Effect of upright and supine posture on hypoxemia during air transport.

BACKGROUND: Change in body position can cause hypoxemia at sea level in patients with lung diseases. Because of concern for the added risk of hypoxemia during air transport, we investigated the effect of body position on arterial oxygen partial pressure (PaO2) in individuals with lung disease under conditions of hypobaric hypoxia. METHOD: The study groups consisted of 8 patients with chronic obstructive lung disease, 4 patients with interstitial lung disease, and 6 healthy subjects. We obtained samples from radial artery catheters at sea level (SL) and altitude (ALT) simulation of 8000 ft (2438 m) in a hypobaric chamber in supine and upright postures. RESULTS: Altitude exposure did not result in a significant change in mean supine minus mean upright PaO2 (dPaO2); however, some individuals had large changes at SL. Moreover, the variance for dPaO2 was significantly smaller at ALT compared to SL with all groups combined (F test, p < 0.05). We found no correlation between dPaO2 at SL vs. ALT (p = 0.293; r = 0.262; n = 18). At both SL and ALT, dPaCO2 correlated negatively with dpH. At SL, dPaO2 did not correlate with either dPaCO2 or dpH; at ALT dPaO2 correlated with dpH (p < 0.05) and correlated negatively with dPaCO2 (p < 0.01). CONCLUSION: We conclude that significantly less postural variation in PaO2 occurs at moderate ALT compared to SL. In our patients with diffuse bilateral pulmonary disease, postural change did not contribute significantly to hypoxemia experienced at ALT. We infer that greater ventilatory response to hypoxemia at ALT in either posture may explain this finding.