An anaesthetic machine (British English) or anesthesia machine (American English) is a medical device used to generate and mix a fresh gas flow of medical gases and inhalational anaesthetic agents for the purpose of inducing and maintaining anaesthesia.

In the developed world, the most frequent type in use is the continuous-flow anaesthetic machine or "Boyle's machine", which is designed to provide an accurate supply of medical gases mixed with an accurate concentration of anaesthetic vapour, and to deliver this continuously to the patient at a safe pressure and flow.

This is distinct from intermittent-flow anaesthetic machines, which provide gas flow only on demand when triggered by the patient's own inspiration.

This device has unidirectional valves which suck in ambient air, which can be enriched with oxygen from a cylinder, with the help of a set of bellows.

Many of the early innovations in anaesthetic equipment in the United States, including the closed circuit carbon-dioxide absorber (a.k.a.

[3] Open circuit forms of equipment, such as the Magill attachment, require high fresh gas flows (e.g. 7 litres/min) to prevent the patient from rebreathing their own expired carbon dioxide.

Recirculating (rebreather) systems, use soda lime to absorb carbon dioxide, in the scrubber, so that expired gas becomes suitable to re-use.

With a very efficient recirculation system, the fresh gas flow may be reduced to the patient's minimum oxygen requirements (e.g. 250ml/min), plus a little volatile as needed to maintain the concentration of anaesthetic agent.

Increasing fresh gas flow to a recirculating breathing system can reduce carbon dioxide absorbent consumption.

There is a cost/benefit trade-off between gas flow and use of adsorbent material when no inhalational anaesthetic agent is used which may have economic and environmental consequences.

The design of these devices takes account of varying: ambient temperature, fresh gas flow, and agent vapor pressure.

[5] The plenum vaporizer is an elegant device which works reliably, without external power, for many hundreds of hours of continuous use, and requires very little maintenance.

(Technically, although the dial of the vaporizer is calibrated in volume percent (e.g. 2%), what it actually delivers is a partial pressure of anesthetic agent (e.g. 2kPa)).

The drawover vaporizer is driven by negative pressure developed by the patient, and must therefore have a low resistance to gas flow.

The design of the drawover vaporizer is much simpler: in general it is a simple glass reservoir mounted in the breathing attachment.

The drawover vaporizer may be mounted either way round, and may be used in circuits where re-breathing takes place, or inside the circle breathing attachment.

Pressure and demand from dental surgeons for a more reliable method of administering ether helped modernize its delivery.

By the 1980s, the anesthetic vaporizer had evolved considerably; subsequent modifications lead to a raft of additional safety features such as temperature compensation, a bimetallic strip, temperature-adjusted splitting ratio and anti-spill measures.

A modern anaesthetic machine includes at minimum the following components:[2] Systems for monitoring the patient's heart rate, ECG, blood pressure and oxygen saturation may be incorporated, in some cases with additional options for monitoring end-tidal carbon dioxide and temperature.

However, they were designed to be operated without mains electricity, using compressed gas power for the ventilator and suction apparatus.