Investigation of the time delay between variations in heart rate and blood pressure

Relationships between fast variations in heart rate and blood pressure are known mainly to reflect parasympathetic activity. To assess parasympathetic function, relationships are investigated using a mutual information analysis, by assuming an information channel between blood pressure (BP) and inter-R-wave interval (RR) derived from the electrocardiogram. The differential sequences of BP and RR are coded into 'alphabets' for analysing fast variations. The averaged mutual information (AMI) is obtained as a function of delay d (beats) of the differential sequence of RR with reference to BP. The AMIs for supine and standing positions are compared. For 23 patients with renal failure, the AMI against d shows posture-independent and delayed-transfer characteristics from fast variations in BP to those in RR. The closest relationship is found with a 2-beat delay with an out of phase relationship. The 28 control subjects exhibit the 2-beat delayed transfer characteristics in the standing position, but are almost synchronous in the supine position. The mutual information analysis of the delay between fast variations in BP and RR explicitly shows a deficit in the parasympathetic regulation in the patients with renal failure.