Middle Cerebral Artery Blood Flow Velocity Studied during Quiet Breathing, Reflex Hypercapnic Breathing and Volitionally Copied Eucapnic Breathing in Man

The cerebrovascular circulation can be assessed non-invasively in man using transcranial doppler (TCD) velocimetry (1). The change in flow velocity pulse wave is proportional to the change in blood flow in that vessel, assuming cross sectional area of the artery studied to be constant. Such changes have been shown to correlate well with changes in cerebral perfusion in response to hypercapnia (3). TCD estimates of cerebral blood flow is increased when subjects performed both physical and cognitive activities (6) or when they were presented with visual stimuli (2). This suggests that TCD is capable of detecting changes in cerebral blood flow which accompany cerebral activity. In this study we have tested the hypothesis that volitionally controlled breathing is accompanied by an increase in middle cerebral artery (MCA) blood flow velocity (bfv) independent of changes in PCO2.