Effects of Rapid Ascent to High Altitude on Vascular Tone

Background: Acute changes in endothelial function and sympathetic activation with hypoxia are considered to be the important features of the acute syndromes of high altitude. The purpose of the study was to investigate the effects on vascular tone as a determinant of acute mountain sickness (AMS) after rapid ascent to high altitude. Method: Heart rate (HR), arterial blood pressure (BP), oxygen saturation (SpO2) and vascular tone, quantified by the reflection index (RI) obtained using photoplethysmography (PPG) technique, were studied over 3 days in 47 unacclimatized healthy subjects ascending from 495 to 3,658m. AMS occurrence was evaluated by the Lake Louise score system. Results: At 3,658m, two age groups demonstrated a significant decrease of SpO2 and a significant increase of HR in day 1 and showed gradual partial recovery in day 2 and day 3. We observed a mild increase in both SBP and DBP, and the increase was a little greater in the elderly and in subjects who developed AMS in both groups. RI fell during the first day in both age groups and got back towards normal in the following days. However subjects with AMS in both age groups showed a blunted and delayed response in RI to acute high-altitude exposure. Conclusions: The hypoxia driven effects on RI associated with AMS could in part be explained by a short-term imbalance between local endothelial derived vasodilatation and sympathetically mediated vasoconstriction. The specific feature of RI may provide an important mechanistic insight into the differential effects of functional adaptation to high-altitude hypoxia or capacity to adapt.