Effect of upright and supine posture on hypoxemia during air transport

Background: Change in body position can cause hypoxemia at sea level ill 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 attitude (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.