Scottish Intensive Care Society

ICM Induction programme ©

Figure 1. Relationship between PaCO2 and VA (alveolar ventilation)

Respiratory failure

Hypoventilation

Two things cause alveolar hypoventilation:

1. A reduction in minute ventilation.
2. An increase in the proportion of dead space ventilation which can either be anatomical or physiological.

The characteristic of hypoventilation is CO2 retention (Type 2 respiratory failure). PaCO2 is inversely related to alveolar ventilation, see fig 1. A rise in PaCO2 in the alveoli leads to an increase in oxygen requirements because the accumulated CO2 molecules displace O2 molecules(alveolar gas equation).

Almost every cause of respiratory failure might eventually cause CO2 retention because of respiratory muscle fatigue. Some conditions normally present with Type 1 respiratory failure, but when severe present with marked CO2 retention (e.g. cardiogenic pulmonary oedema). This is because of a profound V/Q mismatch.

Hypoxaemia caused by hypoventilation is easily corrected with low dose oxygen supplementation. If high dose oxygen is required, there is an additional or alternative cause of hypoxaemia.

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