VENTILATORY SENSITIVITY TO CO2 IN HYPEROXIA AND HYPOXIA IN OLDER AGEDHUMANS

Findings from studies of the effects of aging on the human respiratory controller are equivocal. This study assessed the ventilatory respons e to CO2 in hyperoxia and hypoxia in groups of younger (YS) and older (OS) humans. Two protocols were used. In the first, end-tidal PCO2 (PE T(CO2)) was clamped at 1-2 Torr above rest (eucapnia), and, in the sec ond, PET(CO2) was clamped at 7-8 Torr above resting PET(CO2) (moderate hypercapnia). End-tidal PO2 was clamped at 100 Torr throughout except for two 2-min periods at 500 and 50 Torr. The ventilatory responses f or each subject at each PO2 were fitted to the linear equation, VE = S (PET(CO2) - B), where VE is minute ventilation, S is the response curv e slope, and B is the response curve threshold. In eucapnia, there wer e no differences in hypoxic and hyperoxic VE between YS and OS. In hyp ercapnia, hypoxic VE was 24% lower in OS [39.93 +/- 2.71 (SE) l/min] t han in YS (52.16 +/- 3.17 l/min). In hypoxia, S was significantly lowe r in OS (3.25 +/- 0.38 l.min-1.Torr-1) than in YS (4.76 +/- 0.37 l.min -1.Torr-1). We conclude that, in older humans, VE is lower in hypoxia during moderate hypercapnia, resulting mainly from a decreased periphe ral chemoreflex CO2 sensitivity.