Ventilation (VE) rises throughout 40 min of constant elevated end-tidal PCO2 without reaching steady state (S. Khamnei and P. A. Robbins. Respir. Physiol. 81: 117-134, 1990). The present study investigates 8 h of euoxic hypercapnia to determine whether VE reaches steady state within this time. Two protocols were employed: 1) 8-h euoxic hypercapnia (end-tidal PCO2 = 6.5 Torr above prestudy value, end-tidal PO2 = 100 Torr) followed by 8-h poikilocapnic euoxia; and 2) control, where the inspired gas was air. VE was measured over a 5-min period before the experiment and then hourly over a 16-h period. In the hypercapnia protocol, VE had not reached a steady state by the first hour (P < 0.001, analysis of variance), but there were no further significant differences in VE over hours 2-8 (analysis of variance). VE fell promptly on return to eucapnic conditions. We conclude that, whereas there is a component of the VE response to hypercapnia that is slow, there is no progressive rise in VE throughout the 8-h period.

contribute to the enhancement of AHVR in acclimatiza- tion to altitude. The purpose of the present study was to investigate whether such a reduction in the activity of inhibitory dopaminergic mechanisms could also be detected in humans. The study compared the effects of dopaminer- gic blockade on AHVR with and without a prior period of sustained hypoxia. To determine whether there are any changes in the sensitivity to dopamine, the study also compared the inhibitory effects on AHVR of low- dose dopamine infusions with and without prior sus- tained hypoxia. METHODS Subjects. Eleven healthy adults, seven men and four women, were studied. Their average age was 23.4 6 1.1 (SD) yr,