Oxygen

Common indications

1. To increase tissue oxygen delivery in states of hypoxaemia.
2. To accelerate reabsorption of pleural gas in pneumothorax.
3. To reduce the half-life of carboxyhaemoglobin in carbon monoxide poisoning.

 

Mechanisms of action

• An abnormally low partial pressure of oxygen (PO2) in arterial blood (PaO2), termed hypoxaemia, may be a consequence of a wide range of disease processes. Its effect is to reduce the delivery of oxygen to tissues (hypoxia), forcing them to use anaerobic metabolism for energy generation. Supplemental oxygen therapy increases the PO2 in alveolar gas, driving more rapid diffusion of oxygen into blood. The resultant increase in PaO2 increases delivery of oxygen to the tissues, which in effect ‘buys time’ while the underlying disease is corrected. In pneumothorax, supplemental oxygen therapy has an additional benefit of reducing the fraction of nitrogen in alveolar gas. This accelerates the diffusion of nitrogen out of the body. Since pleural air is composed mostly of nitrogen, this increases its rate of reabsorption. In carbon monoxide (CO) poisoning, oxygen competes with CO to bind with haemoglobin and thereby shortens the half-life of carboxyhaemoglobin, returning haemoglobin to a form that can again transport oxygen to tissues.

 

Important adverse effects

• The most common adverse effects of oxygen are related to the delivery device (e.g. the discomfort of a facemask) or its lack of water vapour (dry throat). 
• The latter can be improved by using a humidification system. 
• Except in pneumothorax and carbon monoxide poisoning, there is little to be gained from an abnormally high PaO2 and, indeed, there is some evidence that this may be harmful. 
• However, this concern should not lead you to withhold oxygen in critical illness or states of severe hypoxaemia, in which oxygen may be life-saving.

 

Warnings

• Patients with chronic type 2 respiratory failure (e.g. those with severe COPD) exhibit a number of adaptive changes in response to persistent hypoxaemia and hypercapnoea. If exposed to high inspired oxygen concentrations, this finely balanced adaptive state may be disturbed, resulting in a rise in the blood carbon dioxide concentration. 
• This may lead to respiratory acidosis, depressed consciousness, and worsened tissue hypoxia. This necessitates a different approach to oxygen therapy (see Prescription and Administration). 
• Oxygen accelerates combustion and therefore presents a fire risk if it is brought into close proximity with a heat source or naked flame, including from smoking.

 

Important interactions

• There are no clinically important interactions.