Relationship Between Pulse Oximetry and Arterial Oxygen Saturation During Rest and Exercise at Extreme Altitude

Manferdelli, Giorgio, Denis J. Wakeham, Keval D. Patel, Eric T. Hedge, Michael G. Leahy, Abel Thomas, James Berry, Tony G. Babb, Peter Hackett, Renie Guilliod, Benjamin D. Levine, Christopher M. Hearon, Jr., and Andrew R. Tomlinson. Relationship between pulse oximetry and arterial oxygen saturation during rest and exercise at extreme altitude. High Alt Med Biol. 00:00–00, 2026. Introduction: Pulse oximetry (SpO 2 ) is commonly used to guide oxygen use, including at high altitude, but its accuracy during severe hypoxemia remains uncertain. This study aimed to compare arterial oxygen saturation (SaO 2 ) and SpO 2 while using graded levels of supplemental oxygen at simulated extreme altitude in a hypobaric chamber. Methods: Four healthy, unacclimatized adults completed rest and 60 Watts of exercise at different flow rates of supplemental oxygen (6, 4, 2, 1, and 0 l/min nominal flow) at 282 mmHg barometric pressure (Pb; equivalent to 8,100 m) and 253 mmHg Pb (equivalent to 8,848 m) with simultaneous measurement of arterial blood gases (radial artery catheter) and SpO 2 (forehead pulse oximetry). Exercise was only performed at 282 mmHg. Results: Across all altitudes and conditions, SaO 2 and SpO 2 demonstrated a very strong correlation (all r 2 ≥ 0.95, p < 0.001), with SpO 2 showing a consistent positive bias (pooled: 1.7% [−4.4% to 7.7%]; 282 mmHg rest: 0.4% [−4.5% to 5.3%]; 282 mmHg exercise: 3.1% [−2.2% to 8.5%]; 253 mmHg rest: 2.9% [4.9% to 10.6%]). The magnitude of the SpO 2 bias was higher when only SaO 2 <70% was considered (pooled: 5.2%). The root mean square error was 2.6 for SaO 2 ≥70% and increased to 6.2 when SaO 2 was <70%. Conclusions: Although SpO 2 represents a precise noninvasive measure for SaO 2 across a wide range of oxygen saturations (70%–100%), SpO 2 has lower precision and accuracy during severe hypoxemia (<70% SaO 2 ).