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Understand the components & the operating principles

of the anaesthesia machine, namely, gas supply, O2

safety devices, and flowmeters.

Know how to deal with problems arising from these

components.

An anaesthesia system consists of the various

components that communicate with each other

during the administration of inhalation anaesthesia.

A thorough understanding of these parts is essential

to the safe practice of anaesthesia.

1. O2 & N2O Sources: consisting of a pipeline supply

source and a cylinder supply source. The cylinder

source is regulated from 2,200 to approximately 45

Psig, and the N2O cylinder source is regulated from

745 to approximately 45 Psig.

2. Fail-safe valve: is a safety device located downstream

from the N2O supply source. This valve shuts off or

proportionally decreases the supply of N2O if the O2

supply pressure decreases. Recent machines have an

alarm device to monitor the O2 supply pressure.

3. Second-stage O2 regulator: in most Ohmeda machines

have located downstream from the O2 supply source. It

is adjusted to a precise pressure level, such as 14

Psig.

4. Flow control valves: are an important anatomic

landmark because they divide the anaesthesia machine

into two parts.

5. Machine outlet check valve: in many Ohmeda

machines have located between the vaporizers and the

common gas outlet.

1. The high–pressure circuit: is the part of the machine

that is confined to the cylinders and their primary

pressure regulators.

2. The intermediate–pressure circuit: is the part of the

machine that begins at the regulated cylinder supply

sources downstream the flow control valve.

3. The low–pressure circuit: is the part of the machine

that extends from the flow control valves to the

common gas outlet.

1. The high–pressure circuit: is the part of the machine

that is confined to the cylinders and their primary

pressure regulators.

2. The intermediate–pressure circuit: is the part of the

machine that begins at the regulated cylinder supply

sources downstream the flow control valve.

3. The low–pressure circuit: is the part of the machine

that extends from the flow control valves to the

common gas outlet.

1. The high-pressure circuit: consists of those parts

which receive gas at cylinder pressure:

Hanger yoke (including filter and unidirectional valve).

Yoke block.

Cylinder pressure gauge.

Cylinder pressure regulators.

2. The intermediate – pressure circuit: receives gases

at low, relatively constant pressures (37–55 psig,

which is pipeline pressure).

Ventilator power inlet.

Oxygen pressure–failure device (fail-safe) and alarm.

Flowmeter valves.

Oxygen second-stage regulator.

Oxygen flush valve.

3. The low-pressure circuit: includes components

distal to the flowmeter needle:

Valves.

Flowmeter tubes.

Vaporizers.

Check valves (if present).

Common gas outlet.

Most hospitals today have a central piping system to

deliver medical gases such as O2, N2O, & air to the

operating room.

In a survey of approximately 200 hospitals in 1976,

31%% reported difficulties with pipeline systems.

Inadequate oxygen pressure.

Excessive pipeline pressures.

Accidental crossing of oxygen and nitrous oxide

pipelines.

The pipeline inlet fittings are gas-specific Diameter

Index Safety System (D.I.S.S.).

A pipeline pressure gauge

must be located on the

pipeline side rather than on

the machine side of the

check valve. A value of 50

psig, however, does not

guarantee that the pipeline

is supplying the machine.

Anaesthesia machines have reserve E cylinders if a

pipeline supply source is not available or if the

pipeline fails.

Color–coded cylinders are attached to the

anaesthesia machine through the hanger yoke

assembly, which orients and supports the cylinder,

provides a gas – tight seal, and ensures a

unidirectional flow of gases into the machine.

Each hanger yoke is equipped with the Pin Index

Safety System.

A check valve is located downstream from each

cylinder if a double-yoke assembly is used.

The check valve has several functions:

1. It minimizes gas transfer from a cylinder at high

pressure to one with lower pressure.

2. It allows an empty cylinder to be exchanged for a

full one while gas flow continues from the other

cylinder into the machine with minimal loss of

gas.

3. It minimizes leakage from an open cylinder to the

atmosphere if one cylinder is absent.

Each cylinder supply source has a pressure-reducing valve

known as the cylinder pressure regulator.

The O2 cylinder pressure regulator reduces the O2 cylinder

pressure from a high of 2,200 psig to approximately 45 psig.

The N2O cylinder pressure regulator receives pressure of up to

745 psig and reduces it to approximately 45 psig.

The cylinders should be turned off except:

1. During the preoperative machine-checking period .

2. When a pipeline source is unavailable.

O2 & N2O supply sources existed as independent

entities in older models of anaesthesia machines, and

they were not pneumatically or mechanically

interfaced.

Therefore, abrupt or insidious oxygen pressure failure

had the potential to lead to the delivery of a hypoxic

mixture.

Contemporary anaesthesia machines have a number

of safety devices that act together in a cascade

manner to minimize the risk of hypoxia as oxygen

pressure decreases.

Many older anaesthesia machines have a pneumatic

alarm device that sounds a warning when the O2

supply pressure decreases to a predetermined

threshold value, such as 30 psig.

Electronic alarm devices have both audible and visual

alarms. The O2 pressure threshold value for the

Ohmeda Modulus II Plus & the Ohmeda CD is 27 psig,

and for the N.A.Dräger Narkomed 2B, 3, & 4, the value

is 30 ± 3 psig.

It is present in the gas line supplying each of the

flowmeters except that for oxygen. These valves,

controlled by oxygen pressure, shut off or

proportionally decrease the supply pressure of all

other gases (N2O, air, CO2, helium, and N2) as the O2

supply pressure decreases.

The misnomer "fail-safe" has led to the misconception

that the device prevents administration of a hypoxic

mixture.

This is not the case.

They are equipped with the

pressure-sensor shutoff valve.

It is either open or closed with

a threshold pressure of 25 psig.

An O2 pressure > the threshold

value is exerted upon the

mobile diaphragm.

This pressure moves piston,

pin, and valve off the valve

seat.

N2O flow passes freely to the

N2O flow control valve.

They uses the Oxygen Failure Protection Device

(OFPD), which interfaces the O2 pressure with that of

other gases, such as N2O, air, CO2, helium, and N2.

The OFPD is based on a proportioning principle rather

than a threshold principle.

The pressure of all gases controlled by the OFPD will

decrease proportionally with the oxygen pressure.

The OFPD consists of a seat-

nozzle assembly connected

to a spring-loaded piston.

Nozzle

Valve seat

Spring

When the O2 supply

pressure is 50 psig,it

pushes the piston upward,

forcing the nozzle away

from the valve seat.

N2O advances toward the

flow control valve at 50

psig. 50 Psig

50 Psig

N2O

O2

50 Psig

When the O2 pressure is

zero psig, the spring is

expanded and forces the

nozzle against the seat,

preventing N2O flow

through the device.

0 Psig

50 Psig

N2O

O2

0 Psig

When the O2 pressure is

intermediate at 25 psig.

The force of the spring

partially closes the valve.

The N2O pressure

delivered to the flow

control valve is 25 Psig.

25 Psig

50 Psig

N2O

O2

25 Psig

There is a continuum of

intermed.configurations

between the extremes (0

– 50 Psig) of O2 supply

pressure.

25 Psig

50 Psig

N2O

O2

25 Psig

These intermediate valve

configurations are

responsible for the

proportional nature of

the OFPD.

25 Psig

50 Psig

N2O

O2

25 Psig

Most contemporary Ohmeda machines have a second-

stage oxygen pressure regulator set at a specific value

ranging from 12 to 19 Psig.

O2 flowmeter output is constant when the O2 supply

pressure exceeds the set value.

Ohmeda pressure-sensor shutoff valves are set at a

higher threshold value (20 to 30 Psig).

This ensures that O2 is the last gas flow to decrease if

O2 pressure fails, and act together in a cascade manner

to minimize the risk of hypoxia as oxygen pressure

decreases.

Opening the flow control valve allows

gas to travel through the space between

the float and the flowtube [the annular

space].

The indicator float hovers freely in an

equilibrium position, where the upward

force resulting from gas flow equals the

downward force on the float resulting

from gravity at a given flow rate.

These flowmeters are commonly referred

to as constant – pressure flowmeters

because the pressure decrease across

the float remains constant for all

positions in the tube.

It is composed of a flow control

knob, a needle valve, a valve

seat, and a pair of valve stops.

It can receive its pneumatic input

either directly from the pipeline

source (50 Psig) or from a

second-stage pressure regulator.

The location of the needle valve in

the valve seat changes to establish

different orifices when the flow

control valve is adjusted.

Gas flow increases when the flow

control valve is turned counter-

clockwise, and vice versa.

Extreme clockwise rotation results

in damage to the needle valve and

valve seat.

Therefore, flow control valves

are equipped with valve stops to

prevent this occurrence. The

stops come into contact with

each other at zero flow on most

flow control valves.

The Ohmeda Modulus I, Modulus

II, Modulus II Plus, and Ohmeda

8000 are set at an O2 flow rate of

approximately 200 ml.min-1.

On recent Ohmeda machines,

minimum O2 flow results from

incomplete closure of the O2 flow

control valve.

N.A.Dräger machines, the O2 flow

control valve does close

completely.

1. The oxygen flow control knob is

physically distinguishable [fluted,

projects beyond the control knobs

of the other gases, and is larger in

diameter] from other gas knobs.

2. All knobs are color-coded for the

appropriate gas & the name of the

gas is permanently marked on

each.

3. The knobs are recessed or

protected with a shield or barrier

to minimize inadvertent change

from a preset position.

The flowmeter subassembly consists of:

1. The flow tube.

2. The indicator scale.

3. The indicator float with float

stops.

They are made of glass, have a

single taper in which the inner

diameter of the flow tube

increases uniformly from bottom

to top.

Manufacturers provide double flow

tubes for O2 & N2O to provide

better visual discrimination at low

flow rates.

A fine flow tube indicates flow

approximately 1-200 ml.min-1.

A coarse flow tube indicates flow

approximately 1-10 L.min-1.

The two tubes are connected in

series & supplied by a single flow

control valve.

The total gas flow is that shown on

the higher flowmeter.

Some older machines have two

flow tubes for a single gas

arranged in parallel.

Each of the tubes has a flow

control valve.

The total flow is the sum of the

individual flows.

There are different types of bobbins

or floats, including plumb-bob floats,

rotating skirted floats & ball floats.

Flow is read at the top of plumb-bob

and skirted floats and at the center of

the ball on the ball-type floats.

Flow tubes are equipped with float

stops at the top & bottom of the tube.

The upper stop prevents the float

from ascending to the top of the

tube, plugging the outlet & ensures

that the float will be visible at

maximum flows instead of being

hidden in the manifold.

The bottom float stop provides a

central foundation for the indicator

when the flow control valve is

turned off.

The flowmeter scale

can be marked directly

on the flow tube or can

be located to the right

of the tube.

Gradations are closer together at

the top of the scale because the

annular space increases more

rapidly than does the internal

diameter from the bottom to the

top of the tube.

Rib guides are used in N.A.Dräger

flow tubes with ball-type

indicators to minimize the

compression effect.

They are tapered glass ridges that

run the length of the tube.

There are usually 3 rib guides,

which are equally spaced around

the inner circumference of the

tube.

The annular space from the

bottom to the top of the tube

increases almost proportionally

with the internal diameter,

resulting in a nearly linear scale.

1. The flowmeter subassembly for each gas on the Ohmeda Modulus

I, Modulus II, Modulus II Plus, and CD is housed in an independent,

color-coded, pin-specific module.

2. The flow scale and the chemical formula or name of the gas is

permanently etched on the backing to the right of the flow tube.

3. Flowmeter scales are individually hand-calibrated by use of the

specific float to provide a high degree of accuracy.

4. The tube, float, and scale make an inseparable unit. The entire set

must be replaced if any component is damaged.

5. N.A.Dräger does not use a modular system for the flowmeter

subassembly. The flow scale, the chemical symbol, and the gas-

specific color codes are etched directly onto the flow tube. The

scale in use is obvious when two flow tubes for the same gas are

used.

1. Leaks:

It is a substantial hazard because the flowmeters are

located downstream from all machine safety devices

except the O2 analyzer.

It can occur at the O rings between the glass flow tube

& the metal manifold and even in the glass flow tubes.

Gross damage to glass flow tubes is usually apparent,

but subtle cracks & chips may be overlooked, resulting

in errors of delivered flows.

1. Leaks:

Eger et al (1963) demonstrated that in the presence of a

flowmeter leak, a hypoxic mixture is less likely to occur

if the oxygen flowmeter is located downstream from all

other flowmeters.

O2 escapes through the leak & N2O flows toward the

common outlet, particularly at high N2O/ O2 flow ratios.

Ohmeda Mod II Anaesthesia Machine

2. Inaccuracy:

Flow error can occur even when flowmeters are

assembled properly with appropriate components.

1. Dirt or static electricity can cause a float to stick, and

the actual flow may be higher or lower than that

indicated.

2. Sticking is more common in the low flow range

because the annular space is smaller.

2. Inaccuracy:

3. A damaged float can cause inaccurate readings

because the precise relationship between the float

and the flow tube is altered.

4. Back-pressure from the breathing circuit can cause a

float to drop so that it reads less than the actual flow.

2. Inaccuracy:

5. Finally, if flowmeters are not aligned properly in the

vertical position, readings can be inaccurate because

tilting distorts the annular space.

3. Ambiguous Scale:

Before the standardization of flowmeter scales and

the widespread use of oxygen analyzers, at least two

deaths resulted from confusion created by

ambiguous scales.

The operator read the float position beside an

adjacent but erroneous scale in both cases.

3. Ambiguous Scale:

Today this error is less likely to occur because

contemporary flowmeter scales are marked either

directly onto or to the right of the appropriate flow

tube, therefore confusion is minimized.

3. Proportioning System:

Manufacturers have equipped newer machines with

proportioning systems in an attempt to prevent

delivery of a hypoxic mixture.

N2O & O2 are interfaced either mechanically or

pneumatically so that the minimum O2 concentration

at the common outlet is 25%%.


4. Ohmeda Link-25 Proportion Limiting Control System.

It allows independent

adjustment of either

valve, yet automatically

intercedes to maintain a

min.25%% O2 concentration

with a maximum N2O/O2

flow ratio of 3:1.

The N2O & O2 flow control

valves are identical.

A 14-tooth sprocket is

attached to the N2O flow

control valve, and a 28-

tooth sprocket is attached

to the O2 flow control

valve.



A chain physically links

the sprockets.

When N2O flow control

valve is turned through

two revolutions, O2 flow

control valve will revolve

once because of the 2:1

gear ratio.



It is used on the

N.A.Dräger Narkomed 2A,

2B, 3, and 4.

it is a pneumatic N2O/O2

interlock system designed

to maintain a FG O2 conc.

of at least 25 ±± 3%%.


4. N.A. Dräger Oxygen Ratio Monitor Controller (ORMC).

It limits N2O flow to prevent delivery of a hypoxic

mixture.

This is unlike the Ohmeda Link-25, which actively

increases O2 flow.



It is composed of an O2

chamber, a N2O chamber

& a N2O slave control

valve; all are inter-

connected by a mobile

horizontal shaft.



O2 chamberN2O chamber

The pneumatic input into

the device is from the O2

& N2O flowmeters.



These flowmeters are

unique because they have

specific resistors, located

downstream from the flow

control valves, which create

back-pressures directed to

the O2 & N2O chambers.



Resistor

The value of the O2 flow

tube’s resistor is 3–4 times

that of the N2O flow tube’s

resistor& the relative value

of these resistors

determines the value of

the controlled FG O2 conc.



Resistor

The backpressure in the

O2 and N2O chambers

pushes against rubber

diaphragms attached to

the mobile horizontal

shaft.

Diaphragms

Movement of the shaft

regulates the N2O slave

control valve, which

feeds the N2O flow

control valve.N2O slave control valve

If the O2 pressure is proportionally > the N2O

pressure, the N2O slave control valve opens more

widely, allowing more N2O to flow.

As the N2O flow is increased manually, the N2O

pressure forces the shaft toward the O2 chamber.

The valve opening becomes more restrictive and

limits the N2O flow to the flowmeter.



The N2O slave control valve is closed because of

inadequate O2 back-pressure.



1.1. Wrong Supply Gas: Wrong Supply Gas:

In the Link-25 system, the N2O and O2 flow control

valves will continue to be mechanically linked, and

a hypoxic mixture will proceed to the common

outlet.


The oxygen rubber diaphragm of the ORMC will

recognize adequate " O2 " pressure, and flow of

both the wrong gas plus N2O will result.


The oxygen analyzer is the only machine monitor

that will detect this condition in both systems.

2.2. Defective Pneumatics Or Mechanics: Defective Pneumatics Or Mechanics:

Pneumatic integrity in the Ohmeda system depends

on properly functioning second–stage pressure

regulators. A N2O/O2 ratio other than 3:1 will result if

the regulators are not precise.

The chain connecting the two sprockets must be

intact; a 97% N2O concentration can occur if it is cut

or broken.

2.2. Defective Pneumatics Or Mechanics: Defective Pneumatics Or Mechanics:

In the N.A.Dräger system, a functional OFPD is

necessary to supply appropriate pressure to the

ORMC.

The mechanical aspects of the ORMC [e.g. rubber

diaphragms, flow tube resistors & N2O slave

control valve] must likewise be intact.

3.3. Leaks Downstream: Leaks Downstream:

The oxygen analyzer is the only machine safety

device that can detect the problem.

N.A.Dräger system recommends a preoperative

+ve pressure leak test to detect such a leak.

Ohmeda recommends a preoperative –ve pressure

leak test because of the check valve located at the

common outlet.

3.3. Leaks Downstream: Leaks Downstream:

4.4. Inert Gas Administration: Inert Gas Administration:

Administration of a third inert gas, such as helium,

N2, or CO2, can cause a hypoxic mixture because

contemporary proportioning systems link only N2O &

O2.

The oxygen analyzer is the only machine safety

device that can detect the problem.

Understanding of the components & the operating

principles of the gas supply system, safety devices,

and flowmeters, is a part from the corner stone in

anaesthesia practice.

1. Andrews JJ: Inhaled Anesthetic Delivery Systems. In:

Miller RD, ed., Anesthesia, 6th ed., Philadelphia, Churchill

Livingston, pp. 273 – 317, 2005.

2. Andrews JJ: Delivery system for inhaled anesthetics. In:

Barash PG, Cullen BF, Stoelting RK, eds., Clinical

Anesthesia, 4th ed., Philadelphia, Pennsylvania, Lippincott

Williams and Wilkins, pp 567 – 594, 2001.

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