Gases used in anaesthesia are usually supplied under high pressure either in cylinders or as a piped gas supply. The cylinders are made from molybdenum steel, in which gases and vapours are stored under pressure. The cylinders undergo flattening, bend and impact tests (1 in 100), pressure tests and tensile tests (1 in 100). The shape and colour of the plastic disc around the neck indicates when the cylinder was last examined. The most commonly used sizes are:
H - free-standing, attached to the anaesthesia machine by a flexible hose. These are most economical, but reduce the mobility of the anaesthesia machine. The hose to the anaesthetic machine should be at a regulated pressure of 50 psi (i.e the regulator should be attached to the cylinder and not to the anaesthetic machine).
E - attached directly to the anaesthesia machine via a yoke.
Gases found on anaesthesia machines
Oxygen - stored as a gas at 13,700 kPa
Nitrous oxide - stored as a liquid with vapour on the top at a pressure of 4400 kPa. The filling ratio of this cylinder is the weight of the fluid in the cylinder divided by the weight of the water required to fill it. In the UK, the filling ratio is 0.75. In hotter climates, the ratio is 0.67
Helium - used in laser surgery around the airway to reduce the flammability of the breathing mixture
Air - commonly found on modern medical machines to dilute oxygen when other gases are not desired.
Other gas sources
A central supply of liquid oxygen may also be provided in large hospitals. Other sources of oxygen include oxygen generators (which chemically generate oxygen from, for example, hydrogen peroxide) and oxygen extractors (which extract oxygen from atmospheric air). Although these are not widely used in developed countries, they may be useful in areas where supplies of compressed gas are hard to obtain.
Identification of gases
Correct identification of the gas being supplied to the anaesthesia machine is clearly vital if potentially lethal accidents are to be avoided. A number of different measures are taken to ensure this:
Colour-coding of cylinders provides a rapid means of identifying their contents. However, the definitive indicator of the contents is always the label.
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||Black and white|
A gas-specific pin-index system is provided on small cylinders: pins on the yoke of the anaesthetic machine mate with holes drilled in specific positions on the valve of the cylinder to provide a mechanical means of preventing incorrect connection.
Index positions: Oxygen 2 and 5, nitrous oxide 3 and 5, air 1 & 5.
Gas-specific connectors are used on large (G and H) cylinders that make it impossible to attach a regulator or fitting to the wrong cylinder:
A pressure gauge indicates the pressure within each cylinder. In the case of oxygen (below), and other gases, which do not liquefy at normal temperatures, this enables estimation of the contents, since the amount of gas contained in the cylinder is proportional to the pressure.
This is not the case for vapours, such as nitrous oxide, which are liquefied by the high pressure within the cylinder (above). Here, the pressure remains relatively constant until all of the liquid is evaporated, after which the pressure drops rapidly as the remaining gas is removed. The contents of these cylinders can only be estimated by weighing the cylinder.
The oxygen concentrator
Click here for information on the oxygen concentrator.