The effect of an acute increase in central blood volume on hypercapnia‐induced attenuation in dynamic cerebral autoregulation

Abstract

It is well established that hypercapnia attenuates dynamic cerebral autoregulation. The impact of central blood volume on cerebral blood flow is controversial. The purpose of the present study was to examine whether hypercapnia‐induced attenuation in dynamic cerebral autoregulation is modified by acute central hypervolemia. Nine young healthy subjects voluntarily participated in this study. In order to measure dynamic cerebral autoregulation during normocapnic and hypercapnic (5%) conditions, the change in middle cerebral artery mean blood flow velocity (MCA Vmean) was analyzed during acute hypotension caused by two methods: 1) thigh‐cuff occlusion release (without change in central blood volume) and 2) lower body negative pressure (‐50 mmHg, LBNP) release (with acute increase in central blood volume). As expected, hypercapnia decreased the rate of regulation, as an index of dynamic cerebral autoregulation (0.236±0.053 and 0.167±0.074 sec‐1, respectively, P=0.025). In contrast, this attenuation was disappeared during central hypervolemia (P=0.574). This phenomenon may be associated with hypervolemia‐induced attenuation of dynamic cerebral autoregulation (P=0.009). These findings suggest that an acute change in central blood volume modifies carbon dioxide‐induced changes in dynamic cerebral autoregulation.