CONTINUOUS FLOW ANAESTHESIA MACHINES, INCLUDING UPGRADING TO MODERN MACHINES AND SAFETY FEATURES

Dr P N ViswanathanProf and HOD,

Department of Anaesthesia,Mysore Medical College,

MysoreAnaesthesia machines are evolved from simple pneumatic to sophisticated, computer

based fully integrated anaesthesia systems. It is essential for the anesthesiologist to achieve a

thorough understanding of anaesthesia machines for the safe practice of anaesthesia.

Development of the Anaesthesia Machine :

The early half of 20th century saw the development of a large number of machines.

Teters was the earliest gas oxygen outfit that made possible the use of rebreathing, positive

pressure and addition of ether vapour to gases. Gwathmey introduced the sight feed type of

flow meters. Heidbrink in USA ( 1913 ) introduced the Heidbrink apparatus.

Dr. H. Edmund. G. Boyle ( 1917 ) in Britain introduced the early version of Boyles

apparatus after using Gwathmeys machines, in France during the first world war, He found

that Gwathmeys machine hnd several defects, mainly leakage at gas unions. He suggested

changes which were put in to effect by coxeters. The early Boyles machine was designed on

a wooden frame mounted on castors. It had a triple sight feed water bubble flow meter for

delivering nitrous oxide, oxygen and carbon dioxide and measuring their rates of flow. There

were chloroform and ether bottle mounted their outlet was connected to a two gallon rubber

bag with a Barth 3 way or similar stop cock and rubber padded face piece. Breathing circuits

incorporating Heidbrink expiratory valves were used much later. A metallic chain from the

back bar up to the ground level was an accessory of the machine as antistatic.

Modifications of the early version of Boyles machine :

1926 - Bypass control valves for regulating the amount of ether or chloroform

were incorporated.

1931 – 33 - The dry flow meter and the pressure reducing regulator were added.

1928 - Dr. Brain C. Sword introduced the circle absorber.

1937 - Rotameter superseded the coxeter bobbin meter after Richard salt

suggested the use of rotameter.

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1953 - Colour coding of the cylinders was introduced.

1955 - Pin index safety system was added.

1958 - The Bodok seal was added.

Other older machines :

Merret machine was manufactured in 1958. The anaesthetic head was capable of

being adjusted to give practically any closed or semi closed breathing circuit. A

safety trilene interlock was present.

Ohio machine ( 1958 ) incorporated within the breathing circuit a water manometer to

measure the positive pressure in the breathing circuit, with a facility for closed circle

system.

Gillis machine : It was the British version of the machines of 1950’s. It was a

portable machine with facilities for closed circuit anaesthesia.

New model of Boyles machine :

It has the table and the frame of stainless steel. Cylinder yokes and pressure

regulators are permanently fixed to the table frame and the tubings between the Adams

regulator and the rotameter is made of metal. A permanently fixed halothane vaporizer is

present instead of trichrloroethylene bottle. The take off mount is transposed from its usual

place to a lower point at a corner of the front of the table.

Overview of Anaesthetic machines :

A typical machine consists of :

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1. A rigid metal box frame work on wheels, to this a surface of compressed gas either

pipeline or cylinders are attached.

2. Pressure regulators to reduce the high pressure to working pressure of the machine

and keep the pressure constant.

3. Secondary pressure regulators.

4. Pressure gauges to show pipeline and cylinder pressures.

5. Flow meters to measure and adjust various gas flows.

6. Vapourizers for addition of volatile anaesthetic agents.

7. Oxygen failure warning devices to warn of failure of O2 supply and to avoid hypoxic

mixture delivery.

8. High flow oxygen flush, a flow meter bypass system for administration of high flow

pure O2 in an emergency.

9. Common gas outlet. A single outlet delivering the gases and vapours in to an attached

breathing system.

The anaesthesia gas machine consists of

Pneumatic system.

Electric system.

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Schematic of flow arrangements of a contemporary anesthesia machine.

Pneumatic System :

The pneumatic part of the machine can be divided into

High pressure system

Intermediate pressure system

Low pressure system.

The high pressure system :

This receives gases from a cylinder at a high variable pressure and reduces that

pressure to a lower more constant pressure suitable for use in the machine.

It includes a) Hanger yoke

b) Cylinder pressure indicator

c) Pressure reducing device.

a) Hanger Yoke

Its functions are to orient and support the cylinder, provide a gas tight seal and to

ensure a unidirectional flow of gases in the machine.

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Body : The body of the yoke is threaded into the frame of the machine. It provides

support for the cylinder.

Retaining Screw : This is threaded to 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 achieved. The cylinder is then supported by the retaining screw, the

nipple and the pin index pins.

Nipple : It is that part of the yoke through which the gas enters the machine. If

it is damaged, it is impossible to obtain a tight seal with the cylinder valve.

Index pins : The pins of the pin index safety system are below the nipple. The

holes in to which the pins fit must have specific depth. If they extend too far in to the

body of the yoke it may be impossible to insert an incorrect cylinder in to the yoke.

Pins on the yoke fit into the holes on the cylinder valve. It consists two pins, 4 mm &

6mm on the yoke of the anaesthesia machine.

A 9 / 16 inch circumference semi circle is made and six equidistant points are made

on the arc ( 7th if entonox is to be used ). The pin index for gases that can be used are:

Oxygen 2, 5

Nitrous oxide 3, 5

Cyclopropane 3, 6

Air 1, 5

Nitrogen 1, 4

Entonox ( 50% air + 50% nitrous oxide ) 7

Carbon dioxide ( < 7.5% ) 2, 6

Carbon dioxide ( > 7.5% ) 1, 6

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Hanger yoke for an oxygen tank showing pin – indexed safety system.

Washer / Bodak Seal : It is placed around the nipple to effect a seal between the

cylinder valve and the yoke. Broken washers should not used. No more than one

washer should be used as they may prevent establishment of a tight seal or nullify the

pin index safety system.

Filter : ( 100 maximum ) This is installed between the cylinder and the reducing

device to prevent particulate matter from entering the machine.

Check valve assembly :

The check valve assembly allows gas from a cylinder to enter the machine but

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

This allows an empty cylinder to be replaced with a full one without having to

turn off the in use cylinder. It also prevents the transfer of gas from one

cylinder to another with a lower pressure in a double yoke.

It consists of a plunger that slides away from the side of greater pressure.

When the cylinder pressure exceeds the pressure of the machine, the plunger is

pushed to right and gas passes around it and in to the machine. When the

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machine pressure exceeds cylinder pressure, it moves to left, blocking the flow

of gases.

Check valves are not permanent seals for empty yokes and may allow a small

amount of gas to escape. As soon as the cylinder is exhausted it must be

replaced by a full one, if not available, a yoke plug ( dummy cylinder block or

plug ) should be placed in the empty yoke.

b) Cylinder pressure indicator ( Gauge ) ( Bourdon pressure gauge )

This displays the pressure of the gas in the cylinder. If there is more than one yoke

for a gas, one indicator may be provided for each yoke or one indicator for a group of yokes

of the same gas. The indicators may be located on the front of the machine on a panel or on

the cylinders. If the indicator is circular, the lowest pressure indication must be between 6 O’

clock and 9 O’ clock position on a clock face. The scale must be at least 33% more than the

maximum filling pressure. It is constructed with a special heavy glass window and a safety

back designed to act as a pressure fuses, so that gas is released from the back of the casing if

there is a sudden rise in pressure.

Bourdon tube gauge

c) Pressure reducing device ( Regulator )

It is a device that reduces a high variable input gas pressure to a constant, lower out

put pressure. Cylinder O2 enters the machine at pressure of upto 1900 Psig (depends on how

full the cylinder is) and this is reduced to a constant output pressure of 45 Psig by the O2

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cylinder pressure regulator. Reducing devices are preset at factory and located under the

machines work surface. Failure of the pressure regulator can transmit excessively high

pressures to machine low pressure system. To protect against this there is a pressure relief

valve incorporated into the regulator, where by excess pressures are vented to atmosphere.

Intermediate pressure system

This includes

a) Pipeline inlet connections

b) Pipeline pressure indicators

c) Piping

d) Gas power outlet

e) Oxygen pressure failure devices

f) Oxygen flush

g) Secondary pressure regulators

h) Flow control valves.

a) Pipeline inlet connections :

Pipeline inlets for O2 & N2 O are mandatory. Each pipeline source is attached to the

machine via a gas specific connection. Theses inlets are fitted with threaded non inter

changeable diameter index safety system ( DISS ). Each outlet must contain a check

valve to prevent the flow of gas from machine into piping system and a filter to

prevent impurities from entering the machine.

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Diameter Index Safety System.

Connectors are of different diameters, making them gas specific.

b) Pipeline pressure indicators :

These are usually found on the front of the machine on a panel and are colour coded.

If it is on the pipeline side of the check valve it measures only pipeline pressure and if

the hose is disconnected it reads zero even if the cylinder valve is open. If the

indicator is on the machine side of the check valve it cannot be depended on to give

true indication of the pipeline pressure unless the cylinder valves are closed.

O2 pressure - 420 Kpa

N2 O pressure - 310 Kpa

c) Piping :

Connections between components inside the machine are made from metal tubings.

These must be intended to withstand four times the intended service pressure without

rupturing. The leaks between the pipeline inlet or cylinder pressure reducing system

and the flow control valve should not exceed 25 ml min-1 at normal service pressure.

If the pressure reducing system is included, the leak should not exceed 150 ml min -1 at

maximum inlet pressures.

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d) Gas power outlet :

Most machines are equipped with a connection to supply oxygen or air to a ventilator

or jet ventilation system.

Most machines currently have a direct connection to the ventilator and do not have a

discrete outlet.

If there is a outlet it should be fitted with a DISS fitting and should have a check valve

so that gas can only flow from the machine.

Mater Switch : ( Pneumatic component )

Master switch is a two or three position switch with mutually exclusive positions.

Pneumatic portion is in the intermediate pressure system down stream from the

cylinder and pipeline supplies. Turing on the switch causes both electric and

pneumatic functions of the machine to be activated. O2 flush is independent of this

feature.

e) Oxygen pressure failure devices :

i) Oxygen failure safety device.

ii) Oxygen supply failure alarm.

i) Oxygen failure safety device :

Oxygen failure safety device shuts off or proportionally decreases and ultimately

interrupts the supply of N2 O and other gases if the oxygen supply pressure decreases. These

are located upstream of the flow control valves. These are present on most anaesthesia

machines but may not be present on some older machines or may be present for only one gas.

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Drager oxygen failure protection device

ii) Oxygen supply failure alarm :

The machine standard specifies that whenever the O2 supply pressure falls below a

manufacturer specified threshold ( usually 30 psig or 205 Kpa ) a medium priority alarm

should be eunincated in 5 seconds after being activated it may be silenced for a period not

exceeding 120 seconds. If the low pressure condition is corrected, the alarm sound will

cease. These can be either electric or pneumatic alarms.

Limitations :

Both O2 failure safety device and alarm depend on pressure and not flow, they have

some limitations.

These do not offer total protection against a hypoxic mixture being delivered because

they do not prevent anaesthetic gas from flowing if there is no flow of oxygen.

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They aid in preventing hypoxia by problems in machine circuitry upstream

( disconnected O2 hose, low oxygen pressure in the pipeline, depletion of O2

cylinders) but not against equipment problems ( leaks ) or operator errors ( closed or

partially closed O2 flow control valve ) that occurs downstream.

f) Oxygen flush valve :

It receives O2 from the pipeline inlet or cylinder reducing device and directs a high

unmetered flow directly to the common gas outlet. The flow should be between

35 – 75 L min-1. The standard requires that the oxygen flush be a single purpose, self

closing device which is permanently marked to show its function and designed to

minimize accidental activation,

Problems

1) Risk of barotraumas to the patient.

2) Dilution of anaesthetic gases and awareness.

3) Delivery of high pressures if activated during an inspiration delivered by

anaesthesia ventilator.

g) Second stage reducing device :

The purpose of this device is to eliminate fluctuations in pressure supplied to the flow

indicator caused by fluctuations in pipeline pressure. These reduce the pressure to 26

psig for N2 O and 14 psig for O2, after they receive the gas from pipeline or cylinder

reducing device.

h) Flow control valve : ( Needle valve, pin valve, flow adjustment control )

This controls the rate of flow of a gas through its associated flow indicator by manual

adjustment of a variable orifice.

Current standards requires one flow control valve for each gas that must be adjacent

to or identifiable with its flow indicator.

Components :

Body

Stem and seat

Control knob

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Control knobs are labelled and colour coded. The O2 control knob should look and

feel different from others. Turing counter anticlockwise increases flow and clockwise

decreases flow. They should operate smoothly.

Problems with flow control valves :

Inadvertent alteration.

Inability to turn the knob.

Leak through open flow control valve.

Failure to allow adequate gas flow.

Low pressure system :

This is downstream of the flow control valves. Pressure here is only slightly above

atmosphere. It includes,

a) Flow meters.

b) Vapourizer circuit control valves.

c) Back pressure safety device.

d) Low pressure piping.

e) Common gas outlet.

a) Flow indicators ( Flow meters ) :

Flow meter measures the volume of gas while gas is in motion. Thorpe tube is an

older term for flow meters.

Components :

Tube

Float

Indicator

Stop

Scale.

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Schematic of oxygen flow meter and flow control valve.

i) Tube :

It is made of glass. Gas passes between the ball and the inner wall of the tube.

Because the tube is tapered the flow increases from below upwards. The tubes can be single

taper or dual taper tubes. Dual tapers have one for fine and one for coarse flows.

ii) Float :

The float or bobbin is a free moving device within the tube. Three types of floats are

available.

Non rotating floats.

Rotating floats ( rotameter )

Ball floats.

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iii) Stop :

This is at the top of the flow meter and prevents the float from plugging the outlet,

which could lead to tube damage, also prevents the indicator from ascending to a point where

it can not be seen.

iv) Scale :

The machine standard requires that the flow indicator scale be marked on the tube or

be located on the right side of the tube when viewed from front. It should be calibrated in

liters min-1. Flows upto 1 L min-1 may be expressed in milliliters or in decimal fractions of a

liter per minute with a zero before the decimal point.

v) Lights :

These are optional.

Safety devices :

One of the hazards with flow meters is the possibility that the operator will set flows

so that a hypoxic mixture will be delivered.

1) The oxygen flow control knob is touch coded.

2) Mandatory minimum oxygen flow : Some machines require a minimum flow

of O2 before other gases will flow ( 50 to 250 ml min-1 )

Oxygen ration monitoring and proportioning systems

Minimum oxygen ratio device ( Hypoxic guard )

a) Mechanical linkage ( Ohmeda link 25 proportion limiting control system )

A mechanical linkage involving N2 O and O2 flow control valves is present, to ensure

an adequate percentage ( at least 25% ) of O2 in the gas mixture created. The link 25 system

permits the N2 O and O2 flow control valves to be set independently of one another, but

whenever a setting of nitrous oxide concentration mare than 75% is attempted, the O2 flow is

automatically increased to maintain at least 25% O2 in the resulting mixture.

It only interconnects O2 and N2 O flow control valves. If the anaesthesia machine has

flow controls for helium or air, a gas mixture containing less than 25% O2 could be set at the

level of flow meters.

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Ohmeda link 25 proportion limiting system

b) Pneumatic linkage : ( Drager ORMC )

It is a pneumatic interlock designed to keep fresh gas flow of at least 25% + 3 % O2,

by limiting nitrous oxide flow ( unlike link 25 which increases O2 flow ). It also rings alarm

to prevent hypoxic mixture when the machine is used in N2 O / O2 mode and inactivated in all

gases mode ( i.e., when helium / air might be switched in to the system )

North American Drager Oxygen Ratio Monitor Controller

Other flow meters ( optional )

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Auxiliary oxygen flow meter for attaching a nasal canula or other supplemental O2

delivery devices.

Common gas outlet flow meters.

Scavenging flow meters.

Electronic flow meters :

Gas machines with electronic flow meters have no glass tubes and the flow rate is

indicated with a bar graph on a monitor screen. They are 5 to 10 times as accurate at

metering gas flow than glass flow tubes.

Vaporizers :

A vaporizer is designed to add a controlled amount of an inhalational agent, after

changing from liquid to vapour, to the fresh gas flow. They are located in the low pressure

system between the flow meters and the common gas outlet. They are mounted on the back

bar of the machine using vaporizer mounting devices. These can be permanently mounted or

may be changed.

Unidirectional check valve :

Positive pressure from the breathing system is transmitted back to the machine when

ventilation is assisted or controlled use of oxygen flush valve may also create a positive

pressure. Such an increase in pressure may affect the concentration of the volatile anesthetics

issuing from the vaporizer, can increase leaks and can cause inaccurate flow meter reading.

To minimize these problems unidirectional valve is incorporated between the vaporizer and

the common gas outlet, upstream of where the O2 flush flow joins the fresh gas flow.

Pressure relief device :

This is situated downstream of the vaporizers either on the back bar or near the

common gas outlet to prevent high pressure from being transmitted in to machine and to

protect the patient from high pressures from the machine. It opens to atmosphere and vents

gases if a preset pressure is exceeded. If the pressure in the back bar exceeds 35 Kpa, the

valve opens. It limits the ability of the machine to provide jet ventilation.

Low pressure piping :

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This has a large number of connections and is subject to leaks and breakages. The

piping in the machine between the flow control valve and the common gas outlet must not

exceed 30 ml min-1 at a pressure of 3 Kpa with the vaporizer either on / off / detached

position.

Common ( fresh ) gas outlet :

This receives all the gases and vapors from the machine. Most machine outlets have

15mm female connection with a co-axial 22 mm male connection. The fresh gas supply tube

of the breathing system attaches to common gas outlet.

Electrical system :

Many modern machines are powered by electricity supplied through the anaesthesia

machine. These includes :

a) Master switch – Activates both pneumatic and electric functions on

the machines.

b) Power failure indicator – Visual or other indicators to alert loss of

power.

c) Batter back up – A back up source of power.

d) Electrical outlets – To power monitors that may be attached to the

machine.

e) Circuit breakers – It is activated when the maximum electric load is

exceeded.

New gas machines :

Advanced ventilators are among the biggest differences between new and older gas

machines.

Various models manufactured by different companies are available. The features of

these machines are as follows :

1) Monitoring included :

Volume, pressure, inspired oxygen.

Gas monitoring.

Ultrasonic flow sensors in the breathing circuit displays tidal volume, minute volume,

respiratory rate, respiratory volume wave form.

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Integrated patient monitoring.

2) Ventilator : Piston ventilator with tidal volume corrected for leaks, patient and breathing

circuit compliance and fresh gas flow. Various modes like SIMV, volume control, pressure

control, manual / spontaneous are available.

3) Machine checklist : From cold start up, 1 minute power on self test, then 5 minute

ventilator self test or manual FDA check list.

4) Flow meters : Traditional glass flow tubes or electronic flow meters.

5) Vaporizers : Variable by pass vaporizers in most of the machines.

6) Breathing circuit : The absorbed head is warmed ( most of the machines ) and the

circuit can be of lower, medium or higher volume with 1.5 L absorbent volume.

7) Scavenger : Open, closed or passive scavenger interface.

8) Electrical power failure : Most machines have 30 minute battery reserve with fresh gas,

vaporizers and ventilator operational. Patient monitoring is not preserved in most of the

machines.

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Fabius G S Anaesthesia Gas Machine

Julian Anaesthesia Gas Machine ( Closeup )

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Modulus SE Anaesthesia Gas Machine

FDA anaesthesia gas machine check list ( 1993 )

This is a modification of the original. The anaesthesia gas machine must be equipped

with an ascending billows ventilator and certain monitors. If not so equipped, the checklist

must be modified.

1) Verify back up ventilation equipment is available and functioning.

2) Check oxygen cylinder supply.

One cylinder must be at least half full ( 1000 Psi )

It is not necessary to

i) Check any other cylinders beside oxygen.

ii) Bleed the pressure off the cylinder pressure gauge after checking.

Leave cylinder closed after checking.

Check suction, AMBU bag, extra circuit, location of circuit breakers,

scavenger laps, any loose pipeline, colour / date of absorbent, electrical

components.

3) Check central pipeline supplies.

Check for proper connection at wall.

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Check pipeline pressure gauge – should read 50 Psi.

Not necessary to unhook pipeline connections at wall.

4) Check initial status of the low pressure system.

Remove oxygen analyzer sensor.

Check liquid level and fill vaporizers if needed.

Check vaporizer interlock.

5) Perform leak check of low pressure system.

Leaks as low as 100 ml min-1 may lead to critical decrease in the concentration

of the volatile anaesthetic or permit hypoxic mixtures.

Negative pressure leak test 10 seconds is recommended.

Repeat for each vaporizer.

6) Turn master switch on.

7) Test flow meters.

Check for damage, full range, hypoxic guard.

8) Calibrate oxygen monitor.

9) Check initial status of the breathing system.

10) Test ventilation systems and unidirectional valves.

11) Perform leak check of the breathing system.

The usual high pressure check.

Let the gas out of the circuit through the APL valve.

12) Adjust and check scavenging system.

13) Check, calibrate, set alarm limits of all monitors.

14) Check the final status of the machine.

Vaporizer off

Bag / vent switch to bag mode.

APL open.

Zero flow of flow meters.

Suction adequate.

Breathing system ready.

NOTE : May omit or abbreviate # 1 – 9 between cases.

Minimum test under life threatening conditions : ( Trauma / Emergency C. Section )

Situations do arise in anaesthesia where there is neither time nor opportunity to fully

check the anaesthesia gas machine.

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The following check list is recommenced for such situations.

1) High pressure test of the breathing circuit.

2) Check suction.

3) Observe and / or palpate breathing bag during preoxygenation.

Ensures :

Adequate flow of oxygen.

Good mask fit

Patient is breathing.

The bag / vent switch is on ‘Bag’ not ‘Vent’ mode.

Required components of an anaesthesia work station :

The current anaesthesia gas machine ( work station ) standard is ASTM F 1850. The

European standard in EN740. F 1850 specifies what is needed for an anaesthesia work

station.

Required components include :

Battery back up for 30 minutes.

Alarms – High, medium and low priority.

High priority alarms may not be silenced for more than two minutes and must

sound if user adjustable limits are exceeded or if continuing high pressure is

sensed.

Disconnect alarms may be based on low pressure, exhaled volume or carbon

dioxide.

Required monitors.

Exhaled volume

Inspired O2

Anaesthetic vapour concentration

Pulse oximetry, blood pressure and ECG monitoring.

Pressure in the breathing circuit limited to 12.5 Kpa.

The electrical supply cord must be non detachable or resistant to detachment.

The machine must have at least one oxygen cylinder attached.

The hanger yoke must be pin indexed, have a clamping device that resists leaks and

contain a filter. It must have a check valve to prevent transfilling and a cylinder

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pressure gauge. The machine must use pipeline gas as long as pipeline pressure is

greater than 50 Psi.

Flow meters.

Single control for each.

Each flow control next to a flow indicator.

Uniquely shaped oxygen flow control knob.

Valve stops such that excessive rotation will not damage the flow meter.

Oxygen flow indicator to the right side of a flow meter bank.

Oxygen enters the common manifold down stream of other gases.

Oxygen flush, capable of 35.75 L min-1 flow, and not passing through any vaporizers.

Vaporizers.

Concentration calibrated.

Interlock must be present.

Liquid level indicated, designed to prevent over filling.

Should use keyed filler devices.

No discharge of the liquid anaesthetic occurs at even maximum fresh gas flow.

Only one common outlet at 22 mm external land 15mm internal diameter.

Pipeline gas supply

Pipe line pressure gauge.

Inlets for at least O2 & N2 O.

DISS protected

In line filter

Check valve.

Check list must be provided

Digital data interface must be provided.

An auxiliary oxygen flow meter is strongly recommended.

REFERENCES :

1. The evolution of the Boyle apparatus, 1917 – 67, Anaesthesia Vol 23 No. 1

January 1968.

2. Anaesthetic Devices ( 1900 – 1925 ) Indian J. Anaesth. 2003 ; 47 ( 4 ) : 263 – 264.

3. Anaesthetic Equipments ( 1950 – 1975 ) Indian J. Anaesth. 2004 ; 48 ( 1 ) : 25 – 27.

4. The Anaesthesia Gas Machine, www.udmercy. Edu/crna/agm/.

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5. Understanding Anaesthetic Equipment. Dorsch and Dorsch 4th Edition.

6. Anaesthetic Equipment. Principles and Applications. Jan Ehrenwerth and

James B.Eisenkraft.

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