One day long ago, when I, if not the world, was young, I was sitting on the floor of the intensive care unit. It was midnight and one of our ventilators had gone on strike, though I thought I could fix it without calling in an engineer. (They will not let you do this sort of thing nowadays). As I sat there with bits of ventilator around me, I thought to myself "I've done this before", and so I had. The problem was that the water feed to the humidifier had stopped, and the reason was that the needle valve above the float in the water reservoir had stuck shut. Not too long previously, my car had been suffering fuel starvation, and yes, the needle valve above the float in the carburettor's petrol reservoir had stuck shut. I fell to musing on the associations between car engines and anaesthetic machines, not for the first time.

What's the difference between your car engine and my anaesthetic machine? Not much, you might say, they both emit some pretty noxious fumes. You are, however, supposed to recover from my noxious agents. Fumes apart, there are engineering problems common to both, and the solutions are not always the same.

Fuel mixture is one. A car runs on a mixture of air and petrol vapour, traditionally by way of a carburettor, a device through which air is drawn by suction from the engine. A small tube leads from a chamber of liquid fuel, enabling the fuel to be sucked into the airstream, where it is vaporised to form the mixture which is then ignited in the cylinder. The flow of air through the carburettor remains constant until the driver changes speed. A careful design and choice of needle jets ensures a nearly optimum proportion of fuel to air in the mixture.

On my anaesthetic machine there are also arrangements for turning a liquid into a vapour mixed with a carrier gas. The liquid is an anaesthetic drug (older ones included ether and chloroform; newer ones have names like isoflurane and desflurane) and is vaporised in a mixture of oxygen and nitrous oxide for delivery to the lungs, whence it is transferred to the bloodstream and so to the brain. The mixing device in this case is known as a vaporiser and works in a completely different way. Some of the liquid contained in a reservoir chamber is drawn up on a wick to allow it to evaporate more readily into the space in the chamber above the liquid. The incoming gas is split into two channels, some passing through this chamber to pick up the vapour, and some bypassing it. The split is determined by a valve which can be adjusted to provide the desired concentration. (An anaesthetic is not maintained with a more or less constant mixture, as in a car, so the vaporiser needs a calibrated knob for adjustment). With this simple arrangement there are still problems like the mixture varying with temperature and with pressure. Temperature-stabilising and temperature-compensating devices have to be built in, which means that even cheap and cheerful vaporisers start at around £500.

Some vaporisers, like carburettors, are driven by suction from the patient side ("draw-over vaporisers"). They can be simple and cheap, and one relatively lightweight model is used by all three British Armed Forces for work in the field. Unfortunately they do not deliver a consistent concentration over a wide range of gas flows (a human breathing in creates a gas flow that varies in a short space of time between zero and 30 litres a minute) and they are considerably less accurate than the alternative design in which the carrier gas is forced under pressure through the vaporising chamber/bypass arrangement. This is analogous to a turbo-charged engine, though it is done not to increase the delivery, but to make it more consistent. The engineering is even more expensive, and models of this design cost over £1000.

A supply of nitrous oxide is a standard item on my machine, used to provide pain relief on its own, and to act as a carrier gas for the volatile anaesthetics. You will also find a cylinder of nitrous on some cars, of the dragster variety. To obtain huge amounts of power, you need not only a rapid delivery of fuel, but also a rapid delivery of oxygen to burn it. One solution is turbocharging, using pressure to deliver a larger quantity of fuel-air mixture. Another very effective way is to use more oxygen (air being only 21% oxygen). Nitrous oxide is unstable when heated and liberates oxygen. This is a bad thing in anaesthesia, but just the job for dragsters. Why use nitrous instead of oxygen itself? Because nitrous oxide is liquid at normal temperatures, and a liquid has a much more dense packing of molecules than a gas. There is actually more oxygen stored in a cylinder of nitrous oxide than there is in a cylinder of compressed oxygen. Why not use liquid oxygen? Because it boils at a very low temperature and has to be kept in Thermos flask type insulation. It is OK for a rocket engineer, but jolly dangerous for the rest of us. There is another feature of using nitrous oxide. Any liquid turning to vapour causes the surroundings to cool. This can be a big nuisance on my anaesthetic machine because the pressure regulator tends to freeze unless properly designed, but it is just what you want for the dragster. The cooling effect means that there is again a better packing of molecules, and therefore a more rapid delivery of fuel mixture to the cylinders.

By now you will be thinking "My car has fuel injection. Why are you still interested in obsolete engineering like carburettors?". Injection of liquid anaesthetics into breathing circuits has been tried, and proved successful in experiments. The engineering has to be superb, as even small overdoses can be lethal, and there is only one model of anaesthetic machine commercially available which uses it; the machine is prohibitively expensive and not available in this country.

I will finish with the story of Dr. Goldman and his vaporiser. Dr. Goldman was an anaesthetist who worked in several dental surgeries and had to carry his kit round with him. He also had to pay for his own drugs, and was loath to leave expensive residues in the wicks of conventional vaporisers. He designed a very inefficient but extremely simple vaporiser which consisted of little more than a flow-splitting valve and a reservoir chamber, the latter being the glass bowl of a popular pattern of petrol filter, of which he obviously had a spare above his workbench. It must have pleased him on his rounds to know that if he dropped it he could call in for a replacement at the nearest garage, rather than head for a London instrument maker! Alas, history does not relate the brand of filter he adapted.

Copyright PHP Harris 1999

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