FLOW VELOCITY-MEASUREMENTS AS AN INDEX OF CEREBRAL BLOOD-FLOW - VALIDITY OF TRANSCRANIAL DOPPLER SONOGRAPHIC MONITORING DURING CARDIAC-SURGERY

Background: Transcranial Doppler sonography is increasingly used to mo nitor changes in cerebral perfusion intraoperatively. However, little information is available about the validity of velocity measurements a s an index of cerebral blood now (CBF). The purpose of this study was to compare invasive and Doppler-derived measurements of cerebral hemod ynamic variables during coronary artery bypass graft surgery. Methods: In 15 male patients, measurements of CBF and middle cerebral artery f low velocity (V-MCA) were performed before and after induction of fent anyl-midazolam anesthesia, during hypothermic cardiopulmonary bypass ( CPB), and at the end of the surgical procedure. Transcranial Doppler s onography recordings of systolic, diastolic, and mean V-MCA, and deriv ed parameters such as pulsatility (PI) and resistance (RI) indexes wer e recorded from the proximal segment of the right middle cerebral arte ry. CBF was measured by the Kety-Schmidt inert gas saturation method w ith argon as a tracer. To facilitate comparisons of CBF and V-MCA meas urements, changes between consecutive measurements were expressed as p ercentage values. Calculations of cerebral perfusion pressure and cere bral vascular resistance (CVR) were based on jugular bulb pressure. Th e cerebral metabolic rate for oxygen was calculated from CBF and the a rterial - cerebral venous oxygen content difference. Results: Changes in mean V-MCA paralleled changes in mean CBF except for hemodynamic ch anges associated with hypothermic CPB. At this stage of surgery, mean V-MCA increased while actual CBF decreased. Separate analysis of the p eriods before and after CPB revealed a poor association between percen tage changes in CBF and V-MCA (r = 0.26, P = 0.36; r = 0.51, P = 0.06, respectively). Mean values of CVR, PI, and RI showed consistent chang es after induction of anesthesia. After termination of CPB, mean CVR s ignificantly decreased, whereas mean PI and RI remained virtually unch anged. Neither before nor after CPB was a clinically useful correlatio n found between percentage changes in PI, RI, and CVR (PI r = 0.28, P = 0.34; r = -0.47, P = 0.03, respectively; RI r = 0.16, P = 0.59; r = -0.53, P = 0.06, respectively). Conclusions: Hypothermic CPB seems to alter the relation between global CBF and flow velocity in basal cereb ral arteries. Inconsistency in directional changes in CBF and V-MCA at this stage of surgery might be attributable to changes in middle cere bral artery diameter, red blood cell velocity spectra, and regional no w distribution. Although changes in mean V-MCA before and after CPB ap pear to parallel changes in mean CBF, individual responses of V-MCA ca nnot reliably predict percentage changes in CBF. Furthermore, Doppler sonographic PI and RI cannot provide an approximation of changes in CV R during cardiac surgery.