5. Supply of medical gases

Central gas supply Piped gas (oxygen, nitrous oxide, Entonox and medical air) is supplied through high-quality copper pipelines. The outlets have a non-interchangeable coupling in the form of a Schrader-type valve. The hoses from the gas outlet to the anaesthetic machine are colour-coded. Gas is supplied at a pressure of 4 bar, apart from the medical gas that is used to drive surgical instruments which is supplied at 7 bar. The gases may come from a manifold of large cylinders. They may be arranged in banks of cylinders, each of which should contain enough gas to supply a hospital for at least 2 days.

Oxygen is usually supplied from a liquid oxygen source. Liquid oxygen is stored below its critical temperature (-118 C) at around -160 C and at a pressure of 7 bar, which is the vapour pressure of oxygen at that temperature. The low temperature is maintained both by a vacuum insulated shell and by the fact that as the oxygen evaporates, its temperature will fall. The contents of the storage device can be determined either by weight or by pressure gauges, which measure the pressure difference between the top and bottom of the liquid oxygen.

Oxygen is usually supplied from a liquid oxygen source. Liquid oxygen is stored below its critical temperature (-118 C) at around -160 C and at a pressure of 7 bar, which is the vapour pressure of oxygen at that temperature. The low temperature is maintained both by a vacuum insulated shell and by the fact that as the oxygen evaporates, its temperature will fall. The contents of the storage device can be determined either by weight or by pressure gauges, which measure the pressure difference between the top and bottom of the liquid oxygen.

— The cylinders on an anaesthetic machine are usually size ‘E’, which contain 680 litres of oxygen, 1,800 litres of nitrous oxide and 680 litres of medical air. Designed to withstand very high pressures (they are tested to 250 bar), they are made of molybdenum, chromium steel, manganese and high-carbon manganese steel. (Cylinders for domiciliary oxygen can be made of lightweight aluminium alloy.) — Their features include colour coding (which is not international; oxygen cylinders in the UK are black with white shoulders, whereas in the USA they are green), a pin index system to ensure attachment only to the correct yoke and information about the contents of the cylinder. The coloured plastic collar indicates the date of the last cylinder test (the interval is between 5 and 10 years). The bodok seal is a neoprene washer enclosed within an aluminium surround which provides a gas-tight seal between the cylinder head and the yoke.

— Oxygen is stored as a gas at a pressure of 13,700 kPa (137 bar). — Entonox is a 50:50 gas mixture of oxygen and nitrous oxide at a pressure of 13,700 kPa. — Medical air is stored in cylinders of high tensile steel at a pressure of 13,700 kPa. Larger cylinders (which contain 4,600 litres) are designed for a higher maximum working pressure of 23,000 kPa. — Nitrous oxide is in a mixed liquid and vapour phase whose pressure is 4,400 kPa.

— These provide an alternative method of providing oxygen, although their low flow rates (4 l min/1) and pressures (70 kPa) mean that they are more commonly used to provide domiciliary supplies for individual patients. — They comprise zeolite-containing columns. Zeolites are hydrated aluminium silicates which are ion-exchangers and molecular sieves. The flow of air into the cylinders is directed so that nitrogen and water vapour are absorbed from one cylinder, and absorbed gas from the other is extracted by a vacuum pump. Every 30 seconds a solenoid valve switches the flow to ensure a constant flow of 95% oxygen to the reservoir. The remaining 5% is argon, which appears to have no adverse effects. (Although in higher concentrations it has the same effect as xenon in stimulating erythropoietin release.)

Medical air: this can be supplied from a central compressor or from cylinders. It must be dry, free from particulate matter, including the mineral oils used to lubricate the compressor, and free from bacteria. The air is, therefore, desiccated and filtered.

the critical temperature of a gas is the temperature at which it cannot be liquefied no matter how great the pressure that is applied (oxygen: 118.6 C, nitrous oxide: 36.4 C, xenon: 16.6 C).

The critical pressure of a substance is the pressure required to liquefy a gas at its critical temperature.

Filling ratio: this is the mass of gas used to fill a cylinder divided by the mass of water needed to fill the cylinder completely. It applies to gases that are stored in the liquid phase, and for nitrous oxide, it is 0.75. If the cylinder is to be used in hotter climates, this is reduced to 0.67.0 An overfilled cylinder that is exposed to high ambient temperatures will generate dangerously high pressures.

This is largely common sense. Cylinders should be kept in a secure and dry environment, free from extremes of temperature. Full and empty cylinders should ideally be kept in separate areas to avoid the risk of substitution. Large cylinders are usually stored upright; smaller ones may be laid horizontally.

Entonox: this is a 50:50 N2O/O2 mixture. Cylinders should be stored flat to prevent the risk of delivering 100% nitrous oxide when the cylinder is first used.

Nitrous oxide: you may be asked what happens when a nitrous oxide cylinder empties. In theory, the pressure, which is the vapour pressure, should remain constant until the liquid phase is exhausted, after which the pressure would fall to zero as the cylinder emptied. In practice, because the temperature of the liquid nitrous oxide falls as it vaporizes, the cylinder pressure also drops. The pressure returns to 4,400 kPa only if the gas flow ceases and the cylinder is allowed to return to room temperature

failure of the liquid oxygen source triggers supply from a reserve manifold of large oxygen cylinders, which are also remote from the site of delivery to the patient. There should also be reserve cylinders available in theatre. Should there be a complete failure of oxygen delivery, the anaesthetic machine should discontinue the flow of nitrous oxide and entrain air.