In Europe and other advanced medical communities, medical gases are generally supplied by pipeline, with cylinders available as back up. Large hospitals usually have oxygen supplied and stored in liquid form, since one volume of it provides 840 volumes of gaseous oxygen at 15◦C. It is stored in a secure Vacuum Insulated Evaporator (VIE) on the hospital site. The arrangement is shown in Figure 22.1. The VIE consists of an insulated container, the inner layer of which is made of stainless steel, the outer of which is made of carbon steel. The liquid oxygen is stored in the inner container at about−160◦C (lower than the critical temperature of−118◦C) at a pressure of between 700 and 1200 kPa. There is a vapour withdrawal line at the top of the VIE, from which oxygen vapour can go via a restrictor to a superheater, where the gas is heated towards ambient temperature. Where demand exceeds supply from this route, there is also a liquid withdrawal line from the bottom of the VIE, from which liquid oxygen can be withdrawn; the liquid can be made to join the vapour line downstream of the restrictor and pass either through the superheater or back to the top of the VIE. The liquid can also be made to pass through an evaporator before joining the vapour line. After passing through the superheater, the oxygen vapour is passed through a series of pressure regulators to drop the pressure down to the distribution pipeline pressure of 410 kPa. It should be remembered that no insulation is perfect and there is a pressure relief valve on top of the VIE in case lack of demand and gradual temperature rise results in a pressure build up in the container. There is a filling port and there is usually considerable wastage in filling the VIE; the delivery hose needs to be cooled to below the critical temperature, using the tanker liquid oxygen itself to cool the delivery pipe. The whole VIE device is mounted on a hinged weighing scale and is situated outside the hospital building, protected by a caged enclosure, which also houses two banks of reserve cylinders.

Although a proportion of anaesthetic and surgical equipment is disposable nowadays, there is still a significant amount of cleaning and sterilisation required, and with the emergence of new organisms, the methods used have come under closer scrutiny. It is worth noting, in passing, that hand cleanliness of staff coming into contact with patients has also come under close scrutiny in recent years. Cleaning of equipment involves the physical removal of as much of the infectious agent as possible, usually using water and a detergent, and is an important precursor to disinfection or sterilisation. It can be done manually where automated devices are unavailable. Ultrasonic washers are sometimes used, as are irrigation pumps for flushing out the lumina of tubes. There is a difference in definition between disinfection and sterilisation. Disinfection is merely the killing of non-spore producing micro-organisms. It kills most bacteria except mycobacteria and spores, and it kills some fungi and some viruses. A higher level of disinfection ensures the destruction of mycobacteria, and most fungi and viruses. Sterilisation is required to kill all micro-organisms, including spores, fungi and viruses. Prions are, however, resistant to most sterilisation procedures. To disinfect or sterilise the modes of heat, chemicals or radiation are used. Moist heat is much more efficacious at coagulating bacterial protein than dry heat, which requires higher temperatures for longer periods to guarantee effect. Moist heat achieves this by increasing the permeability of the organism’s cellular structure to the heat. A hot water washer or low temperature steam applied to instruments for fifteen minutes kills bacteria, but not spores. Higher temperatures are achievable by pressurising the steriliser (Boyle’s law). The modern autoclave uses steam at 134°C and 2 bar, when 3½ minutes is sufficient to kill all organisms, providing the steam can reach the instruments; however, to dry the equipment, the steam is removed and replaced with sterile air, the total cycle time being 10 minutes. Rubber and plastic materials degenerate after some time with this regime, and a combination of 121°C for fifteen minutes, or 115°C for thirty minutes can be used instead.