Background: Brain hypoperfusion during neurocardiogenic syncope develops as a consequence of hypotension and bradycardia. Transcranial Doppler indicates 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 testing in pediatric patients with neurocardiogenic syncope. We assessed whether a critical reduction in flow velocity (>40%) was predictive of the presyncopal manifestations during the test.
Methods: A 2-MHz transcranial Doppler measured blood flow velocity in the right 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 required development of syncope or presyncope with at least 30% decrease in systolic blood pressure and/or heart rate relative to preceding values. Patients were divided into: group I (isoproterenol-induced positive tests), group II (positive without isoproterenol), and group III (negative tests).
Results: Within the first 3 minutes of the upright position mean cerebral blood flow velocity in groups I, II, and III decreased by 18%, 29%, and 17%, respectively, as the systolic and diastolic blood pressures showed only minimal changes. A decreased mean blood flow velocity of 48% and 45% and an increase in resistance index of 42% and 26% from supine values in the absence 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 symptom (presyncope latency). Mean blood flow velocity during presyncope decreased by 58% and 59%, whereas resistance index was double. A significant correlation (rho = -0.62) was found between presyncope latency and the decreased mean cerebral blood flow velocity. Similar blood flow velocity changes were 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 manifestations. It seems that once this critical threshold is identified during the tilt table testing, supine position may be resumed several seconds before the clinical manifestations of syncope.