Hypoxaemia is defined as the reduction below normal levels of oxygen in arterial blood - a PaO2 of < 8 0 kPa (60 mmHg) or oxygen saturations < 90%. Hypoxia is the reduction below normal levels of oxygen in the tissues. The normal range for arterial blood oxygen is 11-14 kPa (85-105 mmHg), which reduces in old age. It is hypoxia rather than hypoxaemia that causes cell damage. Hypoxaemia usually results in hypoxia. The main causes of hypoxaemia are:

• ventilation-perfusion (V/Q ) mismatch

• intrapulmonary shunt

• hypoventilation

• increased oxygen consumption.

Severe tissue hypoxia results in cell death and organ damage. In the ward setting, simple oxygen therapy may reverse the confusion, agitation, and cardiac ischaemia that is seen in such circumstances.

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