Postural hypoxemia in cystic fibrosis.

We measured arterial blood gases in the sitting and supine position in 33 patients with cystic fibrosis (CF). Arterial PO2 ranged from 48 to 97 mm Hg in the sitting position and fell by 1 to 23 mm Hg in 26 patients, rose by 1 to 4 mm Hg in five, and was unchanged in two when supine. Mean PO2 change for the group was a 6.5 (+/- 6.8) mm Hg decrease (sitting to supine), and substantial falls were more likely to occur in patients with mild to moderate, rather than severe, obstructive lung disease. There were no significant changes in PCO2 or pH with posture. Postural hypoxemia was not completely explained by greater airway closure as measured by the single-breath nitrogen method in 11 patients or by worse radiographic change in the upper lung zones. Arterial PO2 should always be compared in the same body position in CF patients, preferably while supine. Postural hypoxemia also may contribute to the lower PO2 at night in CF patients.