Dr Q Milner,
Registrar in Cardiac Anaesthesia, Papworth Hospital, Cambridge, UK.

Introduction		Pressure relief valves
Medical gas supply		Emergency oxygen flow
Flow meters		Checking the machine
Vaporisers

Continuous flow anaesthetic machines date back to the first availability of compressed gases, and despite numerous modifications the modern apparatus retains many of the features of the original Boyle's machine, a British Oxygen Company trade name in honour of the British anaesthetist H E G Boyle (1875-1941). This article describes the basic principles of continuous flow apparatus and will be followed in the next edition of Update by a description of anaesthetic circuits.

Basic Design: Pressurised gases are supplied by cylinders or pipelines to the anaesthetic machine which controls the flow of gases before passing them through a vaporiser and delivering the resulting mixture to the patient via the breathing circuit.

 
Medical gas supply.

Cylinders: Anaesthetic gas cylinders are made of molybdenum steel and should be regularly checked by the manufacturer for faults.

Different gases are supplied at specific pressures. The standard E size oxygen cylinders attached to anaesthetic machines are supplied at 134 bar pressure and contain 680 litres of oxygen.

During compression nitrous oxide becomes a liquid, which then evaporates to form a gas as it is released. This process causes cooling of the cylinder. The pressure in an E size nitrous oxide cylinder is 44 bar which releases 1800 litres of nitrous oxide during use. Unfortunately there is no international colour coding system for the contents of anaesthetic gas cylinders (or pipelines). In the USA oxygen is supplied in green cylinders, in the UK the cylinders are black with white shoulders and in Germany they are blue. Under circumstances where the supply of oxygen is unreliable the only true method of identifying the contents of an oxygen cylinder is to use an oxygen analyser.

In order to ensure that the correct cylinder is attached to the yoke of the anaesthetic machine a series of pins on the machine yoke is made to fit an identical pattern of indentations on the cylinder.

A small metal and neoprene seal (Bodok Seal) ensures a gas-tight fit between the cylinder and anaesthetic machine yoke. Under no circumstances may oil or grease be used as a seal; the pressurised gases give off heat as they are released from the cylinder and may cause explosions if oil is used. Before attaching a full cylinder to the machine briefly open and close the cylinder valve to clear any dirt from the port.

Anaesthetic machines operate at 4 bar pressure and therefore the compressed medical gases in cylinders pass through reducing valves to bring the gas pressure to a constant 4 bar. Non-return valves prevent empty cylinders still attached to the machine from refilling from fresh cylinders. Until recently cylinders of cyclopropane and carbon dioxide were also commonly found on anaesthetic machines. Cyclopropane is now rarely used and carbon dioxide should only be attached if the anaesthetist wants to use it for a specific reason.

Pipeline Supply: Larger hospitals supply medical gases from a central store via pipelines in the floor and walls. The central store may be a bank of large cylinders, or in the case of oxygen, a large insulated tank of liquid oxygen maintained at approximately minus 165 degrees centigrade. The piped medical gases are delivered to specific ports located in the wall of the operating theatre and anaesthetic room. Non-interchangeable spring loaded valves (Schraeder Valves) are inserted into the wall ports and connect to the anaesthetic machine via flexible but non-crushable tubing. The valves are specific for each gas and the tubing is colour coded and permanently bonded to the individual valve.

The pressure in anaesthetic gas pipelines is 4 bar (the same as the working pressure of the anaesthetic machine). All anaesthetic machines, including those on pipeline gases, should have reserve gas cylinders attached, but turned off.

 
Flow meters:

The gases from both cylinders and pipelines flow through narrow steel tubing to the rotameters where the flow rate of the gases is controlled by a needle valve. The flow rate of the individual gas is shown by a float or bobbin in a vertical glass tube. These tubes are individually calibrated by the manufacturer and are not interchangeable. The control knob for oxygen is larger and different in shape to the nitrous oxide control. In the UK, the left rotameter is oxygen whilst in the USA oxygen is on the right!

After the gases have passed through the rotameters the different gas tubes are joined together with oxygen added last so that the chances of an hypoxic mixture resulting from a leak of gases is minimised. Many machines now also link the flow of nitrous oxide to that of oxygen to ensure that a minimum of 25% oxygen will always be delivered.

(Continued...)

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