23. Safety Features of the Anaesthetic Machine

Question 1

What are the principal functions of the anaesthetic

machine?

Answer 1

> To receive compressed gases
from their supplies (pipeline or cylinder)

> To accurately and
continuously deliver a gas and
volatile mixture of the
desired composition

> To avoid delivering hypoxic gas mixtures

> To deliver a gas mixture to the patient at a safe pressure (to avoid
barotrauma)

Question 2

What are the safety features of an anaesthetic machine?

Answer 2

One of the most important safety features
is the presence of a trained,
competent anaesthetist and a
serviced and checked anaesthetic machine.

A pre-use check to ensure the 
correct functioning of the 
anaesthetic machine
and equipment is essential 
to patient safety. 

The importance of this check is
recognised internationally and

is included in the World Health Organization
Safer Surgical Checklist.

Prior to commencing any anaesthetic,
it is essential
to have a self-inflating bag,
an alternative source of oxygen (e.g. a cylinder),
relevant airway equipment
and emergency drugs immediately available.

The anaesthetic machine itself
has numerous safety features built in
and in
order not to miss anything,

it is best to breakdown the anaesthetic machine

into systems and work your way
from the back of the anaesthetic machine
to the common gas outlet.

Question 3

Power supply and battery back-up

Answer 3

Power supply and battery back-up

> Modern anaesthetic machines have visual
and audible indicators to alert
the anaesthetist of a power failure.

> A back-up, re-chargeable battery
is present, and this must be checked
as part of the Association of Anaesthetists of Great Britain and Ireland
(AAGBI) anaesthetic machine check 
to ensure it is charged.

Question 4

Gas supplies

Answer 4

1
> Pipeline – 
colour-coded, 
flexible hosepipes 
black = air, 
white = oxygen
and blue = nitrous oxide) 

connect to the wall via a Schrader valve
(gas specific and non-interchangeable)

and connect to the back of the
anaesthetic machine via
non-interchangeable screw threads

(NIST – gas specific and permanently fixed)

2
> Cylinders – colour-coded gas cylinders

(oxygen = black body with white shoulder,
nitrous oxide = blue body with blue shoulder
and air = black body with white and black shoulders)

act as an emergency source of
gases should primary
piped gas delivery fail.

They connect to the back
of the anaesthetic machine via a
pin-indexed system

(oxygen = 2.5,
nitrous oxide = 3.5
air = 1.5)

incorporating a Bodok seal to
make the connection gas-tight.

They are now made of molybdenum steel,
which is lighter and stronger
than its carbon steel predecessor.

> Pressure regulators reduce the
pressure of cylinder gases to
approximately 400 kPa

(i.e. the same as piped gas pressure),

thereby protecting the anaesthetic machine.
from damage due to high gas
pressures.

> Pipeline and gas cylinder pressure
indicators
(traditional machines used a
Bourdon gauge)

Question 5

Gas flow measurement and control

Answer 5

> Flow control needle valves govern
the transition from high- to lowpressure
systems,

reducing the pressure from 4 bar
to just above
atmospheric pressure as
gas enters the flowmeter block.

> Flowmeters can be mechanical or electronic.

• Mechanical –
rotameters are the conventional mechanical flowmeter.

They are constant pressure,
variable orifice flowmeters,
which allow fresh gas flow rates
to be regulated and measured

(calibrated to individual gases as the
density and viscosity of the gases are
important).

Oxygen is the last gas to be added 
to the fresh gas
flow, which prevents 
delivery of a hypoxic gas mixture
 should a
proximal crack in the
 flowmeter occur.

Rotameters are produced with
anti-static material to prevent
the bobbin ‘sticking’,
which could result in inaccurate fresh gas flow measurement.

The control knobs are labelled and colour-coded.

The oxygen control knob is larger,
protrudes further and is
grooved to allow differentiation from the air
and nitrous oxide control knobs.

• Electronic – 
modern anaesthetic machines use
microprocessors to control gas flow 
and the flow is indicated either electronically
 by a numerical display or 
using virtual flow tubes. 

There is a pneumatic back-up in the
event of a power failure,
which continues the delivery
of fresh gas.

Some systems allow low flow rates
(<500 mL/min) to be utilised,
reducing cost and pollution.

> Anti-hypoxic mixture devices prevent
 the inadvertent delivery of a
hypoxic-inspired gas mixture,
and they can be mechanical,
 pneumatic or
electronic.

• Mechanical devices use a
chain to link the nitrous oxide flow to a
minimum oxygen flow.

• Pneumatic devices use a ratio mixer valve.
• Electronic devices use a paramagnetic oxygen analyser to continuously sample the gas mixtures from the flowmeters.

Question 6

Vaporisers

Answer 6

> They sit on the back bar of the 
anaesthetic machine and 
convert volatile liquid into vapour 
and add a controlled amount of
 volatile to the fresh gas flow. 

The common manifold systems 
that prevent the use of more
than one vaporiser at any time 
are the Selectatec type (Ohmeda) and the
Interlock type (Drager).

> They are colour-coded
(blue = desflurane,
yellow = sevoflurane and
purple = isoflurane)

and have a liquid-level indicator.

The modern devices have a
non-spill reservoir allowing up to 180 ° of tilt.

> Filling devices are geometrically coded
and agent specific,
designed to
prevent incorrect vaporiser filling

.
> Back-bar pressure relief valves
 are situated downstream of the 
vaporisers and vent off gas mixtures
 at pressures greater than 35 kPa.

This prevents barotrauma to the
flow meters and vaporisers
but not the patient.

Question 7

Oxygen failure warning device

Answer 7

> British standards specify that the
alarm should be powered solely by the
oxygen supply pressure.

It is activated when the 
oxygen supply pressure
falls below 2 bar and 
when this has occurred 
flow of all other gases
ceases and atmospheric air is entrained.

The alarm produces a sound of
at least 60 dB for a minimum 7 seconds.

Question 8

Oxygen flush

Answer 8

> 100% oxygen is supplied from the
high-pressure circuit upstream,

bypassing flowmeters and vaporisers,
and is delivered at rates between

35 and 75 L /min and
a pressure of about 400 kPa.

> There is a risk of barotrauma
and of anaesthetic agent dilution with its use.

Question 9

Adjustable pressure-limiting valve

Answer 9

> Allows excess gas to escape
when a preset pressure is exceeded,
thereby reducing risk of barotrauma to the patient.

Question 10

Monitoring

Answer 10

> Monitors have now become integrated
within modern anaesthetic
machines.

 Oxygen, inhalation agent and
 end-tidal carbon dioxide
concentration analysers,
gas volume and
 airway pressure measurements
are all essential 
and are monitored and 
displayed on LED screens.

> Prioritised preset alarms with
audible and visual components exist.

Alarm limits can also be individualised
and the monitor settings can be changed

(e.g. local anaesthesia,
cardiopulmonary bypass or
to detect pacing spikes).

Question 11

Common gas outlet

Answer 11

> Standardised 22 mm male outer diameter
15 mm female internal diameter

connection accommodating
only breathing system attachments
(circle or T-piece).