Altitude illness is related to low hypoxic chemoresponse and low oxygenation during sleep

Altitude illness remains a major cause of mortality. Reduced chemosensitivity, irregular breathing leading to central apnoeas/hypopnoeas, and exaggerated pulmonary vasoconstriction may compromise oxygenation. All factors could enhance susceptibility to acute mountain sickness (AMS). We compared 12 AMS-susceptible individuals with recurrent and severe symptoms (AMS+) with 12 "AMS-nonsusceptible" subjects (AMS-), assessing sleep-breathing disorders in simulated altitude as well as chemoresponsive and pulmonary vasoconstrictive responses to hypoxia. During exposure to simulated altitude, mean blood oxygen saturation during sleep was lower in AMS+ subjects (81.6±2.6 versus 86.0±2.4%, p<0.01), associated with a lower central apnoea/hypopnoea index (18.2±18.1 versus 33.4±24.8 events·h(-1) in AMS+ and AMS- subjects, respectively; p=0.038). A lower hypoxic (isocapnic) chemoresponsiveness was observed in AMS+ subjects (0.40±0.49 versus 0.97±0.46 L·min(-1)·%; p<0.001). This represented the only significant and independent predictive factor for altitude intolerance, despite a higher increase in pulmonary artery systolic pressure in response to hypoxia, a lower lung diffusing capacity and a higher endothelin-1 level at baseline in AMS+ subjects (p<0.05). AMS+ subjects were more hypoxaemic whilst exhibiting fewer respiratory events during sleep owing to lower hypoxic (isocapnic) chemoresponsiveness. In conclusion, the reduction in peripheral hypoxic chemosensitivity appears to be a major causative factor for altitude intolerance.