Nocturnal Hypoventilation
Hypoventilation occurs when alveolar ventilation does not result in sufficient oxygen uptake or carbon dioxide output. Occasional nocturnal hypoxemia occurs in normal subjects without significant physiological consequences, however, in patients with CF, normally occurring variations in gas exchange during sleep can be consequential with underlying advanced lung disease. Several mechanisms have been identified that may lead to hypoventilation in CF patients. Patients with CF have an increased incidence of kyphosis, which results in decreased compliance or elasticity of the chest wall54. In addition, CF patients are often malnourished and have hyperinflation.   Though both malnutrition and hyperinflation should lead to respiratory muscle weakness in patients with moderate to severe lung disease, the findings are not substantiated12. In a study of 64 adults with mild to moderate lung disease , FEV1 65±19%, when compared to normal controls, these subjects had normal respiratory muscle strength as measured by PI/PE max55. CF patients may also be at increased risk of hypoventilation due to opioid use that are often prescribed for pain control in advanced CF lung disease56. Finally, increased risk of sleep disordered breathing may also lead to nocturnal hypoventilation. In normal healthy individuals, during sleep there is an increase in upper airway obstruction and decrease in the central drive to breathe. In individuals with CF, although V-Q mismatch may lead to nocturnal hypoxemia, the major contributor to gas exchange abnormalities results from hypoventilation37 . These findings were more pronounced during REM sleep, which is also associated with reduced minute ventilation mainly due to a decrease in tidal volume11,31.