Respiratory

Respiratory Failure

Gas exchange fails in one of two ways: the lung cannot load oxygen into blood (type 1), or the ventilatory pump cannot clear carbon dioxide (type 2).

Definition

Respiratory failure is impairment of gas exchange severe enough to cause hypoxaemia, hypercapnia or both. Type 1 is a PaO2 below 8 kPa with a normal or low PaCO2. Type 2 is a PaCO2 above the normal range of 4.6 to 6.1 kPa, whatever the oxygen level. pH separates acute from chronic: below 7.35 with a raised PaCO2 is acute respiratory acidosis, whereas a normal pH with a raised bicarbonate is chronic compensation.

First principles

Oxygenation and ventilation fail separately

Oxygen loading depends on alveoli meeting perfused capillaries. Carbon dioxide clearance depends almost entirely on how much air moves per minute. Oxygen therapy corrects only the first, which is why type 2 failure is defined on the blood gas and never on the oximeter.

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