Does hysteresis in cardiorespiratory variables during high altitude ascent and descent quantify the magnitude of acclimatization?

Acute mountain sickness (AMS) is common during ascent to high altitude. We aimed to quantify ventilatory acclimatization in the context of high altitude hypoxia by comparing oxygen saturation (SpO2) and end-tidal CO2 (PETCO2) during ascent and descent. We hypothesized (1) the hysteresis area formed by differential cardiorespiratory values during ascent and descent would quantify the magnitude of acclimatization, (2) prolonged stays above 5000m would lead to larger areas than shorter stays, and (3) larger areas would be correlated with lower AMS symptoms. In 2012, a single male participant (P1) ascended to 5050m in Nepal and stayed approximately three weeks. P1 returned in 2016 as part of a group of 20 participants (P20), and stayed only one night above 5000m. In P1, the calculated SpO2 hysteresis area was 72% larger in 2012 than 2016. In P20, there was a weak positive correlation between SpO2 and PETCO2 hysteresis areas (r=0.43, P=0.06), but these values were not correlated with AMS severity when arriving above 5000m (r=0.06, P=0.8; r=0.3, P=0.2, respectively). We conclude that calculation of hysteresis in cardiorespiratory variables may hold promise as a simple method to quantify the magnitude of ventilatory acclimatization when staying at altitude for prolonged periods. * Indicates faculty mentor