INTERACTIONS BETWEEN CO2 CHEMOREFLEXES AND ARTERIAL BAROREFLEXES

We studied interactions between CO2 chemoreflexes and arterial baroref lexes in 10 supine healthy young men and women. We measured vagal caro tid baroreceptor-cardiac reflexes and steady-state fast Fourier transf orm R-R interval and photoplethysmographic arterial pressure power spe ctra at three arterial pressure levels (nitroprusside, saline, and phe nylephrine infusions) and three end-tidal CO2 levels (3, 4, and 5%, fi xed-frequency, large-tidal-volume breathing, CO2 plus Oz). Our study s upports three principal conclusions. First, although low levels of CO2 chemoreceptor stimulation reduce R-R intervals and R-R interval varia bility, statistical modeling suggests that this effect is indirect rat her than direct and is mediated by reductions of arterial pressure. Se cond, reductions of R-R intervals during hypocapnia reflect simple shi fting of vagally mediated carotid baroreflex responses on the R-R inte rval axis rather than changes of baroreflex gain, range, or operationa l point. Third, the influence of CO2 chemoreceptor stimulation on arte rial pressure land, derivatively, on R-R intervals and R-R interval va riability) depends critically on baseline arterial pressure levels: ch emoreceptor effects are smaller when pressure is low and larger when a rterial pressure is high.