[Circulatory changes of the middle cerebral artery during syncopes produced by the head-up tilt test]

Velocity of the middle cerebral artery was recorded with transcranial pulsed doppler in 10 neurally-mediated syncopes elicited by 60 degrees head-up tilt test. Mean arterial blood pressure decreased by -36 +/- 16 percent (P < 0.0001) and heart rate by -6 +/- 27 percent (P < 0.05). Mean middle cerebral artery velocity decreased by -48 +/- 16 percent (P < 0.0001); Pourcelot's resistance index increased by 74 +/- 47 percent (P < 0.0001) and Gosling's pulsatility index by 218 +/- 111 percent (P < 0.0001). However, resistance normalized with mean arterial pressure showed no significant increase (+ 14 +/- 28 percent; NS). The decrease in diastolic velocity as well as mean arterial blood pressure and heart rate was significantly lower when loss of consciousness occurred. Contrary to those of other authors, our results do not support the hypothesis that syncopes result from a paradoxical cerebral vasoconstriction since no significant changes were observed in resistance normalized with mean arterial pressure. Thus, decrease in cerebral perfusion is likely to be the result of a decrease in cerebral perfusion pressure.