CEREBRAL HEMODYNAMIC-CHANGES INDUCED BY SYMPATHETIC-STIMULATION TESTS

Sympathetic neuronal activity is primarily responsible for the neuroge nic control of cerebral autoregulation The stimulation of sympathetic nerves causes both large arterial constriction and small vessel dilati on in experimental animals. However, the role of the sympathetic nervo us system in the control of cerebral hemodynamics has yet to be clarif ied in humans. In order to assess the effect of sympathetic activation on human cerebral hemodynamics, we performed a simultaneous transcran ial Doppler (TCD) monitoring of bilateral middle cerebral arterial flo w velocity in 16 healthy male volunteers (mean age 26) during well-kno wn sympathetic activation measures such as isometric hand-grip exercis e (IHE) and cold presser test (CPT). Blood pressure was checked manual ly before and at each minute during tests. The mean arterial pressure (MAP) was calculated as (systolic pressure +2 X diastolic pressure)/3. There was a significant increase in MCA flow velocities during both s ympathetic activation tests. The percent increase of diastolic velocit y (36% with IHE and 24% with CPT) was significantly higher than systol ic velocity (21% with IHE and 9% with CPT). The pulsatility index was significantly decreased during the tests (from 0.75 to 0.58 with IHE a nd from 0.81 to 0.63 with CPT). These results suggest that sympathetic activation increases MCA flow velocities, related with a reduction in small vessel resistance and/or a constriction of large arteries.