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.