SummaryA technique employing a Doppler ultrasound probe to measure cerebral blood velocity was used to study the cerebral circulation continuously in 30 newborn mongrel dogs. Utilizing a transfontanelle approach, the probe was maintained in fixed position throughout a given experiment. In 20 animals, changes in systolic, diastolic, and mean cerebral blood velocity during hypo-and hypercarbia were directly correlated ( P < 0.01) with changes in regional cerebral blood flow (rCBF) determined in 12 regions of the brain by the ['4C]iodoantipyrine autoradiography technique. In an additional 10 dogs, multiple determinations of systolic, diastolic, and mean blood velocity were made over a wide range of Paco2 values and found to be directly related to the Paco2 ( P < 0.001). These data suggest that changes in cerebral blood velocity are closely related to changes in cerebral blood flow. We also calculated the pulsatility index (PI) from the peak systolic and end diastolic velocities and found a poor, but direct (r = 0.28, P < 0.05) relationship between the PI and Paco2 rather than the indirect relationship, which has been suggested in published clinical studies. We conclude that the Doppler technique may be valuable in monitoring dynamic events of the neonatal cerebral circulation if a constant probe position is maintained. Our results suggest, however, that the PI is not a reliable index of cerebral vascular resistance.Abbreviations AUTC, area under the curve CBF, cerebral blood flow rCBF, regional cerebral blood flow CVR, cerebral vascular resistance PI, pulsatility index Currently there is no acceptable noninvasive method of assessing instantaneous changes in CBF in human newborns. The experimental methods for determining CBF in animals using electromagnetic flowmeters (2 1 ), radioactive tracers (3 1 ), and microspheres (30) clearly are not adaptable to humans. The injection or inhalation of radioactive xenon has been used in human adults (13,22) and newborns (19,20), although its use in infants is limited because the technique is invasive and involves a radioactive substance. Occlusion plethysmography in newborn infants, although noninvasive, includes extracranial as well as intracranial blood flow (4,5,17). With all of these methods only single or intermittent determinations can be made, and instantaneous changes in CBF cannot be determined.Recently, transcutaneous Doppler ultrasound has been used to study the cerebral circulation in human adults (8, 28) and newborns ( I ) by determining changes in blood velocity. The advantages of this technique are that it is noninvasive and provides continuous information. Its limitations include the difficulty of standardizing the angle between the Doppler probe and the blood vessel, and defining the relationship between blood velocity and blood flow.Bada et al. (1) were the first investigators to recognize the value of Doppler ultrasound in monitoring cerebral hemodynamics through the open anterior fontanelle in the human newborn. They attempted to minimize the Doppler's lim...