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

Ogoh S, Ainslie PN, Miyamoto T. Onset responses of ventilation and cerebral blood flow to hypercapnia in humans: rest and exercise. J Appl Physiol 106: 880-886, 2009. First published January 8, 2009; doi:10.1152/japplphysiol. 91292.2008.-The respiratory and cerebrovascular reactivity to changes in arterial PCO2 (Pa-CO2) 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 (V-mean) to hypercapnia (5.0% CO2-40% O-2-balance N-2) at rest and during dynamic exercise (similar to 1.01/min O-2 consumption). Each onset response was evaluated using a single-exponential regression model consisting of the response time latency [CO2-response delay (t(0))] and time constant (tau). At rest, t(0) and tau data indicated that the MCA V-mean onset response was faster than the ventilatory ((V) over dot E) response (P < 0.001). In contrast, during exercise, t(0) of (V) over dot E and MCA V-mean onset responses were decreased. In addition, despite the enhanced Pa-CO2 response to CO2 administration (P = 0.014), tau of MCA V-mean tended to increase during exercise (P = 0.054), whereas tau of (V) over dot(E) 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 (V) over dot E onset response, and 2) during exercise, despite higher rates of increasing Pa-CO2, the lack of change in the onset response of cerebral blood flow and reduced washout of CO2 may act to augment the (V) over dot E onset response.