Objectives

The objectives of this chapter are to:

• understand the causes of hypoxaemia

• be familiar with devices available to increase the inspired oxygen concentration

• understand the function and use of the self-inflating bag-mask

• understand the function and use of the Mapleson C breathing system

• understand how to monitor oxygenation

• understand the principle of pre-oxygenation.

Causes of hypoxaemia

The strict definition of hypoxaemia is a partial pressure of oxygen in the arterial blood (PaO2) below normal; however, a value of <8kPa or 60mmHg (equivalent to an arterial oxygen saturation of approximately 90%) is often used to define hypoxaemia requiring treatment. In nearly all patients hypoxaemia can usually be improved, at least initially, by increasing the inspired oxygen concentration.

Although the cause of hypoxaemia is usually multifactorial, there are several distinct mechanisms:

• alveolar hypoventilation

• mismatch between ventilation and perfusion within the lungs

• pulmonary diffusion defects

• reduced inspired oxygen concentration.

Alveolar hypoventilation

Insufficient oxygen enters the alveoli to replace that taken up by the blood. Both the alveolar partial pressure of oxygen PaO2 and arterial partial pressure of oxygen (PaO2) decrease. In most patients, increasing the inspired oxygen concentration will restore alveolar and arterial PO2. When an adult's tidal volume decreases below approximately 150ml there is no ventilation of the alveoli, only the ‘dead space’, which is the volume of the airways that plays no part in gas exchange. No oxygen reaches the alveoli, irrespective of the inspired concentration, and profound hypoxaemia will follow. At this point ventilatory support and supplementary oxygen will be required.