5. Anaesthetic Machine Flashcards
Core Information
The modern anaesthetic machine delivers accurate mixtures of anaesthetic gases and
inhalational agents at variable, controlled flow rates and at low pressure. It accomplishes
this via a number of features that are best described by tracing the gas flow
through the system from the cylinder or pipeline to the fresh gas outlet
Gas pipelines
Gas pipelines:
These are colour-coded for the UK,
but there is no international consistency.
A Schrader coupling system ensures that the pipeline connections are
non-interchangeable.
Reducing valves reduce the pressures to 400 kPa (4 bar).
The pipeline hose connection to the rear of the anaesthetic machine is permanent. The
threads are gas-specific
(NIST – non-interchangeable screw thread),
and a one-way valve ensures unidirectional flow.
Gas cylinders:
Gas cylinders: these are also colour-coded for the UK,
but here too there is no international standard.
They are made from molybdenum steel alloy.
They are robust and undergo rigorous regular hydraulic testing (as does the cylinder outlet valve).
A pin-index system, which is unique to each gas,
prevents connection to the wrong yoke,
and side guards on each yoke ensure that the cylinders are vertical.
Bodok (bonded disk) seals ensure a gas-tight fit.
A Bourdon pressure gauge indicates cylinder pressure.
Pressure regulators
Pressure regulators:
primary pressure regulators/reducing valves decrease the high cylinder pressures to 4 bar,
and a relief valve is located downstream in case of regulator failure.
Adjustable pressure-limiting (APL) valves are part of the breathing
system rather than the anaesthetic machine itself,
but are designed to minimize the risk of barotrauma by venting gas when a pre-set pressure is exceeded.
When fully closed, the APL will open at a pressure of 60 cmH2O.
Flow restrictors
Flow restrictors:
These are placed upstream of the flowmeter block and
protect the low-pressure part of the system from
damaging surges in gas pressure from the piped supply.
They may sometimes be used downstream of the vaporizer
back bar to minimize backpressure associated with IPPV.
Flow control valves
: these govern the transition from the high-pressure to the low pressure system,
and reduce the pressure from 4 bar to just above atmospheric as gas
enters the flowmeter block.
Oxygen failure and interlock devices
: systems vary, but a British Standard specifies that the failure alarm should be
powered by the O2 supply pressure in the machine pipeline and activated by a pressure reduction to below 2 bar. In most systems the gas
mixture is then vented, and an audible warning tone is activated. The same valve
opens an air-entrainment valve so that the patient cannot be exposed to a hypoxic
mixture resulting from failure of O2 delivery. An interlock system between the O2
and N2O control valves prevents the administration of a hypoxic mixture. The
machine cannot deliver a nitrous oxide concentration greater than 75%.
Emergency oxygen flush:
Emergency oxygen flush:
O2 is supplied direct from the high-pressure circuit upstream of the vaporizer block
and provides 35–75 l min1
(if the O2 flowmeter needle valve is opened fully, it delivers about 40 l min1).
Both methods may cause barotrauma in vulnerable patients
Flowmeters:
these are constant pressure variable orifice flowmeters
(Rotameter is a trade name) which are calibrated for a specific gas.
Accuracy is to within 2.5%.
The tubes have an antistatic coating to prevent sticking,
and there are vanes etched into the bobbin to ensure rotation.
In the UK the oxygen knob is always on the left, is larger, is hexagonal in profile and is more prominent than the others. (This is said to be because
Boyle, who designed one of the original anaesthetic machines, was left-handed). This
position does, however, put the patient at risk of breathing a hypoxic mixture if there is
damage to a downstream flowmeter tube. Thus, modern O2 flowmeters feed distal to
other gases should there be a proximal leak.
CO2 has disappeared from most
machines; where it is still delivered, it is usually governed to prevent a flow of greater
than 500 ml min1. Not all modern machines use flowmeters; some control gas flow
by microprocessors and produce an electronic display.
Vaporizers and back bar
Vaporizers and back bar: the most common vaporizers are temperature compensated
variable bypass devices which allow accurate and safe delivery of the
dialled concentrations.
A locking mechanism on the back bar prevents more than
one vaporizer being used at the same time. A spring-loaded non-return valve on the
back bar prevents retrograde flow caused by the pumping effect of IPPV.
A pressure
relief valve on the downstream end of the back bar protects against increases in the
pressure within the circuit.
Common gas outlet:
Common gas outlet: this receives gases from the back bar and from the emergency
O2 flush. It has a swivel outlet with a standard 15-mm female connection.
Clinical Applications
Protection from barotrauma: the key features from the preceding list include
Pressure-reducing valves, both pipeline and cylinders.
Flow restrictors.
Flow control valves.
Pressure relief valves downstream of the vaporizer back bar
Protection from hypoxia: the key features from the preceding list include
Protection from hypoxia: the key features from the preceding list include
Gas pipelines, colour coding and NIST connections.
Gas cylinders, colour coding and pin indexing.
Oxygen failure devices.
Interlock system.
Emergency oxygen flush.
