Introduction		Oxygen Therapy
Oxygen Manufacture and Storage		Conclusion
Hypoxia		References
Oxygen Delivery Systems

Introduction

Oxygen has been used in clinical practice for more than 200 years. It is probably the most widely prescribed medication in pre-hospital and hospital environments. If appropriately used it is life-saving and part of first-line treatment in many critical conditions. It is important that oxygen not only reaches the lungs but is delivered to the tissues. Therefore a good cardiac output, circulation and haemoglobin is vital and is why attention to the circulation is an early part of initial resuscitation ( The physiology of oxygen delivery, Update in Anaesthesia 1999;10:3). As with any drug, oxygen should be used when indicated, in appropriate dosage (concentration), and correctly administered for a planned duration. 

Oxygen manufacture and storage

When cooled to very low temperatures gases change to either solids, (carbon dioxide), or liquids (oxygen and nitrogen). Oxygen has to be cooled to below -118°C to change to a liquid. When the gas changes form to a liquid, it occupies a much smaller volume. Therefore when a small volume of liquid oxygen is warmed it will make a very large volume of oxygen gas. Oxygen can be stored as either a gas in cylinders or as a liquid in a special container. In the liquid form, a very large quantity of oxygen can be transported or stored in a low volume, although there are problems in keeping the liquid cold as explained below.

Vacuum Insulated Evaporator (VIE). A VIE is a container designed to store liquid oxygen. It has to be designed to allow the liquid oxygen inside to remain very cold. It consists of two layers, where the outer carbon steel shell is separated by a vacuum from an inner stainless steel shell, which contains the oxygen (figure 1). The oxygen temperature inside is about -170°C and the container is pressurised to 10.5 atmospheres (10.5 bar). Gaseous oxygen above the liquid is passed through the superheater to raise the temperature to ambient (outside) levels. It then flows into the hospital pipeline system giving a continuous supply of piped oxygen to outlets on the wards and in theatre. Heat is always able to get into the container and provides the energy to evaporate the liquid oxygen, changing it into oxygen gas which is continuously drawn off into the pipeline system. This escape of gas into the pipeline system prevents the pressure inside the container from rising. If the pressure rises too much (above 17 bar), oxygen is allowed to escape via a safety valve into the atmosphere.

In contrast, if the pressure inside the container falls because of heavy demand in the hospital for oxygen, liquid oxygen can be withdrawn, passed through the evaporator and returned to the VIE in the gaseous form to restore the pressure. The amount of oxygen available in the container is estimated by weighing the container with an in-built device.

The VIE system is used in large hospitals which have a pipeline system, and where liquid oxygen can be supplied by road tanker.

Oxygen cylinders. Oxygen can be stored under pressure in cylinders made of molybdenum steel. Cylinders may be combined to form a bank attached to a manifold. The advantages of combining large cylinders into a bank include a reduction in cost, transportation and constant change of exhausted cylinders. Oxygen cylinders come in several sizes (table 2). In UK oxygen cylinders are black with white shoulders. The pressure inside at 15°C is 137 bar.

Oxygen concentrators An oxygen concentrator is a device which extracts oxygen from atmospheric air using canisters of zeolite. Nitrogen is filtered out and oxygen produced. The function and successful economics were described in detail. ( Oxygen concentrators for district hospitals, Update in Anaesthesia 1999;10:11). When ether is used, the oxygen concentrator should be positioned 1.5m above the floor. 

Hypoxia

Hypoxaemia is when the oxygen tension in arterial blood is less than 80mmHg (10.6kPa). Hypoxia is a deficiency of oxygen at the tissue level. Traditionally, hypoxia has been divided into 4 types.

1. Hypoxic hypoxia in which oxygen tension of arterial blood is reduced
2. Anaemic hypoxia in which the arterial oxygen tension is normal but the amount of haemoglobin(Hb) available to carry oxygen is reduced.
3. Stagnant or ischaemic hypoxia in which blood flow to the tissues is so low that oxygen is not delivered to the tissues despite normal arterial oxygen tension and Hb concentration.
4. Histotoxic hypoxia in which oxygen is delivered to the tissues but a toxic agent prevents the cells using the oxygen.

Recognition of hypoxia. Recognition of tissue hypoxia is not always easy as there are a number of different signs and symptoms. Clinical signs and symptoms include:

• Altered mental status (agitation, confusion, drowsiness, coma)
• Cyanosis
• Dyspnoea, tachypnoea or hypoventilation
• Arrhythmias
• Peripheral vasoconstriction often with sweaty extremities
• Systemic hypotension or hypertension depending on the underlying diagnosis
• Nausea, vomiting and other gastrointestinal disturbance

Cyanosis means blueness of the tissues and is due to an excessive amount of deoxygenated Hb in the peripheral blood vessels. Cyanosis appears whenever the arterial blood contains more than 1.5grams of deoxygenated Hb in each 100mls of blood (normal Hb15g/100ml). Cyanosis can often be detected in a patient with a normal haemoglobin level when the oxygen saturation is less than 90%. When the oxygen saturation falls in anaemic patients, cyanosis is often absent.

As the clinical signs are non-specific, the best method of assessing oxygenation is to measure peripheral arterial oxygen saturation (SaO2<95% is abnormal) and oxygen partial pressure in the arterial blood (PaO2<80mmHg (10.6kPa). Pulse oximeters and blood gas analysis have become more widespread throughout the world. Hypoxia at tissue level may still exist even when SaO2 and PaO2 are within normal limits, if there is a low cardiac output, anaemia or failure of tissues to use oxygen (e.g. cyanide poisoning). In this situation the blood lactate concentration rises due to anaerobic metabolism. Lactate can be measured in some laboratories. 

(Continued ...)

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