Predictive value of very low frequency at spectral analysis among patients with unexplained syncope assessed by head-up tilt testing

Background: The role of heart rate variability (HRV) in the prediction of vasovagal syncope during head-up tilt testing (HUTt) is unclear. Aim: To evaluate the ability of the spectral components of HRV at rest to predict vasovagal syncope among patients with unexplained syncope referred for HUTt. Methods: Twenty-six consecutive patients with unexplained syncope were enrolled in the study. All patients underwent HRV evaluation at rest (very low frequency [VLF], low frequency [LF], high frequency [HF] and LF/HF ratio) and during HUTt. HUTt was performed using the Westminster protocol. Continuous electrocardiogram and blood pressure monitoring were performed throughout the test. Results: Eight (31%) patients developed syncope during HUTt. There were no baseline differences in terms of clinical features and HRV variables among patients who developed syncope and those who did not, except for VLF (2421 vs 896ms2; P<0.001). In the multivariable logistic regression analysis, including age and sex, VLF was the only independent variable associated with syncope during HUTt (odds ratio 1.002, 95% confidence interval 1.0003-1.0032; P=0.02). The area under the curve at rest was 0.889 for VLF, 0.674 for HF and 0.611 for LF. A value of VLF>2048ms2 was the optimal cut-off to predict syncope during HUTt (sensitivity 87.5%, specificity 72.2%). Conclusions: VLF at rest predicted the incidence of syncope during HUTt. Further studies are warranted to confirm these preliminary data.