Hypoxic potentiation of the ventilatory response to dynamic forearm exercise

The slope of the relationship between ventilation (V̇I) and O₂ consumption, as derived in progressive-intensity exercise tests, is increased markedly by systemic hypoxia. The mechanisms underlying the hypoxic potentiation of the ventilatory response to exercise have not been established, partly because several factors that can increase respiratory drive (e.g., metabolic rate, cardiac output, circulating catecholamine levels) change significantly and simultaneously under these conditions. In an effort to avoid these confounding changes, we sought to determine whether hypoxia potentiates the ventilatory response to dynamic forearm exercise in humans. Forearm exercise increased the O₂ consumption by only 80-90 ml/min; nevertheless, hypoxia resulted in a significant potentiation of V̇I that was mediated by a marked increase in breathing frequency. These observations led us to hypothesize that the hypoxic potentiation of V̇I is due to an exaggerated stimulation of chemosensitive afferent nerve endings within the exercising muscles ('muscle chemoreceptors'). We tested this hypothesis in separate experiments under conditions of forearm ischemia so that the stimulus to the muscle chemoreceptors in normoxic and hypoxic exercise would be the same. The magnitude of the change in V̇I evoked by hypoxic ischemic exercise was significantly greater than the sum of the separate changes evoked by normoxic ischemic exercise and hypoxic ischemic rest. We conclude that the combination of dynamic forearm exercise and hypoxia potentiates V̇I and that this effect is mediated by neural structures that govern respiratory frequency. Moreover the potentiated ventilatory response cannot be attributed to an exaggerated stimulation of intramuscular chemoreceptors.