The anaesthetic machine

In 1917, Dr Henry Edmund Gaskin Boyle OBE developed his continuous-flow anaesthesia machine, the design of which is the forerunner of all modern anaesthetic machines. This prototype has undergone significant changes to increase the efficiency of anaesthesia and patient safety. Gases (oxygen, nitrous oxide and air) arrive at the machine via the hospital's piped medical gases and vacuum system via colour-coded tubing. Cylinders attached to the back of the machine provide a back-up supply. Cylinder gases pass through pressure-regulating valves into the ‘back bar’ of the machine, but pipeline gases are supplied at ‘back bar pressure’ of 4 bar. From the back bar, gas flow rate is set using a needle valve that regulates flow into the flowmeter of which Rotameter™ is one trademarked example. Rotameters are fixed pressure, variable orifice flowmeters which are accurate to within ±2.5%.Many modern anaesthetic machines have electronic gas mixers rather than conventional rotameters. The gases then pass through a vaporizer where volatile anaesthetic is added to the fresh gas flow. This mixture is delivered, via the common gas outlet, to a patient breathing circuit, nowadays usually a ‘circle system’. This contains a carbon dioxide absorber to stop patients re-breathing and allows higher efficiency than other systems. Waste gases are scavenged from the circle. Monitoring, ventilators and suction apparatus are mounted on the machine. The anaesthetic machine should be thoroughly tested prior to use.