Characterization of multivariate cardiovascular dynamics during head-up tilting based on Kullback-Leibler divergence

In this paper, multivariate cardiovascular dynamics during head-up tilting is investigated as an augmenting process of sympathetic activities. Characteristically in spectral profiles, a respiration-induced rhythm in each variable is reduced, and alternatively the Mayer wave components in blood pressure and heart rate signals become distinct, as a tilting angle increases. These spectral features are shown to be originated from closed-loop impulse responses which show the longer lasting response from HR to HR and the delayed response from HR to BP in contrast with those from BP to HR and from BP to BP which exhibit no systematic changes. The newly introduced Kullback-Leibler divergence consistently differentiates tilting-angle-dependent multivariate cardiovascular dynamics. The results here suggest that the Kullback-Leibler divergence could provide synthetic characterization of the autonomic regulations