Profound hypoxemia in pulmonary patients in airline-equivalent hypoxia: roles of VA/Q and shunt.

Background: Airliners at cruise altitudes have cabin pressures of 84.3-75.2 kPa, equivalent to 5000-8000 ft. Supplementary oxygen in flight is generally advised for patients whose ground-level arterial oxygen saturation by pulse oximetry (SpO2) is < 92%. However, some pulmonary patients with values above that threshold nevertheless develop profound in-flight hypoxemia (SpO2 < or = 82%) as cabin pressure falls. This experiment measured the contributions of ventilation-perfusion ratio (VA/Q) and pulmonary shunt to that phenomenon.

Methods: Air/nitrogen mixtures were used to expose 18 patients with pulmonary dysfunction to PIO2 = 15 kPa. VA/Q was estimated from right shift of the steep phase of the PIO2 vs. SpO2 curve along the PO2 axis. Shunt was estimated from change of shape and downward shift of the same curve.

Results: During hypoxia, the lowest observed SpO2 was correlated with reduced VA/Q (R2 = 0.89). Of 10 patients with VA/Q < or = 0.69, 9 developed profound hypoxia even though 5 of those had SpO2 > or = 92% at ground level; patients with VA/Q > 0.75 maintained SpO2% > or = 84%. Hypoxemia did not correlate with shunt for the group as a whole, but eight patients with VA/Q > or = 0.73 and shunts in the range 6.6-20.8% showed such a correlation (R2 = 0.8).

Conclusion: In patients with similar ground-level SpO2, profound hypoxemia was more likely to develop in those with reduced VA/Q (< or = 0.69) as opposed to those with shunts of 20%. Measurement of VA/Q and shunt during hypoxic hypoxia improves prediction of patient need for supplementary oxygen on airline flights.