How does anaesthetic machine work?

The anaesthesia machine mixes the correct concentrations of gas and drugs to be inhaled and exhaled

by the patient so they lose consciousness. They are held unconscious and have no sensation while being

operated on, then regain consciousness after the operation.

Physiology Anaesthetic gas affects the nervous system, resulting in a numbing of the nerve pathways. The patient

becomes unconscious, unaware of what is happening, has no pain, is immobile (may need breathing

support) and free from memory while under the influence of the anaesthetic agent.

How it works Air, Oxygen, and Nitrous Oxide are supplied from wall outlets or cylinders. The pressures are reduced by

pressure regulators then pass through an O2 failure alarm. The flow of gas is controlled by flowmeters.

The gas is mixed and passed over a vaporiser containing the anaesthetic agent. An Anti-hypoxic device

ensures that the flowmeters deliver a minimum of 25% O2 to the agent vaporiser. The gas then goes

through an over pressure relief valve and back flow valve, then to the common gas outlet (CGO), ready to

be given to the patient.

There is an emergency O2 flush valve connected to give 100% O2 if required. The gas then enters a

patient circuit and ventilator. Expired gas can be stripped of CO2 in a soda-lime filter and recirculated, or

scavenged out of the room.

Units of

measurement

Flow: L/min, Pressure: kPa and cmH2O

Typical values Dependant on patient size, rate and strength of breathing, circulation and solubility of the anaesthetic.