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 min 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 before the establishment of HVD. Eleven (n = 6 males) young, healthy participants completed three testing visits. Day 1 was a maximal exercise test and days 2 and 3 were randomized experimental visits separated by > 48 h. Each experimental visit began with a 5 min normoxic baseline followed by 15 min of rest and 15 min 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.NEW & NOTEWORTHY Imposing moderate-intensity exercise on acute, sustained isocapnic hypoxia in close temporal proximity showed evidence of hypoxic ventilatory decline in the first 15 min of exercise. The percent decline in ventilation during exercise was smaller than the hypoxic ventilatory decline at rest, indicating a blunting of the ventilatory decline when occurring together with exercise. Duration of pre-exercise hypoxic exposure should be considered when interpreting hypoxic exercise ventilation.