Cerebrovascular mechanisms in neurocardiogenic syncope with and without postural tachycardia syndrome

Background and purpose: Recent transcranial Doppler studies in patients wit h neurocardiogenic syncopes (NCS) have demonstrated that the cerebrovascula r response to sudden systemic hypotension is vasoconstriction instead of co mpensatory vasodilation (autoregulation). We tried to characterize the cond itions leading to this unexpected response in NCS patients further by conti nuously monitoring autoregulation and autonomic parameters during a standar dized tilt-table test (TTT). Methods: Sixteen patients below the age of 50 years with a history of at least three syncopes of undetermined cause and t ilt-table verified NCS and 20 normal controls were studied. Arterial blood pressure (ABP) and heart rate (HR) were monitored by Finapres and cerebral blood flow velocity (CBFV) of the left middle cerebral artery by transcrani al Doppler. Baroreflex sensitivity and autoregulation parameters were measu red continuously, using cross-spectral analysis of Mayer waves (3-9 cycles per minute oscillations) in ABP, HR and CBFV, respectively. Pulsatility ind ices (PI) of CBFV and ABP were determined continuously. Measurements were t aken during 5 min in supine and during 5 min in tilted position. In patient s, tilting was continued for a maximum of 45 min until the onset of syncope or presyncope. Results: According to the maximum increase in heart rate (D elta HR) during the first 5 min of standing, heart rate responses were clas sified as postural tachycardia syndrome (POTS) (Delta HR > 35/min) or as no rmal. Only one out of 20 control subjects showed a POTS (5%) in contrast to seven patients (44%). Patients with a POTS had significantly lower PI valu es in ABP and higher ratios between the PI of CBFV and the PI of ABP both i n supine and in tilted positions. Baroreflex sensitivity during standing de creased significantly in POTS patients when compared to controls. Although autoregulation remained intact during standing, mean CBFV decreased signifi cantly and continuously. The nine patients without a POTS showed almost the same cardiovascular and cerebrovascular responses as the control subjects. All 16 patients showed similar circulatory responses during syncope (sudde n hypotension, relative or absolute bradycardia, reduced CBFV and increased PI in CBFV). Conclusions: The development of a POTS during tilting indicat es a high risk for fainting. The characteristic hemodynamic features in the initial phase of standing in these patients can be interpreted in terms of central hypovolemia (low PI of ABP) with sufficient ABP regulation and inc reased cerebrovascular resistance (defined as the ratio between PI of CBFV and ABP). Cerebral autoregulation seems not to be affected in patients suff ering from NCS. (C) 1999 Elsevier Science B.V. All rights reserved.