Caffeine can affect velocity in the middle cerebral artery during hyperventilation, hypoventilation, and thinking: A transcranial Doppler study

This study examined possible caffeine-mediated changes in blood flow veloci ty in the middle cerebral artery (VMCA) induced by tests of cerebrovascular responsiveness. Transcranial Doppler (TCD) sonography provided simultaneou s bilateral VMCA measures while healthy college students hypoventilated, hy perventilated, and performed cognitive activities (short-term remembering, generating an autobiographical image, solving problems), each in 31-second tests. VMCA measures were obtained from the same persons, in separate testi ng sessions, when they were noncaffeinated and under two levels of caffeine : a smaller amount (from a cola, 45 mg/12 oz) and a larger amount (from cof fee, 117 mg/8 oz). Compared with the no-caffeine control condition, a small er amount of caffeine had no significant effects on global VMCA, but a larg er amount suppressed VMCA by 5.8%. Time-course analyses showed that VMCA (1 ) followed a triphasic pattern to increase over baselines during hypoventil ation regardless of caffeine condition, (2) slowed below baselines during h yperventilation (with the degree of slowing attenuated under caffeine), and (3) increased over baselines during all cognitive activities (ranges 3.8-6 .9%). It is concluded that a large amount of caffeine can suppress VMCA, an d this possibility should be anticipated when TCD is used to assess cerebra l hemovelocity.