Fine tune anesthetic delivery over the range of dial settings/flow
rates.
Aladin Cassette Vaporizer System
A Novel system
Single vaporizer capable of delivering 5 different anaesthetic agents
It is designed for use with Datex-Ohmeda S/5 ADU and similar
machines.
FGF is divided into bypass flow and liquid chamber flow
Liquid chamber flow conducted into agent specific, color coded
cassette in which volatile anesthetic is vaporized
Machine accepts only one cassette at a time
Magnetic Labeling
Conclusion
Quest for understanding vaporizers is never-
ending.
The more we try to learn them,
more developments and newer insights happen.
Very demanding and rapidly developing field.
It pays for every anesthesiologist to remain in
constant accompaniment with them.