Early syncope detection during head up tilt test by analyzing interactions between cardio-vascular signals

Head up tilt test is a well-known medical procedure used to diagnose vasovagal syncope. Several studies have focused on the early prediction of the outcomes of this test to predict syncope before its occurrence. Therefore, the evaluation of the dynamic interactions between several cardiovascular variables can play an important role to improve our understanding on the function of the baroreflex control of arterial blood pressure and can help to enhance the prediction performance. In this aim, the smoothed pseudo-Wigner-Ville distribution (SPWD) approach was used to characterize the time-frequency relationship between time series extracted from ECG (RR-interval) and blood pressure (amplitudes of the systolic blood pressure, the peak amplitude of the first derivative of the blood pressure, dP/dt and the variability of the pulse transit time, PTT). Two new indexes quantifying the relationship between time series were introduced. The first index computed the distribution of time-frequency coherence between 2 time series in different frequency bands and the second one computed the scalar product of the time-frequency spectra of these time series. This approach was applied to predict the outcome of a head up tilt test (HUTT) based on the first 15 min, to early detect subjects that would faint or not during this test. Our results demonstrate that the strongest parameter to predict HUTT result is the dynamic interaction between RR-interval and the amplitude of systolic blood pressure (AmpS), using the scalar product of the auto time-frequency spectra computed with SPWD (sensitivity: 87.5%; specificity: 93.8%). This study also shows that in high frequency band, the similarity between the RR-interval and the AmpS time series is higher during the first 15 min of HUTT in subjects that would faint. As a clinical viewpoint, these results could bring new perspectives to better understand physiological mechanisms during HUTT and syncope, which are still unclear and could lead also to a reduction of HUTT duration, which is currently between 30 and 45 min.