Cerebral blood flow velocity during tilt table test for pediatric syncope

Background. Brain hypoperfusion during neurocardiogenic syncope develops as a consequence of hypotension and bradycardia. Transcranial Doppler indicat es that an increase in cerebral vascular resistance occurs before or during the loss of consciousness. Objective. Cerebral blood flow velocity was studied during tilt table testi ng in pediatric patients with neurocardiogenic syncope. We assessed whether a critical reduction in flow velocity (>40%) was predictive of the presync opal manifestations during the test. Methods. A 2-MHz transcranial Doppler measured blood flow velocity in the r ight middle cerebral artery in 27 pediatric patients (ages, 8 to 18 years) during a three-stage 80 degrees tilt table test protocol. A positive test r equired development of syncope or presyncope with at least 30% decrease in systolic blood pressure and/or heart rate relative to preceding values. Pat ients were divided into: group I (isoproterenol-induced positive tests), gr oup II (positive without isoproterenol), and group III (negative tests). Results. Within the first 3 minutes of the upright position mean cerebral b lood flow velocity in groups I, II, and III decreased by 18%, 29%, and 17%, respectively, as the systolic and diastolic blood pressures showed only mi nimal changes. A decreased mean blood flow velocity of 45% and 45% and an i ncrease in resistance index of 42% and 26% from supine values in the absenc e of hypotension, were detected in groups I and II at 46 seconds (range, 30 -120 seconds) and 50 seconds (range, 0-300 seconds) before any clinical sym ptom (presyncope latency). Mean blood flow velocity during presyncope decre ased by 58% and 59%, whereas resistance index was double. A significant cor relation (rho = -0.62) was found between presyncope latency and the decreas ed mean cerebral blood flow velocity. Similar blood flow velocity changes w ere not detected in group III. Conclusion. A sustained reduction >40% in mean cerebral blood flow velocity in the absence of hypotension always resulted in presyncopal or syncopal m anifestations. It seems that once this critical threshold is identified dur ing the tilt table testing, supine position may be resumed several seconds before the clinical manifestations of syncope.