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.

The machine is commonly used together with a mechanical ventilator, breathing system, suction equipment, and patient monitoring devices; strictly speaking, the term "anaesthetic machine" refers only to the component which generates the gas flow, but modern machines usually integrate all these devices into one combined freestanding unit, which is colloquially referred to as the "anaesthetic machine" for the sake of simplicity. 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.

Simpler anaesthetic apparatus may be used in special circumstances, such as the triservice anaesthetic apparatus, a simplified anaesthesia delivery system invented for the British Defence Medical Services, which is light and portable and may be used for ventilation even when no medical gases are available. 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.

History

The original concept of continuous-flow machines was popularised by Boyle's anaesthetic machine, invented by the British anaesthetist Henry Boyle at St Bartholomew's Hospital in London, United Kingdom, in 1917, although similar machines had been in use in France and the United States. Prior to this time, anaesthesiologists often carried all their equipment with them, but the development of heavy, bulky cylinder storage and increasingly elaborate airway equipment meant that this was no longer practical for most circumstances. Contemporary anaesthetic machines are sometimes still referred to metonymously as "Boyle's machine", and are usually mounted on anti-static wheels for convenient transportation.

thumb|left|Handheld anaesthetic device for [[trichloroethylene, made in the UK, 1947. This device was designed for self-administration by the patient.]]

Many of the early innovations in anaesthetic equipment in the United States, including the closed circuit carbon-dioxide absorber (a.k.a. the Guedel-Foregger Midget) and diffusion of such equipment to anaesthesiologists within the United States can be attributed to Richard von Foregger and The Foregger Company.

Flow rate

In anaesthesia, fresh gas flow is the mixture of medical gases and volatile anaesthetic agents which is produced by an anaesthetic machine and has not been recirculated. The flow rate and composition of the fresh gas flow is determined by the anaesthetist. Typically the fresh gas flow emerges from the common gas outlet, a specific outlet on the anaesthetic machine to which the breathing attachment is connected.

  • High flow anesthesia supplies fresh gas flow which approximates the patient’s minute ventilation, which is usually about 3 to 6 litres per minute in a normal adult.
  • Low flow anesthesia supplies fresh gas flow of less than half the patient's minute ventilation of the patient, which is usually less than 3.0 litres per minute in a normal adult.
  • Minimal flow anesthesia supplies fresh gas flow of about 0.5 litres per minute.
  • Closed system anesthesia supplies fresh gas flow as needed to make up the recirculated gas volume to compensate for the patient’s need for oxygen and anesthetic agents.

Anaesthetic vapouriser

thumb|Anesthetic machine, showing [[sevoflurane (yellow) and isoflurane (purple) vaporizers on the right]]

An anesthetic vaporizer (American English) or anaesthetic vapouriser (British English) is a device generally attached to an anesthetic machine which delivers a given concentration of a volatile anesthetic agent. It works by controlling the vaporization of anesthetic agents from liquid, and then accurately controlling the concentration in which these are added to the fresh gas flow. The design of these devices takes account of varying: ambient temperature, fresh gas flow, and agent vapor pressure. There are generally two types of vaporizers: plenum and drawover. Both have distinct advantages and disadvantages. The dual-circuit gas-vapor blender is a third type of vaporizer used exclusively for the agent desflurane.

Plenum vaporizers

The plenum vaporizer is driven by positive pressure from the anesthetic machine, and is usually mounted on the machine. The performance of the vaporizer does not change regardless of whether the patient is breathing spontaneously or is mechanically ventilated. The internal resistance of the vaporizer is usually high, but because the supply pressure is constant the vaporizer can be accurately calibrated to deliver a precise concentration of volatile anesthetic vapor over a wide range of fresh gas flows. It was designed by Ivan Houghton for military use in 1981. Original design included trichloroethylene for its analgesic properties and halothane for main general anaesthesia; it was later used with isoflurane.

Dual-circuit gas–vapor blender

The third category of vaporizer (the dual-circuit gas–vapor blender) was created specifically for the agent desflurane. It is mounted on the anesthetic machine in the same way as a plenum vaporizer, but its function is quite different. It evaporates a chamber containing desflurane using heat, and injects small amounts of pure desflurane vapor into the fresh gas flow. A transducer senses the fresh gas flow.

  • Flowmeters such as rotameters for oxygen, air, and nitrous oxide
  • Vaporisers to provide accurate dosage control when using volatile anaesthetics
  • A high-flow oxygen flush, which bypasses the flowmeters and vaporisers to provide pure oxygen at 30-75 litres/minute
  • Systems for monitoring the gases being administered to, and exhaled by, the patient, including an oxygen failure warning device

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. Machines should be cleaned between cases as they are at considerable risk of contamination with pathogens.

See also

References

Further reading

  • Virtual Anesthesia Machine (VAM) — a free transparent reality simulation of a generic anaesthetic machine from the University of Florida
  • Virtual Anaesthesia Textbook
  • — resources for UK anaesthetists in training
  • History of Richard von Foregger and the Foregger Company — written by his son, R. Foregger, this website chronicles one of the leading manufacturers and developers of anesthesiology equipment in the early 20th century.
  • Anesthesia Equipment & Instruments: Definitions & Principles of Practical Usage
  • Free resource which graphically explains the principles of anesthesia vaporizers on howequipmentworks.com
  • Anesthetic machine: a presentation
  • The Anesthesia Gas Machine by Michael P. Dosch
  • Precision vaporisers on Anaesthesia.UK
  • Operating Instructions & Maintenance Guidelines for Precision Vaporizers