Differential effects of digitalis on chemoreflex responses in humans

To investigate the effects of digitalis on chemoreflexes in humans, we measured muscle sympathetic nerve activity (microneurography), minute ventilation, oxygen saturation, end-tidal carbon dioxide, mean arterial pressure, heart rate, and central venous pressure during stimulation of peripheral chemoreceptors with hypoxia, during stimulation of central chemoreceptors with hypercapnia, and during a cold pressor test before and after digitalis and placebo in 10 healthy volunteers on two different days (randomized, double-blind, cross-over design). Digitalis did not affect baseline measurements significantly. Despite similar changes in oxygen saturation and end-tidal carbon dioxide during hypoxia and hypercapnia with both placebo and digitalis, digitalis significantly potentiated overall ventilatory responses to hypoxia (+67 +/- 12% before versus +98 +/- 3% after digitalis; mean +/- SEM; P < .01) but did not affect the response to hypercapnia. Sympathetic nerve activity increased by 25 +/- 9% during hypoxia before digitalis and 30 +/- 10% during hypoxia after digitalis (P = NS) and increased by 38 +/- 18% during hypercapnia before digitalis and 26 +/- 11% during hypercapnia after digitalis (P = NS). Digitalis did not significantly change responses to the cold pressor test. Placebo had no effect on ventilatory and sympathetic nerve activity responses. We conclude that digitalis selectively augments ventilatory responses to peripheral chemoreceptor stimulation by hypoxia.