Impaired arterial baroreceptor sensitivity before tilt-induced syncope

Autonomic dysfunction seems to play a central role in the pathophysiology of neurocardiogenic syncope (NCS) but conflicting data have recently become available. We evaluated autonomic nervous system (ANS) function (heart rate variability (HRV), systolic blood pressure variability (SBPV) and baroreceptor gain (BRG)) and non-invasive haemodynamics (cardiac output and total peripheral resistance) in patients with neurocardiogenic syncope. Retrospectively, we evaluated 12 NCS patients (positive head-up tilt without pharmacological provocation) in the basal state and at initial tilt, 12 non-NCS patients with tilt-negative syncope and 12 aged-matched normal controls. Prospectively, we evaluated 16 NCS patients to analyse the haemodynamics and ANS activity throughout the tilt test (beginning of tilt and before syncope occurs). HRV and SBPV were accessed by fast Fourier transforms (FFT) and spontaneous BRG by temporal sequences, slope and a index. Modelflow was used to quantify the non-invasive haemodynamics. None of the autonomic and haemodynamic parameters at baseline or in the first 10 min of tilt was different among the respective NCS, non-NCS syncope and normal control groups, except for SBP, which was higher at baseline in controls. Throughout the tilt test in the prospective NCS group, the heart rate increased (88-95 beats x min(-1), P<0.05), systolic blood pressure decreased (123-109 mmHg, P<0.01), and arterial baroreceptor gain was reduced (7.6 to 5.5 ms mmHg(-1), P<0.01) and the absolute high frequency component of HRV (HF HRV) decreased (150-80 ms(-2), P<0.05), before syncope occurred. There was no change in the low frequency component of HRV (LF HRV), SBPV, cardiac output (CO) or total peripheral resistance (TPR). Tilt-induced syncope could not be predicted by non-invasive haemodynamic or autonomic parameters at rest or in the initial minutes of tilt. The decrease in arterial baroreceptor gain could be a precocious expression of the transient autonomic dysfunction that characterizes the occurrence of neurocardiogenic syncope.