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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.

 

Resp failure vent co2

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

 

A rise in PaCO2 in the alveoli leads to an increase in oxygen requirements because the accumulated CO2 molecules displace O2 molecules (see alveolar gas equation below).

Alveolar gas equation

  • PAO2 = PIO2 - PACO2 / R

PAO2 is the partial pressure of oxygen in alveolar air.

PIO2 is the partial pressure of oxygen in inspired air.

PACO2 is partial pressure of CO2 in alveolar air.

R is the respiratory quotient.

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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