A high concentration of oxygen is needed during and after anaesthesia:

Air already contains 20.9% oxygen, so oxygen enrichment with a draw-over system is a very economical method of providing oxygen. Adding only 1 litre per minute may increase the oxygen concentration in the inspired gas to 35–40%. With oxygen enrichment at 5 litres per minute, a concentration of 80% may be achieved. Industrial-grade oxygen, such as that used for welding, is perfectly acceptable for the enrichment of a draw-over system and has been widely used for this purpose.

To add oxygen to a draw-over system, use a T-piece and reservoir tube at the vaporizer inlet (Figure 15.1).

Figure 15.1

If a ready-made T-piece with reservoir is unavailable, you can easily make an improvised alternative using a small-bore oxygen tube threaded into a large-bore tube. Connect the T-piece and reservoir tube (or your improvised version) to the vaporizer inlet and turn on the oxygen supply. By doing this, the oxygen that flows from the cylinder during expiration is not wasted, but is stored in the reservoir tube for the next inspiration. The reservoir tubing should, of course, be open to the atmosphere at its free end to allow the entry of air and it should be at least 30 cm long.

OXYGEN SOURCES

In practice, there are two possible sources of oxygen for medical purposes:

For remote hospitals that cannot obtain oxygen cylinders on a regular basis, there is a strong case for introducing concentrators. However, cylinders can be used to supply oxygen during power cuts and concentrators cannot. Without electricity, the flow of oxygen from a concentrator will stop within a few minutes.

The ideal oxygen supply system is one based primarily on concentrators, but with a back-up supply from cylinders.



Details of the necessary apparatus for such a system, designed to supply oxygen reliably to up to four children or two adults, are available from the Department of Blood Safety and Clinical Technology, World Health Organization and from the Procurement Office, UNICEF. For details, contact:

Department of Blood Safety and Clinical Technology
World Health Organization
1211 Geneva 27
Switzerland
Fax: +41 22 791 4836 E-mail: bct@who.int
http://www.who.int/bct/dct

Procurement Office
UNICEF
3 United Nations Plaza
NY 10017, USA

OXYGEN CYLINDERS

Cylinders of oxygen are produced by a relatively expensive industrial process. An oxygen cylinder needs a special valve (regulator) to release the oxygen in a controlled way and a flow meter to control the flow. Without a flow meter, the use of oxygen from cylinders is very wasteful; without a regulator it is also extremely dangerous.

Not all oxygen cylinders are the same; there are at least five different kinds of cylinder in use in different countries. A regulator will fit only one type of oxygen cylinder. Precise information on the type of oxygen cylinder in use should be obtained from the local oxygen supplier before ordering regulators. This should be confirmed by someone with technical knowledge who works in the hospital, such as an anaesthetist, chest physician or fully trained hospital technician.

An international standard exists for the identification of oxygen cylinders, which specifies that they should be painted white. Unfortunately, the standard is widely ignored. Medical oxygen cylinders originating in the USA are normally green, while those originating in Commonwealth countries are usually black with white shoulders. Cylinders of industrial oxygen should also be identified clearly, but this is not always the case. Never use any cylinder to supply gas to a patient unless you are sure of its contents.

Getting oxygen to patients requires more than simply having oxygen cylinders available. You must have in place an entire functioning system, comprising not only the apparatus for oxygen delivery, but also people who have been trained to operate it and a system for maintenance, repair and supply of spare parts.

A complete system for using oxygen in cylinders requires:

Safe use of oxygen cylinders

The oxygen supply from a cylinder must be connected through a suitable pressure-reducing valve (regulator). For larger cylinders, this valve is incorporated into the cylinder’s pressure gauge; on a Boyle’s machine both the gauge and the pressure-reducing valve are part of the machine.
Using oxygen from cylinders without a regulator is extremely dangerous.

When connecting a cylinder to the anaesthetic apparatus, make sure that the connectors are free from dust or foreign bodies that might cause the valves to stick. Never apply grease or oil, as it could catch fire in pure oxygen, especially at high pressure. Remember that an oxygen cylinder contains compressed oxygen in gaseous form and that the reading on the cylinder pressure gauge will therefore fall proportionately as the contents are used. A full oxygen cylinder normally has a pressure of around 13 400 kPa (132 atmospheres, 2000 p.s.i.). It should always be replaced if the internal pressure is less than 800 kPa (8 atmospheres, 120 p.s.i.) as failure is then imminent.

Oxygen cylinders are dangerous objects. If they fall over, they may injure or even kill.
Make sure that cylinders are safely stored and mounted. In storage, they should lie horizontally. In use, they should be securely fixed in the vertical position to a wall or be kept standing secured with a restraining strap or chain.

Supplies, equipment and maintenance

Compressed oxygen is expensive and using it may pose logistical and cost problems for small or remote hospitals. In the United Republic of Tanzania, for example, a recent survey showed that 75% of district hospitals had an oxygen supply for less than 25% of the year. A reliable system for cylinder oxygen depends on a good source of supply and reliable year-round transportation. In many countries, oxygen cylinders must be bought rather than rented and frequent losses of cylinders in transit impose additional costs.

Fortunately, since oxygen is needed for a variety of industrial as well as medical applications, it is widely available. Because cylinders of “industrial” oxygen and of “medical” oxygen are produced by the same process (the fractional distillation of air), good-quality industrial oxygen is perfectly safe for medical use. It may also be easier to obtain and less expensive, since a price premium is often levied for “medical-grade” oxygen. However, if you obtain oxygen from an unorthodox source, you must check it for purity before use (a portable analyzer may be used).

Efficient and economical use of oxygen – while still ensuring that the patient receives the maximum benefit – is important. If properly understood, oxygen supplies can be used quite economically. The oxygen concentration of air (21%) generally needs to be increased only to about 40% in order to bring great benefit to the majority of patients who need extra oxygen.

OXYGEN CONCENTRATORS

Oxygen concentrators are suitable for use in all levels of hospital. They provide oxygen more cheaply than cylinders, as well as making oxygen available in hospitals where a regular supply of cylinders is difficult to obtain. It is strongly recommended that only those models listed by the World Health Organization as suitable should be purchased for use in district hospitals. An up-to-date list of suitable concentrators currently known to meet the recommended performance standard is available from the Department of Blood Safety and Clinical Technology, World Health Organization and UNICEF supplies concentrators meeting the Performance Standard at a very economical price (see page 15–7).

Oxygen concentrators designed for use with individual patients normally give a flow rate of up to 4 litres per minute of near-pure oxygen at relatively low pressure. This oxygen can be used in exactly the same way as oxygen from a cylinder:

The oxygen from a concentrator is at relatively low pressure and therefore cannot be used in a compressed gas (Boyle’s) anaesthetic machine.

If there is an electrical power failure, the oxygen flow from a concentrator will continue for about a minute only, so make sure you have a back-up system for use in such emergencies – either a generator to maintain electrical supply, or a cylinder of compressed oxygen.

Oxygen concentrators have been installed in many hospitals where cylinders are not consistently available. Concentrators ensure a more reliable and lower-cost supply of oxygen than cylinders. An oxygen concentrator uses zeolite to separate oxygen from nitrogen in air. The oxygen produced by concentrators is at least 90% pure and can be used in the same way as oxygen from cylinders, with the same beneficial effects.

Oxygen concentrators require much less energy than fractional distillation and have the additional advantage that oxygen is easily produced in the operating room or at the patient’s bedside, provided that there is an electricity supply (a small concentrator uses about 350 W). The purchase price of a concentrator is about half the cost of a year’s supply of oxygen from cylinders and running costs for electricity and spare parts are low.

A complete system for oxygen delivery based on concentrators, requires:

For successful use in a district hospital, a concentrator must:

A hospital planning to use oxygen concentrators should consider buying at least two. Remember that no piece of equipment will last for ever, especially if it is neglected. Hospitals need to plan for regular maintenance – usually after every 5000 hours of use. Servicing the machines is not complicated and can, if necessary, be carried out by the user after simple training.