Onset responses of ventilation and cerebral blood flow to hypercapnia in humans: rest and exercise

Abstract

The respiratory and cerebrovascular reactivity to changes in arterial Pco2 ([Formula: see text]) is an important mechanism that maintains CO2 or pH homeostasis in the brain. It remains unclear, however, how cerebrovascular CO2 reactivity might influence the respiratory chemoreflex. The purpose of the present study was therefore to examine the interaction between onset responses of the respiratory chemoreflex and middle cerebral artery (MCA) mean blood velocity ( Vmean) to hypercapnia (5.0% CO2-40% O2-balance N2) at rest and during dynamic exercise (∼1.0 l/min O2 consumption). Each onset response was evaluated using a single-exponential regression model consisting of the response time latency [CO2-response delay ( t0)] and time constant (τ). At rest, t0 and τ data indicated that the MCA Vmean onset response was faster than the ventilatory (V̇e) response ( P < 0.001). In contrast, during exercise, t0 of V̇e and MCA Vmean onset responses were decreased. In addition, despite the enhanced [Formula: see text] response to CO2 administration ( P = 0.014), τ of MCA Vmean tended to increase during exercise ( P = 0.054), whereas τ of V̇e decreased ( P = 0.015). These findings indicate that 1) at rest, faster washout of CO2 via cerebral vasodilation results in a reduced activation of the central chemoreflex and subsequent reduced V̇e onset response, and 2) during exercise, despite higher rates of increasing [Formula: see text], the lack of change in the onset response of cerebral blood flow and reduced washout of CO2 may act to augment the V̇e onset response.