Hypocapnia and Sustained Hypoxia Blunt Ventilation on Arrival at High Altitude

Hypoxia at high altitude stimulates ventilation but inhibitory influences limit the ventilatory response. Possible inhibitory influences include hypocapnia and depression of ventilation during sustained hypoxia. Our approach was to compare hypoxia ventilatory responses at low altitude with ventilation at high altitude. In 12 subjects we compared responses at acute 10 min isocapnic hypoxia, acute poikilocapnic no CO2 added hypoxia and sustained 30 min hypoxia in Denver, 1600 M, with ventilation measured on each of 5 days on Pikes Peak, 4300 M. On Pikes Peak day 1 ventilation VE10.0 1min, SaO2-82 was less than predicted by either acute isocapnic of poikilocapnic tests. However, sustained poikilocapnic hypoxia SaO282 in Denver yielded ventilation similar to that on Pikes Peak day 1. By Pikes Peak days 4 and 5, end-tidal PCO2, pHa, and arterial oxygen saturation approached planteaus, and ventilation 12.4 1min was as predicted by the acute isocapnic test. Thus, the combination of hypocapnia and sustained hypoxia may have blunted the ventilatory increase on Pikes Peak day 1, but apparently not after 4 or 5 days of acclimatization.