Cardiac output is important in the delivery of oxygen to the tissues. The proportion of oxygen delivery increased by increasing the PaO2 or SaO2 when it is already above the shoulder of the haemoglobin-oxygen dissociation curve is small compared with improving a reduced cardiac output. Therefore in the management of the critically ill patient as much attention should be paid to fluid resuscitation as to the normalisation of hypoxaemia (see Chapter 2).

Oxygen tension in the air is around 20 kPA (154 mmHg) at sea level, falling to 0 5 kPA (3 8 mmHg) in the mitochondria. This gradient is known as the oxygen "cascade" (Figure 4.1). An interruption at any point can cause hypoxia - high altitude, upper or lower airway obstruction, alveolar flooding, abnormal haemoglobins, circulatory failure and mitochondrial poisoning are examples.

Hypoxaemic respiratory failure is characterised by a low PaO2, leading to an elevated alveolar-arterial oxygen gradient and a low PaCO2, reflecting adequate ventilation but inadequate gas exchange. Hypoxaemia is most commonly due to the mismatch of ventilation (V) and perfusion (Q) or intrapulmonary right-to-left shunts (Figure 4.2).

Figure 4.2 Ventilation/perfusion mismatch and shunts

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