Dynamics of the ventilatory response to step changes in end-tidal PCO2 in older humans.

The purpose of this study was to examine the ventilatory response to carbon dioxide (CO2) in young and older men. Six square-wave steps of end-tidal CO2 (PETCO2) were administered in euoxia (PETO2 = 100 torr), hyperoxia (PETO2 = 500 torr), and mild hypoxia (PETO2 = 60 torr). The peripheral and central chemoreflex loops were described by three parameters including a gain (gp and gc), time constant of the response (tau p, tau c), and a time delay (Tp, Tc), respectively. The young and older men showed similar characteristics for Tp and Tc, with Tp being 3 to 5 s shorter than Tc. In hypoxia, the ventilatory responses of the old group were characterised by a significantly smaller gc and a smaller gp. In hypoxia, tau c was significantly shortened from its euoxic value in the young group, but not in the old group. Thus, this study demonstrated that in older men, the ventilatory responses to CO2 in euoxia and hyperoxia are similar to younger men, while in hypoxia the ventilatory responses are characterised by smaller gain terms.