Non-linear coupling among cardiovascular variability signals in neuromediate syncope

Aim of this study is to evaluate the degree of coupling among the cardiovascular variability series and the respiration in subjects susceptible to neurally mediated syncope, in comparison to normal subjects. 21 patients susceptible to syncope and 10 sex and age matched control subjects have been enrolled in the study. ECG, respiration activity, and arterial blood pressure were simultaneously recorded at rest (controlled and free breathing) and during 70° head-up TILT test (free breathing). The degree of non-linear coupling among heart rate. variability (HRV), blood pressure variability (BPV) and respiration has been quantified by means of a multivariate embedding-based approach. 11 patients developed syncope during the TILT test. The non-linear coupling among the cardiovascular variables and respiration turned out to be stronger during controlled breathing than during free breathing, both for TILT-positive and TILT-negative group. During late TILT phase, in the TILT-positive group, the non-linear coupling showed a significant increase. If the proposed non-linear coupling indexes can be considered expression of the coupling mechanisms involved in the vagal regulation of the cardiovascular system, an increase of the vagal tone accompanied by a decrease of the sympathetic activity seem to occur before a vasovagal event.