Cerebral blood flow is regulated by neurogenic, myogenic, and metabolic mechanisms. The sympathetic nervous system is responsible for the neurogenic mechanism of cerebrovascular autoregulation, but the effects of sympathetic activation on cerebral vessels have not yet been fully clarified. We performed transcranial Doppler (TCD) monitoring of the time-averaged maximum velocity (Vm) and pulsatility index (PI) in the middle cerebral artery during a sympathetic activation utilizing the cold pressor test (CPT) in two groups of eight healthy volunteers and ten elderly hypertensives. A ratio of mean arterial blood pressure (MABP) to Vm (MABP/Vm) was also calculated as an index of the resistance of the cerebrovascular bed. MABP significantly increased in both groups during CPT. Vm did not significantly change in either group. PI significantly decreased during CPT only in the healthy volunteers. During CPT, MABP/Vm increased to a greater extent in the group of elderly hypertensives than in the group of healthy volunteers. Despite increases in the MABP/Vm ratio, the PI decreased during CPT in healthy volunteers. Although PI has been used as an index of cerebrovascular resistance, a recent study has indicated that PI is correlated to the compliance of the proximal insonated artery. Our results suggest that the decrease in the PI observed during CPT in healthy volunteers reflects the reactivity of the proximal arteries, which regulate vascular tone.
