anesthesia workstation , electrical components , high pressure

ANESTHESIA MACHINE , ELECTRICAL COMPONENTS, HIGH PRESSURE SYSTEM , INTERMEDIATE PRESSURE SYSTEM

MODERATOR: DR. H. SAHARIA

( ASST. PROFFESOR)

PRESENTER: DR. KUNAL AGARWAL (PGT)

DEPT OF ANESTHESIOLOGY & CRITICAL CARE .GMCH

24TH-NOV-2012

Anaesthesia machine: a short introduction

Anaesthesia gas machine is a device which delivers a precisely known but variable gas mixture,including anaesthetizing and life sustaining gases. The most common type of anaesthetic machine in use in the developed world is the continuous-flow anaesthetic machine, which is designed to provide an accurate and continuous supply of medical gases (such as oxygen and nitrous oxide), mixed with an

Contd..

accurate concentration of anaesthetic vapour (such as isoflurane), and deliver this to the patient at a safe pressure and flow. Modern machines incorporate a ventilator, suction unit, and patient-monitoring devices

• The original concept of Boyle's machine was invented by the British anaesthetist H.E.G. Boyle in 1917.

• Prior to this time, anaesthetists often carried all their equipment with them, but the development of heavy, bulky cylinder storage and increasingly elaborate airway equipment meant that this was no longer practical for most circumstances.

• The anaesthetic machine is usually mounted on anti-static wheels for convenient transportation.

HISTORY…

• Originally boyle introduced nitrous oxide-oxygen anaesthesia through this machine and it was a two gas system with watersight-feed type of flowmeter.

• In 1920,modification was made by incorporating a vaporising bottle to flowmeters.

• A second vaporising bottle and bypass controls were further added in 1929.

• In 1933 dry bobbin type of flowmeter was introduced in place of watersight-feed type.

• In 1937 rotameters displaced dry-bobbin type of flowmeters.

• The basic design has been called upon to perform more complicated functions since 1990, with the advent of computer-controlled monitors into the operating room, especially pulse oximetry, capnography, and gas analysis.

• Our gas machines have become top-heavy with the monitors we have added to their basic design.

• But despite numerous modifications the modern apparatus retains many of the features of the original Boyle's machine.

PHYSICAL TERMS AND UNITS

Flow - Flow is the measure of volume per unit time. In anaesthesia , common units are liters per minute ( L min -1 )

Force - is that property which when applied to a mass causes acceleration , or a change in the direction of motion. i.e. F = ma

Pressure - is the intensity of force over a defined area such that:

Pressure = total force (F) / total area (A) =force per unit area.

Pressure retains some traditional units in common use such as pounds per square inch (p.s.i.) , millimeters of mercury (mm Hg), centimeters of water(cm H2O) , and atmospheres).

1 bar = 100 kPa = approx. 760 mmHg = approx. 1000cm H20

= approx 14.7 p.s.i.

Gauge pressure – describes the pressure above atmospheric pressure.

Absolute pressure includes atmospheric pressure.

The pressure in a full O2 cylinder can be quoted as137 bar gauge pressure or 138 bar absolute pressure.

Critical temperature - The temperature , above which a gas cannot be liquefied , however much pressure is increased is known as its critical temperature. It is 36.5 oC for nitrous oxide and -118oC for O2

Filling Ratio – To prevent cylinders , the contents of which are normally in liquid form , from being overfilled , a filling ratio (or filling density) is specified. This is the ratio of the total weight of gas in a cylinder to the weight of water that the cylinder would hold at 160C. For nitrous oxide it is 0.67- 0.75. Note that the filling ratio is not the proportion of the cylinder occupied by liquid.

STANDARDS FOR ANESTHESIA MACHINES AND WORKSTATIONS

Standards for anesthesia machines and workstations provide guidelines to manufacturers regarding their minimum performance, design characteristics, and safety requirements. During the past 2 decades, the progression of anesthesia machine standards has been as follows:

1979: American National Standards Institute, Z79.8-19793

1988: American Society for Testing and Materials, F1161-884

1994: ASTM F1161-94 (reapproved in 1994 and discontinued in 2000)5

2005: International Electrical Commission (IEC), 60601-16

2005: ASTM F1850-00 (reapproved)2

CONTD…..

To comply with the 2005 ASTM F1850-00 standard, newly manufactured workstations must have monitors that measure the following parameters:

continuous breathing system pressure,

exhaled tidal volume,

ventilatory CO2 concentration,

anesthetic vapor concentration,

inspired oxygen concentration,

oxygen supply pressure,

arterial hemoglobin oxygen saturation

arterial blood pressure,

and continuous electrocardiogram.

CONTD…. The anesthesia workstation must have a prioritized alarm

system that groups the alarms into three categories: high, medium, and low.

These monitors and alarms may be enabled automatically and made to function by turning on the anesthesia workstation, or the monitors and alarms can be enabled manually and made functional by following a pre-use checklist.

APL valve

SYSTEM COMPONENTS

Electrical Components

Many components of modern anesthesia machines are powered by electricity. Turning the machine ON enables these devices.

MASTER SWITCHOn most anesthesia machines, a master (main power) switch activates both the pneumatic and electrical functions.

On most machines, when the master switch is in the OFF position, the only electrical components that are active are the battery charger and the electrical outlets.

Master switch. Turning the master switch to the ON position activates both pneumatic and electrical functions of the machine as well as certain alarms and safety features.

POWER FAILURE INDICATORMost machines are equipped with a visual and/or audible indicator to alert the anesthesia provider to the loss of mains power .

The machine will usually give an indication when mains power is lost.

RESERVE POWERSince electricity is crucial for many machine functions, a backup source of power for the occasional outage is necessary.

The anesthesia provider should check the battery status during the preuse checkout procedure.

While some older anesthesia machines used replaceable batteries, most new machines use rechargeable batteries.

The duration of battery backup depend on the power usage. A noninterruptible power source may be added to the anesthesia machine for a longer backup .

An extra source of power may be attached to the anesthesia machine to extend the life of the electrical power in the machine

ELECTRICAL OUTLETS Most modern anesthesia machines have electrical outlets.

These are intended to power monitors and other devices.

As a general rule, these outlets should only be used for anesthesia monitors.

Other appliances should be connected directly to mains power.

If the power requirements exceed that for which the outlet was designed, a circuit breaker will be activated .

Convenience electrical outlets on the back of the anesthesia machine. These should be used only for anesthesia monitors and not for general operating room use.

CIRCUIT BREAKERS

There are circuit breakers for both the anesthesia machine and the outlets .

When a circuit breaker is activated, the electrical load should be reduced and the circuit breaker reset.

DATA COMMUNICATION PORTS

Most modern anesthesia machines have data communications ports.

These are used to communicate between the anesthesia machine, monitors, and the data management system.

View Figure

MEDICAL GAS CYLINDERS

COMPONENT OF MEDICAL GAS CYLINDERS

BODY

constructed of steel alloy containing molybdenum(0.15% -0.25% ) & / chromium (0.8%-1.1%)

alloys added for strength ,minimize wt. & wall thickness.

Cylinders that have marked ‘3AA’ are manufactured by using steel.

The marking ‘3AL’or ‘3ALM’ indicates that the cylinders made from aluminium.

Aluminum cylinders are MRI compatible

Cylinders have flat or concave bases .

The other end may taper into a neck that is fitted with tapered screw threads that attach to the valve.

CONTD…VALVE filled and discharged through a valve it is attached to the neck made of bronze or brass

PORT The port is the point of exit for the gas. It should be protected in transit by a covering. When installing a small cylinder on an anesthesia machine, it is

important not to mistake the port for the conical depression on the opposite side of the valve.

The conical depression is designed to receive the retaining screw on the yoke. Screwing the retaining screw into the port may damage the port and/or index pins.

CONTD…

STEM

Each valve contains a stem, or shaft, that is rotated during valve opening or closing.

To close the valve, the stem seals against the seat that is part of the valve body.

When the valve is opened, the stem moves upward, allowing gas to flow to the port.

CONTD…..

1. PACKED VALVE:

here, stem is sealed by a resilient packing such as Teflon, which prevents leaks around the threads.

-This type of valve is also called direct acting, because turning the stem causes the seat to turn.

-In a large cylinder valve, the force is transmitted by means of a driver square. This type of valve is capable of withstanding high pressures.

2. DIAPHRAGM VALVE -The disks, or diaphragms, separate upper and lower stems,

which may be permanently attached to the diaphragms. -The upper stem is actuated by a manual or automatic

means, and the lower stem shuts or permits flow through the valve.

-This type of valve has the following advantages:• -can be opened fully using a one half to three

quarters turn• -The seat does not turn & therefore less likely

to leak• -No stem leakage can occur because of

diaphragm.

CONTD…

CONTD….

Handle or HandwheelA handle or handwheel is used to open or close a cylinder valve. It is turned counterclockwise to open the valve and clockwise to close it. This causes the stem to turn.

A handle (cylinder wrench) is used to open a small cylinder valve. Handles come in a variety of shapes

CONTD….

PRESSURE RELIEF DEVICES :

Every cylinder is fitted with pressure relief (safety relief/safety) device whose purpose is to vent the cylinders content to atmosphere if the pressure of enclosed gas increases to dangerous level.

TYPES

● rupture disc

● fusible plug

● combination of both

● pressure relief valve(spring loaded)

CONTD….

Rupture disc:

-non reclosing device held against an orifice.

-when predetermine pressure is reached the disc ruptures & allow the cylinder content to be discharged.

-it protects against excess pressure as a result of high temp or overfilling.

CONTD..

FUSIBLE PLUG

The fusible plug is a thermally operated, nonreclosing pressure relief device with the plug held against the discharge channel.

It offers protection from excessive pressure caused by a high temperature but not from overfilling.

The yield temperature is the temperature at which the fusible material becomes sufficiently soft to extrude from its holder so that cylinder contents are discharged.

Figure 1.6 Pressure relief valve. When the set pressure is exceeded, the pressure in the cylinder forces the spring to the left, and gas flows around the safety valve seat to the discharge channel

pressure relief valve(spring loaded)

PIN INDEX SAFTEY SYSTEM

NONINTERCHANGEBLE SAFTEY SYSTEMS

Fig :Pin Index Safety System. The bottom figure shows the six positions for pins on the yoke. The pins are 4 mm in diameter and 6 mm long, except for pin 7, which is slightly thicker. The seven hole positions are on the circumference of a circle of 9/16 inch radius centered on the port.

Indian standard specification for yoke type valve connection for oxygen & nitrous oxide:

-two line one mid-horizontal & other vertical drawn over the yoke

-meeting point taken as center of gas inlet hole

-hole is drawn as diameter of 7mm which is fitted to with BODOK SEAL

-distance between center & lower part of yoke is 20.6mm.

-from the center a circle is drawn with a radius of 14.3mm. In that arc pins numbering 1 to 6 are positioned.

The pin should be diameter of 4.75 mm & 6mm long

PISS

oxygen air N2O CO2 nitrogen

PIN INDEX SYSTEM

OXYGEN 2,5

NITROUS OXIDE 3,5

CYCLOPROPANE 3,6

AIR 1,5

NITROGEN 1,4

NITROUS+OXYGEN 7

CARBON DIOXIDE 1,6

Bodok seal

-cylinders are fitted with yoke with a sealing washer called BODOK SEAL

-it is made up of non combustible material and has a metal periphery which

make it long lasting.

-it should be less than 2.4mm thick prior to compression.

-only one seal should be use between the valve & yoke

SIZE OF CYLINDERS

Cylinder classified using a letter code

A type cylinders are smallest

However AA (smaller than A) also available.

SIZE D & E is the cylinder most commonly used

TYPICAL MEDICAL GAS CYLINDRES, VOLUMES, WEIGHT cylinder size

Cylinder Dimensions (O.D. × Length in Inches)

wt .(lb)EMPTY

Air(litres)

CO2(litres)

oxygn (litres)

NITROUSOXIDE

(litres)

A 3X7 0.23 76 189

B 3 1/2 x 13

5 370 200

D 4 1/2 x 17

11 375 940 400 940

E 41/4 x 26 14 625 1590

660 1590

M 7 x 43 63 2850 7570

3450 7570

G 8 1/2 x 51 97 5050 12300

13800

H 91/4 X 51 119 6550 6900 15800

COLOUR OF CYLINDERGAS USA INTERNATIONA

Loxygen Green White

Shoulder& Black Body

Carbon dioxide

Gray Gray

Nitrous oxide

Blue Blue

helium Brown Brown

Nitrogen Black Black

Air Yellow Gray Body ,Shoulder black/white quartered

CONTENT AND PRESURE

FIG:A nonliquefied gas such as oxygen will show a steady decline in pressure until the cylinder is evacuated. Each cylinder, however, will show a steady decline in weight as gas is discharged.

FIG:The relationship between cylinder weight, pressure, and contents. A gas stored partially in liquid form, such as nitrous oxide, will show a constant pressure (assuming constant temperature) until all the liquid has evaporated, at which time the pressure will drop in direct proportion to the rate at which gas is withdrawn.

CONTENTS AND PRESSURE

PNEUMATIC SYSTEM

PNEUMATIC SYSTEM

Gases are supplied under tremendous pressure for the convenience of storage and transport.

The anaesthesia machine receives medical gases from a gas supply; controls the flow of desired gases reducing their pressure, to a safe level.

Gases used during ventilation must be supplied to the patient in a controlled way to avoid any harm to the patient.

PNEUMATIC SYSTEM

So the pressure inside a source ( cylinder or pipeline ) must be brought to a certain level before it can be used for the purpose of ventilation.

And it needs to be supplied in a constant pressure, otherwise the flow meter would need continous adjustment.

This is achieved by bringing down the pressure of a gas supply in a graded manner with the help of three pressure reducing zones .

Thus the pneumatic part of the machine can be conveniently divided into three parts:high ,intermediate and low pressure systems.

MACHINE COMPONENTSHigh-pressure circuit oxygen cylinder 2200 psi >> 45 psi

N2O cylinder 750 psi >> 45 psi

Intermediate-pressure circuit oxygen cylinder 45 psi

N2O cylinder 45 psi

oxygen pipeline 50 psi

N2O pipeline 50 psi

>>14-26 psi (low)

CONTD..Low pressure system: from flow control valve to common gas outlet.

THE ANESTHESIA MACHINE

IntermediateHigh Intermediate Low Pressure Circuit

HIGH PRESSURE SYSTEM• Receives gasses from the high pressure

E cylinders attached to the back of the anesthesia machine (2200 psig for O2, 745 psig for N2O)

• Consists of:– Hanger Yolk (reserve gas

cylinder holder)– Check valve (prevent reverse

flow of gas)– Cylinder Pressure Indicator

(Gauge)– Pressure Reducing Device

(Regulator)• Usually not used, unless pipeline gas

supply is off

HANGER YOKE ASSEMBLY

The Hanger yoke assembly

1) Orients and supports the cylinder

2) Provides a gas-tight seal

3) Ensures uni-directional gas flow

The workstation standard recommends that there be at least one yoke each for oxygen and nitrous oxide.

If the machine is likely to be used in locations that do not have piped gases, it is advisable to have a double yoke, especially for oxygen.

HANGER YOLK

Hanger-yoke assembly

HANGER YOKE ASSEMBLYThe hanger yoke assembly is composed of several parts:

1)Body, which is the principal framework and supporting structure

2) The retaining screw, which tightens the cylinder in the yoke

3) The nipple, through which gas enters themachine

4) The pin index system, which prevents attaching of incorrect cylinder

HANGER YOKE ASSEMBLY5) The washer, which helps to form a seal between the cylinder and the yoke

6) The filter, to remove dirt from the gas in the cylinder

7) Check valve assembly, to ensure unidirectional flow of gas

BODY It is threaded into the frame of the machine.

It provides support for the cylinder(s).

Commonly the swinging gate type is used.

When a cylinder is mounted onto or removed from a yoke, the hinged part can be swung to side.

RETAINING SCREW

It is threaded into the distal end of the yoke.

Tightening the screw presses the outlet of the cylinder valve against the washer and the nipple so that a gas tight seal is obtained.

The cylinder is then supported by the retaining screw, the nipple, and the index pins.

The conical point of the retaining screw is shaped to fit the conical depression on the cylinder valve.

NIPPLE & INDEX PINSNipple

It is a part of the yoke through which the gas enters the machine.

It fits into the port of the cylinder valve. If it is damaged, it may be impossible to obtain a tight

seal with the cylinder valve.

Index Pins

These are situated below the nipple. These help to prevent mounting of incorrect cylinder to

yoke. The holes into which the pins are fitted must be of a

specific depth. If they extend too far into the body of the yoke, it may

be possible to mount a incorrect cylinder.

WASHER & FILTERWasher

It is placed around the nipple to effect a seal between the cylinder valve and the yoke.

A broken or curled washer should not be used. No more than one washer should be used.

Filter It is used to prevent particulate matter from

entering the machine. It is to be placed between the cylinder and the pressure

reducing device.

CONTD..

PLACING CYLINDER IN YOKE. THE CYLINDER IS SUPPORTED BY THE FOOT AND GUIDED INTO PLACE MANUALLY

Swinging gate–type yoke. Note the washer around the nipple and the index pins below.

CHECK VALVE ASSEMBLY It allows gas from a cylinder to enter the

machine but prevents gas from exiting the machine when there is no cylinder in the yoke.

It allows an empty cylinder to be replaced with a full one without having to turn off the `in–use` cylinder.

Prevents transfer of gas from one cylinder to the other with a lower pressure in a double yoke.

CHECK VALVE ASSEMBLY CONT….. It consists of a plunger that slides away from the

side of the greater pressure.

It is not designed to act as a permanent seal for empty yoke and may allow small amount of gas to escape.

As soon as a cylinder is exhausted it should be replaced by a full one or a dummy plug.

CHECK VALVE ASSEMBLY

CYLINDER PRESSURE INDICATOR It is made up of Bourdon tube which is a metal tube bent

into a curved, sealed, and linked to a clock like mechanism.

The other end of the tube is connected to the gas source and a increase in pressure of gas inside the tube causes it to straighten.

Through a clock like mechanism these motions are transmitted to the indicator.

Gauges are requried to be calibrated in kPa but psi may also be used.

Now-a-days digital pressure indicators has also come up.

CYLINDER PRESSURE INDICATOR(BOURDEN TUBE)

CYLINDER PRESSURE GAUGE Each hanger yoke or a group of hanger yoke should be

provided with a gauge that will display the pressure of the cylinder.

The indicator may be located near the cylinder or on the front panel of the machine.

The indicator is circular, the lowest pressure indication must be between the 6 o`clock and the 9 o`clock position on a clock face.

The scale must be at least 33% more than the maximum filling pressure.

It must be clearly and permanently marked with the name or chemical symbol of the gas it monitors and should be identified by the color assigned to that gas.

PRESSURE REDUCING DEVICE (REGULATOR) The pressure in a cylinder varies. The anesthesia machine

is fitted with devices (reducing valves, regulators, reducing regulators, reduction valves, regulator valves) to maintain constant flow with changing supply pressure.

These reduce the high and variable pressure found in a cylinder to a lower (40 to 48 psig, 272 to 336 kPa) and more constant pressure suitable for use in an anesthesia machine.

The machine standard requires reducing devices for each gas supplied to the machine from cylinders.

Separate yokes for the same gas may be connected to one reducing device.

INTERMEDIATE PRESSURE SYSTEM

INTERMEDIATE PRESSURE SYSTEM

Begins at the regulated cylinder supply source at 45 psig

,includes the pipeline sources at 50 to 55 psig and extends to

the flow control valve.

Depending on manufacturer & specific

machine design,second stage regulators may be used to

decrease the pipeline supply pressures to the flow control

valves to even lower pressures as 14 or 26 psig with in the

intermediate pressure circuit.

INTERMEDIATE PRESSURE SYSTEMReceives gasses from the

regulator or the hospital pipeline at pressures of 40-55 psig

Consists of:

• Pipeline inlet connections• Pipeline pressure indicators• Piping• Gas power outlet• Master switch• Oxygen pressure failure

devices• Oxygen flush• Additional reducing devices• Flow control valves

MASTER SWITCH (PNEUMATIC COMPONENT )

The pneumatic portion of the master switch is located in the intermediate pressure system downstream of the inlets for the cylinder and pipeline supplies

The oxygen flush is usually independent of this switch.

The master switch may be a totally electronic switch that when activated controls the various pneumatic components in the anesthesia machine.

When the master switch is turned off ,the pressure in the intermediate system will drop to zero

THE PIPELINE INLET CONNECTION IS THE ENTRY POINT FOR GASES FROM THE PIPELINES.

THE ANESTHESIA WORKSTATION STANDARD REQUIRES PIPELINE INLET CONNECTIONS FOR OXYGEN AND NITROUS OXIDE.

MOST MACHINES ALSO HAVE AN INLET CONNECTOR FOR AIR.

THESE INLETS ARE FITTED WITH THREADED NON INTERCHANGEABLE DIAMETER INDEX SAFETY SYSTEM (DISS) FITTINGS

PIPELINE INLET CONNECTIONS

Each inlet must contain a unidirectional (check) valve to prevent reversed gas flow from the machine into the piping system (or to atmosphere if no hose is connected).

Problems have been reported with this check valve.

Each pipeline inlet is required to have a filter with a pore size of 100 μm or less. The filter may become clogged, resulting in a reduction in gas flow

PIPELINE INLET CONNECTIONS

PIPELINE INLET CONNECTIONS

PIPELINE PRESSURE INDICATORS Indicators to monitor the pipeline pressure of each gas are required by

the anesthesia workstation standard.

They are usually found on a panel on the front of the machine and may be color coded for the gases that they monitor

Some machines have digital pressure indicators. The pipeline pressure is displayed either continuously or on demand

On some newer electronic machines, LEDs indicate pipeline pressure.

A green LED indicates that the pipeline is connected and the pressure is adequate.

If the LED is OFF, the pressure is inadequate or the pipeline is not connected.

If the transducer is inoperable, the LED is dark. A digital pressure can be displayed on the screen to augment the information from the LEDs.

PIPELINE PRESSURE INDICATORS

The workstation standard requires that the indicator be on the pipeline side of the check valve in the pipeline inlet.

If the indicator is on the pipeline side of the check valve, it will monitor pipeline pressure only. If the hose is disconnected or improperly connected, it will read “0” even if a cylinder valve is open

If the indicator were on the machine (downstream) side of the check valve, it would not give a true indication of the pipeline supply pressure unless the cylinder valves were closed. If a cylinder valve is open and the pipeline supply fails, there will be no change in the pressure on the indicator until the cylinder is nearly empty.

PIPELINE PRESSURE INDICATORS The indication of an adequate pressure on the pipeline

indicator does not guarantee that gas is not being drawn from a cylinder. If for any reason the gas pressure coming from a cylinder via a pressure regulator exceeds the pipeline pressure and a cylinder valve is open, gas will be drawn from the cylinder. Therefore cylinder valves should always remain closed when the pipeline supply is in use.

Pipeline pressure indicators should always be checked before the machine is used. The pressure should be between 50 and 55 psig (345 and 380 kPa). The indicators should be scanned repeatedly during use.

PIPING Piping is used to connect components inside the machine.

It must be able to withstand four times the intended service pressure without rupturing.

The anesthesia workstation standard specifies that leaks between the pipeline inlet or cylinder pressure reducing system and the flow control valve not exceed 25 mL/minute. If the yoke and pressure reducing system are included, the leakage may not exceed 150 mL/minute.

Piping cross connections inside the machine have been reported. Disconnections in the piping may occur but are rare

OXYGEN PRESSURE FAILURE DEVICES One of the most serious mishaps that occurred with early

machines was depletion of the oxygen supply (usually from a cylinder) without the user awareness.

The result was delivery of 100% anesthetic gas.

Numerous inventions have been devised to prevent this problem.

Among these are devices that shut off the supply of gases other than oxygen (oxygen failure safety device) or alarm when oxygen pressure has fallen to a dangerous level

The anesthesia workstation standard requires that whenever the oxygen supply pressure is reduced below the manufacturer-specified minimum, the delivered oxygen concentration shall not decrease below 19% at the common gas outlet.

OXYGEN FAILURE SAFETY DEVICE The oxygen failure safety valve (oxygen failure safety device,

low pressure guardian system, oxygen failure protection device, pressure sensor shutoff system or valve, fail safe, pressure sensor system, nitrous oxide shut off valve) shuts off or proportionally decreases and ultimately interrupts the supply of nitrous oxide if the oxygen supply decreases.

On many modern machines, the air supply is also cut off .

When the pneumatic system is activated, oxygen pressure reaches the oxygen failure safety device, allowing other gases to flow. Turning OFF the pneumatic system causes oxygen in the machine to be vented to atmosphere. The resulting decrease in oxygen pressure causes the oxygen failure safety device to interrupt the supply of other gases to their flow control valves.

OXYGEN SUPPLY FAILURE ALARM The anesthesia workstation standard specifies that whenever

the oxygen supply pressure falls below a manufacturer-specified threshold (usually 30 psig (205 kPa)), at least a medium priority alarm shall be enunciated within 5 seconds. It shall not be possible to disable this alarm.

Because both the oxygen failure safety device and alarm depend on pressure and not flow, they have limitations that are not always fully appreciated by the user.

In OHMEDA machines, the alarm is actuated at around 28 psig (190 kPa). On DRAGER machines, the threshold is between 30 and 37 psig.

Alarms may be either electronic or pneumatic (directs a stream of oxygen in a resorvoir through a whistle).

CONTD… They aid in preventing hypoxia caused by problems

occurring upstream in the machine circuitry (disconnected oxygen hose, low oxygen pressure in the pipeline, and depletion of oxygen cylinders)

These devices do not offer total protection against a hypoxic mixture being delivered, because they do not prevent anesthetic gas from flowing if there is no flow of oxygen.

Equipment problems (such as leaks) or operator errors (such as a closed or partially closed oxygen flow control valve) that occur downstream are not prevented by these devices.

They do not guard against accidents from crossovers in the pipeline system or a cylinder containing the wrong gas.

GAS POWER OUTLET One or more gas power (auxiliary gas) outlets may be

present on an anesthesia machine. It may serve as the source of driving gas for the anesthesia ventilator or to supply gas for a jet ventilator. Either oxygen or air may be used.

In the past, the power outlet was usually present when the ventilator was an add-on part of the anesthesia machine, and one of several different ventilators could be used.

However, with modern anesthesia machines, the ventilator is an integral part of the machine and the breathing system and is connected to the ventilator with internal piping. Therefore, the power outlet is not found in many anesthesia machines today.

GAS SELECTOR SWITCH SOME MACHINES HAVE A GAS SELECTOR SWITCH THAT PREVENTS AIR AND NITROUS OXIDE FROM BEING USED TOGETHER. TWO TYPES OF SWITCHES ARE SHOWN.

SECOND-STAGE PRESSURE REGULATOR Some machines have pressure regulators in the intermediate

pressure system just upstream of the flow indicators.

Ohmeda uses a second-stage O2 pressure regulator which receive gas from either the pipeline or the pressure regulator and reduce it further to around 26 psi (177 kPa) for nitrous oxide and 14 psi (95 kPa) for oxygen.

The purpose of this pressure regulator is to eliminate fluctuations in pressure supplied to the flow indicators caused by fluctuations in pipeline pressure (ensures constant oxygen flowmeter input until supply pressure is less than 12-16 psi)

By reducing the pressures below the normal fluctuation range, the flow will remain more constant. Not all anesthesia machines are equipped with this device.

OXYGEN FLUSH The oxygen flush (oxygen bypass, emergency oxygen bypass) receives oxygen

from the pipeline inlet or cylinder pressure regulator and directs a high unmetered flow directly to the common gas outlet.

It is commonly labeled “02+.”

On most anesthesia machines, the oxygen flush can be activated regardless of whether the master switch is turned ON or OFF.

The anesthesia workstation requires that the oxygen flush be a single-purpose, self-closing device operable with one hand and designed to minimize unintentional activation. A flow between 35 and 75 L/minute must be delivered.

An oxygen flush consists of a button and stem connected to a ball. The ball is in contact with the seat. When the button is depressed the ball is forced away from the seat, allowing the oxygen to flow to the outlet. A spring opposing the ball will close the valve when the button is not depressed.

The button is commonly recessed or placed in a collar to prevent accidental activation.

CONTD……. Oxygen flush activation may or may not result in other gas flows

being shut off and may result in either a positive or negative pressure in the machine circuitry, depending upon the design of the inlet and the flush line into the common gas line.

This pressure will be transmitted back to other structures in the machine, such as flow indicators and vaporizers, and may change the vaporizer output and the flow indicator readings.

The effect caused by oxygen flush activation will depend on the pressure generated, the presence or absence of check valves in the machine, and the relationship of the oxygen flush valve to other components.

The anesthesia workstation standard requires that the connection of the flush valve delivery line to the common gas outlet be designed so that activation does not increase or decrease the pressure at the vaporizer outlet by more than 10 kPa or increase the vapor output by more than 20%.

CONTD…..Reported hazards associated with the oxygen flush include

Accidental activation and internal leakage resulting in an oxygen-enriched mixture being delivered.

The flush valve may stick in the ON position obstructing the flow of the gases from the flowmeters.

Barotrauma and awareness during anesthesia have resulted from its activation.

Oxygen flush activation during inspiration delivered by the anesthesia ventilator will result in delivery of high tidal volumes and possible barotrauma. Ventilators that exclude fresh gas flow from the breathing system during inspiration will not present this problem

FLOW ADJUSTMENT CONTROL The flow adjustment controls regulate the flow of oxygen,

air, and other gases to the flow indicators.

There are two types of flow adjustment controls: mechanical and electronic.

The anesthesia workstation standard requires that there be only one flow control for each gas. It must be adjacent to or identifiable with its associated flowmeter.

MECHANICAL FLOW CONTROL VALVE The mechanical flow control valve (needle valve, pin valve, fine

adjustment valve) controls the rate of gas flow through its associated flowmeter

Some also have an ON-OFF function. On some machines, the ON-OFF function is controlled by the master switch.

Mechanical flow control valves are used with both mechanical and electronic flowmeters

MECHANICAL FLOW CONTROL VALVECOMPONENTS

Body. The flow control valve body screws into the anesthesia machine.

Stem and Seat.

The stem and seat have fine threads so that the stem moves only a short distance when a complete turn is made.

When the valve is closed, the pin at the end of the stem fits into the seat, occluding the orifice so that no gas can pass through the valve. When the stem is turned outward, an opening between the pin and the seat is created, allowing gas to flow through the valve. The greater the space between the pin and the seat, the greater the volume of gas that can flow.

To eliminate any looseness in the threads, the valve may be spring loaded. This also minimizes flow fluctuations from lateral or axial pressure applied to the flow control knob.

CONTD

It is advantageous to have stops for the OFF and MAXIMUM flow positions. A stop for the OFF position avoids damage to the valve seat. A stop for the MAXIMUM flow position prevents the stem from becoming disengaged from the body.

Control Knob :

The control knob is joined to the stem. If it is a rotary style knob, the oxygen flow control knob must have a fluted profile and be as large as or larger than that for any other gas. All other flow control knobs must be round. The knob is turned counterclockwise to increase flow. If Other types of flow control valves are present, the oxygen control must look and feel different from the other controls.

Flow Control Knobs

CONTD….. When a machine is not being used, the gas source (cylinder or

pipeline) should be closed or disconnected.

The flow control valves should be opened until the gas pressure is reduced to zero and then closed.

If the gas source is not disconnected, the flow control valve should be turned OFF to avoid the fresh gas desiccating the carbon dioxide absorbent and to conserve gas.

Before machine use is resumed, the control valves should be checked to make certain that they are closed.

Sometimes, a flow control valve remains open after the gas is bled out or opened when the machine is cleaned or moved.

If the gas supply to an open flow control valve is restored and the associated flow indicator is not observed, the indicator may rise to the top of the tube where its presence may not be noticed.

Even if no harm to the patient results, the sudden rise may damage it and impair the flow meter accuracy.

REFERENCES

Understanding Anesthesia Equipments• DORSCH & DORSCH

Clinical Anesthesiology• MORGAN

Drugs and Equipments in Anesthesia Practice• ARUN K PAUL

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