Evaluation of the cerebral hemodynamic response to rhythmic handgrip

The response of the cerebral circulation to exercise has been studied with transcranial Doppler ultrasound (TCD) because this modality provides contin uous measurements of blood velocity and is well suited for the exercise env ironment. The use of TCD as an index of cerebral blood flow, however, requi res the assumption that the diameter of the insonated vessel is constant. H ere, we examine this assumption for rhythmic handgrip using a spectral inde x designed to measure trends in vessel flow. Nineteen normal subjects were studied during 5 min of volitional maximum rhythmic right handgrip at 1 Hz. TCD velocities from both middle arteries (left and right), blood pressure, and end-tidal Pco(2) were recorded every 10 s. A spectral weighted sum was also calculated as a flow index (FI). Averages were computed from the last 2 min of handgrip. Relative changes in velocity, FI, and pressure were cal culated. The validity of Fl was tested by comparing the change in diameter derived from equations relating flow and diameter. Mean blood pressure incr eased 23.8 +/- 17.8% (SD), and velocity increased 13.3 +/- 9.8% (left) and 9.6 +/- 8.3% (right). Although the mean change in FI was small [2.0 +/- 18. 2% (left) and 4.7 +/- 29.7% (right)], the variation was high: some subjects showed a significant increase in Fl and others a significant decrease. Dia meter estimates from two equations relating flow and luminal area were not significantly different. Decreases in FI were associated with estimated dia meter decreases of 10%. Our data suggest that the cerebral blood flow (CBF) response to rhythmic handgrip is heterogeneous and that middle cerebral ar tery flow can decrease in some subjects, in agreement with prior studies us ing the Kety-Schmidt technique. We speculate that the velocity increase is due to sympathetically mediated vasoconstriction rather than a ubiquitous f low increase. Our data suggest that the use of ordinary TCD velocities to i nterpret the CBF response during exercise may be invalid.