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 ventilation‐perfusion (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.