Hypoxic ventilatory decline in young healthy adults persists during moderate-intensity exercise in isocapnic hypoxia.

Hypoxic ventilatory decline (HVD) is part of the mammalian ventilatory response to hypoxia and is characterized by a decline in ventilation that begins after 3-5 mins of sustained, moderate hypoxia. Exercise is a powerful ventilatory stimulus that increases ventilation and chemosensor sensitivity. The extent to which these opposing ventilatory stimuli interact is not fully characterized. We compared the ventilatory response to moderate exercise during two conditions: during pre-established HVD and shortly after the onset of hypoxia but prior to the establishment of HVD. Eleven (N=6 males) young, healthy participants completed 3 testing visits. Day 1 was a maximal exercise test and Days 2 & 3 were randomized experimental visits separated by > 48 hrs. Each experimental visit began with a 5 minute normoxic baseline followed by 15 mins of rest and 15 mins of exercise at 30% of peak power. The experimental visits differed in that, during the 'sustained hypoxia' protocol (oxyhemoglobin saturation ~80%), hypoxia began at the start of rest, whereas in the 'acute hypoxia' protocol, hypoxia began 1 min before exercise. At the onset of exercise, ventilation was significantly lower in the sustained hypoxia protocol (-5.1 L min^-1) suggesting a persistent blunting of ventilation. At end exercise, ventilation was not different between protocols (51.2±11.6 vs 50.5±7.3 L min^-1, for sustained and acute hypoxia, respectively). The percent decline in ventilation (HVD) was greater during the sustained hypoxia (-23±13 vs. -14±9%, p=0.04). We conclude that HVD persists when acute exercise is imposed on sustained isocapnic hypoxia.