Observer of the human cardiac sympathetic nerve activity using noncausal blind source separation

We present a novel method for the blind reconstruction of the cardiac sympa thetic nerve activity (CSNA) in the low-frequency (LF) band (0.04-0.15 Hz) using only heart rate and arterial blood pressure. The originality of the m ethod consists in the application of blind source separation techniques to obtain an observer of CSNA, We show how this observer can be deduced from a linear model of the cardiovascular system by introduction of the fundament al assumptions about the independence of the cardiac sympathetic an parasym pathetic outflow. In cardiovascular applications, the reliability of the observer has been as sessed by verification of the fundamental assumption for the given data. A primer qualitative validation has been performed using the muscle sympathet ic nerve activity as an indirect indicator of CSNA, Very satisfying and pro mising results have been obtained. Moreover, we have performed quantitative validations of the observer in var ious experimental conditions known to elicit selectively cardiac sympatheti c or parasympathetic response, The experimental conditions include a supine -to-60 degrees tilt test, indirect sympathetic stimulation/inhibition by me dication, and sympathetic stimulation by isometric handgrip, We show that t he observer allows to highlight changing levels of the cardiac sympathetic activity in the LF band in all these experimental conditions.