Evaluation of the correlation between cardiac and sympathetic baroreflex sensitivity before orthostatic syncope

The study investigates the two different aspects of the baroreflex control resulting in two baroreflex sensitivity (BRS) indexes: i) sympathetic BRS (sBRS); ii) cardiac BRS (cBRS). sBRS was assessed as the slope of the regression line of the conditional probability of detecting a burst on the integrated muscle nerve sympathetic activity (MSNA) given an assigned diastolic arterial pressure (DAP) on DAP. cBRS was estimated from spontaneous heart period (HP) and systolic arterial pressure (SAP) via a spectral approach in the low (0.04-0-15 Hz) and high (0.15-0.5 Hz) frequency bands respectively. Both sBRS and cBRS were assessed in eight healthy subjects undergoing three experimental sessions: supine resting position (REST), 80 degrees head-up tilt test (TILT) and before the occurrence of pre-syncope symptoms (TILT-PRE). Results showed a decrease of both sBRS and cBRS during TILT and a baroreflex impairment during TILT-PRE. sBRS and cBRS were linearly correlated during TILT but became uncorrelated during TILT-PRE. Findings suggest a failure of both 'baroreflexes' and their disassociation during TILT-PRE.