Pattern of the ventilatory response to transient hypoxia in man: differences from transient hypercapnic test

The transient ventilatory response to carbon dioxide at rest and in exercise in man

A new technique has been developed to measure the transient response to inhaled CO2 using 30 sec pulses at constant inflow. Multiple experiments are ensemble-averaged in order to define the resulting small signals. We measured the peak changes in ventilation (delta V') and in PCO2 (delta PCO2), taking the ratio (delta V'/delta PCO2) as an index of response. Six healthy volunteers performed experiments at rest, 50 W and 100 W exercise. Three runs, each containing three pulses, were performed at each workload and subsequently averaged. Analysis of variance showed no significant difference between successive pulses or among subjects. delta V' did not differ significantly with workload, but delta PCO2 was progressively smaller as workload increased, and hence the response, delta V'/delta PCO2, greater. The delay between the rise in PCO2 and the rise in ventilation was also progressively shorter as workload increased, being 16-18 sec at rest, 7-13 sec at 50 W, and 3-6 sec at 100 W. Our results suggest that there is increased sensitivity to CO2 in exercise, which may be due to progressive activation of the peripheral chemoreceptors as work load increases. The delay at rest is too long for the peripheral chemoreceptor. Therefore, with these small stimuli, the central chemoreceptor must account for the CO2 response at rest.