Cardiovascular control during orthostatic and mental stress: Conditional entropy based analysis

The aim of this study was to characterize the changes in cardiovascular dynamics as a result of orthostatic and mental stress and their combination by linear and information domain analysis of heart rate and blood pressure oscillations. We recorded beat-to-beat RR intervals and systolic blood pressure values in 16 volunteers during mental arithmetics task and head-up tilt test and their simultaneous administration. The analysis included spectral measures and quantification of cardiovascular signals complexity and their mutual causal coupling. Our results demonstrated that orthostatic and mental stress challenges - despite similar heart rate and blood pressure changes - evoked different effects on cardiovascular control system. The novel conditional entropy based measures were sensitive to detect differences in heart rate and blood pressure dynamics responses evoked by mental stress in different body positions.