We studied the response of cerebral blood flow to acute step decreases in arterial blood pressure noninvasively and nonpharmacologically in 10 normal volunteers during normocapnia, hypocapnia, and hypercapnia. The step (approximately 20 mm Hg) was induced by rapidly deflating thigh blood pressure cuffs following a 2-minute inflation above systolic blood pressure. Instantaneous arterial blood pressure was measured by a new servo-cuff method, and cerebral blood flow changes were assessed by transcranial Doppler recording of middle cerebral artery blood flow velocity. In hypocapnia, full restoration of blood flow to the pretest level was seen as early as 4.1 seconds after the step decrease in blood pressure, while the response was slower in normocapnia and hypercapnia. The time course of cerebrovascular resistance was calculated from blood pressure and blood flow recordings, and rate of regulation was determined as the normalized change in cerebrovascular resistance per second during 2.5 seconds just after the step decrease in blood pressure. The reference for normalization was the calculated change in cerebrovascular resistance that would have nullified the effects of the step decrease in arterial blood pressure on cerebral blood flow. The rate of regulation was 0.38, 0.20, and 0.11/sec in hypocapnia, normocapnia, and hypercapnia, respectively. There was a highly significant inverse relation between rate of regulation and PacOj (p<0.001), indicating that the response rate of cerebral autoregulation in awake normal humans is profoundly dependent on vascular tone. (Stroke 1989;20:45-52) C erebral autoregulation is a homeostatic mechanism that minimizes deviations in cerebral blood flow (CBF) when cerebral perfusion pressure (CPP) changes. Cerebral autoregulation acts through vasomotor effectors that control cerebrovascular resistance (CVR). Previous studies have convincingly documented the ability of this physiologic system to maintain relatively constant CBF when CPP is within the range 50-170 mm Hg. 1 -6 Because this is a very fast-acting homeostatic mechanism, 7 the method for measuring autoregulatory responses should ideally have good time resolution. Our knowledge about the dynamic response of cerebral autoregulation in humans is limited because most indicator methods permit sampling of regional CBF at intervals of only minutes. Animal experiments 7 -10 indicate that the main autoregulatory response is produced within seconds. Measurement of dynamic response times and response rates to characterize the effectiveness of autoregulation is of interest not only from a physi-

In this report the authors describe a noninvasive transcranial method of determining the flow velocities in the basal cerebral arteries. Placement of the probe of a range-gated ultrasound Doppler instrument in the temporal area just above the zygomatic arch allowed the velocities in the middle cerebral artery (MCA) to be determined from the Doppler signals. The flow velocities in the proximal anterior (ACA) and posterior (PCA) cerebral arteries were also recorded at steady state and during test compression of the common carotid arteries. An investigation of 50 healthy subjects by this transcranial Doppler method revealed that the velocity in the MCA, ACA, and PCA was 62 +/- 12, 51 +/0 12, and 44 +/- 11 cm/sec, respectively. This method is of particular value for the detection of vasospasm following subarachnoid hemorrhage and for evaluating the cerebral circulation in occlusive disease of the carotid and vertebral arteries.

These data show that in normal human subjects measurement of dynamic autoregulation yields similar results as static testing of intact and pharmacologically impaired autoregulation.