The Management of Chronic Hypoventilation (2)
The development of chronic hypoventilation represents a compromise in which the efficiency of carbon dioxide excretion is improved, te, more carbon dioxide can be excreted with each breath, at the expense of an increase in both alveolar and arterial Pco2 and a decrease in arterial Po2. The tendency to make this compromise is partly determined by the sensitivity of the ventilatory control center for carbon dioxide. Evidence for this derives from studies of family members of individuals with chronic obstructive pulmonary disease (COPD) who are chronic CO£ retainers.
These family members have less ventilatory sensitivity to C02 and to hypoxia than family members of patients with equally severe COPD who are not C02 retainers. Thus, even when a disturbance of ventilatory control is not the primary cause of hypoventilation, a relatively low sensitivity of the respiratory center to C02 may contribute to the development of chronic hypoventilation. Another factor thought to contribute to chronic hypoventilation is respiratory muscle fatigue. Defined as an acute reduction in muscle contractility during exhausting work, respiratory muscle fatigue contributes to acute respiratory failure, and occurs during exercise in patients with severe COPD. However, the role of respiratory muscle fatigue in chronic hypoventilation has not been clearly defined, nor has it been established that intermittent respiratory muscle rest is responsible for the improvement in gas exchange associated with intermittent ventilatory assistance in patients with chronic hypoventilation.