Components of a typical machine
Simple schematic of an anaesthesia machine
A modern machine typically includes the following components:
connections to piped hospital oxygen, medical air, and nitrous oxide. Pipeline pressure from the hospital medical gas system (wall outlet) should be around 400 kPa (60 psi; 4 atmospheres).
reserve gas cylinders of oxygen, air, and nitrous oxide attached via a specific yoke with a Bodok seal. Older machines may have cylinder yokes and flow meters for carbon dioxide and cyclopropane. Many newer machines only have oxygen reserve cylinders. The regulators for the cylinders are set at 300 kPa (45 psi; 3 atmospheres). If the cylinders are left on and the machine is plugged into the wall outlet, gas from the wall supply will be used preferentially, since it is at a higher pressure. In situations where pipeline gases are not available, machines may safely be used from cylinders alone, provided fresh cylinders are available.
a high-flow oxygen flush which provides pure oxygen at 30 litres/minute
pressure gauges, regulators and ‘pop-off’ valves, to protect the machine components and patient from high-pressure gases (referred to as ‘barotrauma’).
flow meters (rotameters) for oxygen, air, and nitrous oxide, which are used by the anaesthesiologist to provide accurate mixtures of medical gases to the patient. Flow meters are typically pneumatic, but increasingly electromagnetic digital flow meters are being used.
one or more anaesthetic vaporisers to accurately add volatile anaesthetics to the fresh gas flow
a ventilator
physiological monitors to monitor the patient’s heart rate, ECG, non-invasive blood pressure and oxygen saturation (additional monitors are generally available to monitor end-tidal CO2, temperature, arterial blood pressure central venous pressure, etc.). In addition, the composition of the gases delivered to the patient (and breathed out) is monitored continuously.
breathing circuits, most commonly a circle attachment, or a Bain’s breathing system, which are breathing hoses connected to a anaesthesia face mask
a heat and moisture exchanger (HME) with or without bacteria-viral filter (HMEF).
scavenging system to remove expired anaesthetic gases from the operating room. Scavenged gases are usually vented to the atmosphere.
suction apparatus
There is generally a small work bench built into the machine where airway management equipment is kept within ready reach of the anaesthetist.
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 (also known as ‘Oxygen Failure Warning Device’ or OFWD). In older machines this was a pneumatic device called a Ritchie whistle. Newer machines have an electronic sensor.
nitrous cut-off: 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.
hypoxic-mixture alarms (hypoxy guards or ratio controllers) to prevent gas mixtures which contain less than 21% 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 penumatic principle or are chain linked. Both are located on the rotameter assembly, unless electronically controlled.
ventilator alarms, which warn of disconnection or high airway pressures
interlocks between the vaporisers 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) 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.
