|This article needs additional citations for verification. (November 2010)|
|Process type||physical change|
|Industrial sector(s)||anesthesia (medicine)|
|Main technologies or sub-processes||vaporization|
|Feedstock||inhalational anaesthetic agents, chiefly nitrous oxide and volatile anesthetics|
|Product(s)||phase transition of feedstock from the liquid phase to the gas phase|
|Leading companies||Leading companies using this industrial process|
|Main facilities||hospitals and outpatient surgery centers|
|Inventor||William T. G. Morton is widely credited for demonstrating the technique of vaporization of diethyl ether. However, many others were involved in its development. Please refer to History of general anesthesia article for more details.|
|Year of invention||16 October 1846|
The anaesthetic machine (UK English) or anesthesia machine (US English) or Boyle's machine is used by anaesthesiologists, nurse anaesthetists, and anaesthesiologist assistants to support the administration of anaesthesia. The most common type of anaesthetic machine in use in the developed world is the continuous-flow anaesthetic machine, which is designed to provide an accurate and continuous supply of medical gases (such as oxygen and nitrous oxide), mixed with an accurate concentration of anaesthetic vapour (such as isoflurane), and deliver this to the patient at a safe pressure and flow. Modern machines incorporate a ventilator, suction unit, and patient monitoring devices.
The original concept of Boyle's machine was invented by the British anaesthetist Henry Boyle (1875–1941) in 1917. Prior to this time, anaesthetists 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. The anaesthetic machine is usually mounted on anti-static wheels for convenient transportation.
Simpler anaesthetic apparatus may be used in special circumstances, such as the TriService Apparatus, a simplified anaesthesia delivery system invented for the British armed forces, which is light and portable and may be used effectively 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. A large number of draw-over type of anaesthesia devices are still in use in India for administering an air-ether mixture to the patient, which can be enriched with oxygen. But the advent of the cautery has sounded the death knell to this device, due to the explosion hazard.
Many of the early innovations in U.S. anaesthetic equipment, including the closed circuit carbon-dioxide absorber (aka: the Guedel-Foregger Midget) and diffusion of such equipment to anaesthetists within the United States can be attributed to Richard von Foregger and The Foregger Company.
In dentistry a simplified version of the anaesthetic machine, without a ventilator or anaesthetic vaporiser, is referred to as a relative analgesia machine. By using this machine, the dentist can administer a mild inhalation sedation with nitrous oxide and oxygen, in order to keep his patient in a conscious state while depressing the feeling of pain.
Components of a typical machine
A modern anaesthesia machine includes the following components:
- Connections to piped hospital oxygen, medical air, and nitrous oxide.
- Reserve gas cylinders of oxygen, air, and nitrous oxide attached via a specific yoke with a Bodok seal.
- A high-flow oxygen flush which provides pure oxygen at 30-75 litres/minute
- Pressure gauges, regulators and 'pop-off' valves, to protect the machine components and patient from high-pressure gases
- Flow meters (rotameters) for oxygen, air, and nitrous oxide, low Flow meters oxygen nitrous oxide
- Updated vaporizers to provide accurate dosage control when using volatile anaesthetics such as isoflurane and sevoflurane
- An integrated ventilator to properly ventilate the patient during administration of anaesthesia
- A manual ventilation bag in combination with an Adjustable Pressure Limiting (APL) valve
- Systems for monitoring the gases being administered to, and exhaled by the patient
- Systems for monitoring the patient's heart rate, ECG, blood pressure and oxygen saturation, in some cases with additional options for monitoring end-tidal carbon dioxide and temperature
- breathing circuits, circle attachment, or a Bain's breathing system
Safety features of modern machines
Based on experience gained from analysis of mishaps, the modern anaesthetic machine incorporates several safety devices, including:
- an oxygen failure alarm (aka 'Oxygen Failure Warning Device' or OFWD). In older machines this was a pneumatic device called a Ritchie whistle which sounds when oxygen pressure is 38 psi descending. Newer machines have an electronic sensor.
- Nitrous cut-off or oxygen failure protection device, OFPD: the flow of medical nitrous-oxide is dependent on oxygen pressure. This is done at the regulator level. In essence, the nitrous-oxide regulator is a 'slave' of the oxygen regulator. i.e., if oxygen pressure is lost then the other gases can not flow past their regulators.
- hypoxic-mixture alarms (hypoxy guards or ratio controllers) to prevent gas mixtures which contain less than 21-25% oxygen being delivered to the patient. In modern machines it is impossible to deliver 100% nitrous oxide (or any hypoxic mixture) to the patient to breathe. Oxygen is automatically added to the fresh gas flow even if the anaesthetist should attempt to deliver 100% nitrous oxide. Ratio controllers usually operate on the pneumatic principle or are chain linked (link 25 system). Both are located on the rotameter assembly, unless electronically controlled.
- ventilator alarms, which warn of low or high airway pressures.
- interlocks between the vaporizers preventing inadvertent administration of more than one volatile agent concurrently
- alarms on all the above physiological monitors
- the Pin Index Safety System prevents cylinders being accidentally connected to the wrong yoke
- the NIST (Non-Interchangeable Screw Thread) or Diameter Index Safety System, DISS system for pipeline gases, which prevents piped gases from the wall being accidentally connected to the wrong inlet on the machine
- pipeline gas hoses have non-interchangeable Schrader valve connectors, which prevents hoses being accidentally plugged into the wrong wall socket
The functions of the machine should be checked at the beginning of every operating list in a "cockpit-drill". Machines and associated equipment must be maintained and serviced regularly.
Older machines may lack some of the safety features and refinements present on newer machines. However, they were designed to be operated without mains electricity, using compressed gas power for the ventilator and suction apparatus. Modern machines often have battery backup, but may fail when this becomes depleted.
The modern anaesthetic machine still retains all the key working principles of the Boyle's machine (a British Oxygen Company trade name) in honour of the British anaesthetist Henry Boyle. In India, however, the trade name 'Boyle' is registered with Boyle HealthCare Pvt. Ltd., Indore MP.
A two-person pre-use check (consisting of an anaesthetist and an operating department practitioner) of the anaesthetic machine is recommended before every single case and has been shown to decrease the risk of 24-hour severe postoperative morbidity and mortality. Various regulatory and professional bodies have formulated checklists for different countries. A free transparent reality simulation of the checklist recommended by the United States Food & Drug Administration is available from the Virtual Anesthesia Machine web site ( see below) after registration which is also free. Machines should be cleaned between cases as they are at considerable risk of contamination with pathogens.
Anesthesia machine vs Anesthesia cart
The Anesthesia machine contains mechanical respiratory support (ventilator) and O2 support as well as being a means for administering anesthetic gases which may be used for sedation as well as total anesthesia. An anesthesia cart holds extra IV push meds for anesthesia, sedation and reversal, extra equipment that the person giving anesthesia/sedation might need, and the hardware for respiratory support and resuscitation.
- Virtual Anesthesia Machine (VAM) — a free transparent reality simulation of a generic anesthesia machine from the University of Florida
- Various anesthesia-related simulations
- anesthesia machine equipments
- Virtual Anaesthesia Textbook
- FRCA UK — resources for UK anaesthetist 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.