Impaired arterial baroreflex function before nitrate-induced vasovagal syncope during head-up tilt test

The aim of this study was to evaluate arterial baroreflex control of heart rate immediately before head-up tilt test (HUT)-induced vasovagal syncope (VVS).

We enrolled 97 otherwise healthy subjects with recurrent unexplained syncope. After 10 min of rest in supine position, they underwent a passive HUT potentiated with nitroglycerin administration after 20 min. Beat-to-beat heart rate and systolic blood pressure were continuously recorded. Sequence method was used to measure two complementary parameters reflecting arterial baroreflex control of heart rate: the baroreflex sensitivity (BRS) and the baroreflex effectiveness index (BEI). Twenty-one patients fainted before nitrate administration (HUT+) and 37 after nitrate administration (NTG+). Immediately before syncope, the NTG+ patients showed significantly lower BRS values than those observed at the end of the test in the patients without syncope (5.5 +/- 2.8 vs. 7.7 +/- 3.4 ms/mmHg; P = 0.004) and a significantly lower BEI (30 +/- 20% vs. 53 +/- 24%; P < 0.001). The HUT+ patients did not show any significant differences in BRS and BEI before syncope from the values observed during the corresponding tilt period in the other groups.

A significant depression in BRS and BEI occurs immediately before syncope in patients who faint after nitrate administration, thus suggesting that arterial baroreflex dysfunction plays a role in mediating nitrate-induced VVS.