Dynamics of the ventilatory response in man to step changes of end-tidal carbon dioxide and of hypoxia during exercise

Abstract

1. Four human subjects exercised in hypoxia (end-tidal partial pressure of O2 (P(ET),O2) ca 55 Torr; heart rate ca 100-130 beats min-1), and the contribution to the respiratory drive of the peripheral and central chemoreflex pathways have been separated on the basis of the latencies and the time courses of the responses to sudden changes of stimulus. 2. The subjects were exposed to repeated end-tidal step changes in PCO2 of ca 3-3.5 Torr (at nearly constant P(ET),O2) and PO2 (between ca 55 and 230 Torr) at three regions along the expiratory ventilation VE-P(ET),CO2 response line (hypocapnia, eucapnia, hypercapnia). The dynamics of the ventilatory responses were calculated using a two-compartment non-linear least-squares optimization method. 3. The component of the response attributable to the peripheral chemoreflex loop may in some subjects contribute up to 75% of the ventilatory drive during mild hypocapnic hypoxic exercise and ca 72% of the total gain following steps of P(ET),CO2 during hypoxic exercise. These data support the notion that the effectiveness of the peripheral chemoreceptor pathway is enhanced in moderate exercise. 4. During hypoxic exercise, the time delays and time constants attributed to the peripheral chemoreflex pathways (ca 3.5 and 9 s respectively) and to the central chemoreflex pathways (ca 9.5 and 47 s respectively) are some of the shortest reported. 5. The dynamics of the peripheral and central chemoreflex pathways appeared to be largely independent of each other. 6. There was a notable absence of systematic change of inspiratory and expiratory durations during the step-induced transients.