Changes in the linear relationship between cardiovascular parameters and neural sympathetic discharge variability before orthostatic syncope

To evaluate the cardiovascular parameters spontaneous fluctuations and neural sympathetic discharge variability relationship during the gravitational stimulus, seven healthy males underwent ECG, blood pressure, respiratory and post-ganglionic sympathetic discharge activity (MSNA) recordings. Data were recorded at rest and during a 15-minute 80°head-up tilt followed by a step wise lower body negative pressure increase up to the onset of presyncope. Spectral and coherence analyses of systolic arterial pressure (SAP) and MSNA variability provided the indexes of vessels sympathetic modulation in the low frequency (LF) band, LF_SAP, and of the coupling between MSNA and SAP at LF, K²_MSNA-SAP(LF), during asymptomatic (T1) and presyncope (T2) periods of tilt. During T2, RR interval, SAP, LFmsna, LF_SAP and K²_MSNA-SAP(LF) values decreased compared to T1. in the presence of unchanged MSNA values. This result suggests that a significant coupling between SAP and MSNA variability in the 0.1 Hz band is mandatory for a proper orthostatic tolerance.