Cerebral blood flow during vasovagal syncope induced by active standing or head up tilt

Vasovagal syncope is usually associated with a sudden drop of blood pressure and/or heart rate. However, occasionally the symptoms of syncope induced by orthostatic stress testing are not associated with obvious haemodynamic changes. The mechanisms of syncope in these patients are not clear.

To evaluate changes in cerebral blood flow velocities during orthostatic stress testing in children and adolescents with vasovagal syncope.

Electrocardiogram, instantaneous arterial blood pressure, and right middle cerebral artery blood flow velocity were recorded at rest, during active standing, and 80 degrees head up tilt. 32 children and adolescents aged between 7 and 18 years with a history of repeated vasovagal syncope and 23 healthy control subjects were studied.

Presyncope occurred in 10 patients during standing, and 13 patients during head up tilt. None of the controls had symptoms during the test. The transcranial Doppler study showed that the symptoms were associated with significant decreases of diastolic cerebral blood flow velocity and an increase of pulsatility. There was no significant change of the systolic cerebral blood flow velocity. The changes of cerebral blood flow velocities occurred in all episodes of presyncope, including those not associated with severe drop of blood pressure or heart rate.

Diastolic cerebral blood flow velocity decreased significantly during episodes of presyncope induced by orthostatic stress. Impairment of autoregulation of cerebral blood flow might play an important role in the pathophysiology of syncope.