Nocturnal Hypoxia: Physiological Consequences in CF
Nocturnal desaturation is common in CF, can be harmful, and often exists
independently of obstructive sleep apnea.35 One
proposed mechanism for hypoxemia is atelectasis of the distal airways
leading to a ventilationâperfusion (V-Q) mismatch.36In addition, sleep hypoxemia and hypercapnia correlate with the degree
of nocturnal ventilatory deficits seen with more severe lung disease in
CF.37 Repeated episodes of hypoxia can lead to
deleterious effects on the right heart and development of pulmonary
hypertension.38 Also, it is important to note that the
cystic fibrosis transmembrane conductance regulatory protein (CFTR) has
been examined as a functional contributor to hypoxia induced pulmonary
hypertension.39 Nocturnal desaturation in COPD
patients has been shown to increase C-reactive protein, which can
increase cardiovascular disease, but similar effects have not been
demonstrated in the CF population.40
Isaiah et al found that the percent predicted FEV1 was
the best predictor of sleep hypoxemia in children with CF referred for
PSG.41 A threshold of FEV1 < 64% was seen
to have good sensitivity and specificity to predict nocturnal hypoxemia
by de Castro-Silva et al.36 Resting awake
SpO2, when low, can be predictive of nocturnal
oxygenation but studies have been unable to find a threshold that
reliably excludes those that will not desaturate.38Fauroux et al have found some correlation of nocturnal hypoxemia with
daytime blood gases, reporting that daytime partial arterial oxygen
pressure correlated with mean and minimal nocturnal
SpO2.37 Six minute walk test
desaturation is not predictive of nocturnal
desaturation.42 The current daytime predictors of
nocturnal hypoxemia continue to be variable in their reliability, and
many continue to follow the recommendation of Frangolias and colleagues
to consider nocturnal pulse oximetry in patients with moderate to severe
CF lung disease.38
It has been suggested that the first clinical action after diagnosing
nocturnal hypoxemia is to optimize treatment of lung
function.41 Treatment of pulmonary exacerbations in
cystic fibrosis results in improvement of nocturnal hypoxemia, but awake
saturation did not immediately return to normal.43Salvatore and colleagues have demonstrated that nocturnal use of inhaled
salmeterol in albuterol responsive CF patients can reduce nocturnal
hypoxemia.44 In addition, it is important to note that
nocturnal oxygen therapy used in isolation may increase
CO2 and could have a negative
impact.45
A Cochrane review evaluating short term oxygen use in advanced CF lung
disease concluded that oxygen therapy during sleep and exercise improves
oxygenation, but is associated with a modest
hypercapnia46.