Two approaches to estimating the relative dynamic stability of physiological processes

We studied the mutual dynamics of systolic blood pressure and pulse intervals. We used two approaches to estimate their stability, based either of phase difference analysis or on time delay stability analysis. In the first approach, we determined the intervals where the time delay of the maximum of the absolute value of the cross-correlation function of two analyzed processes remains stable. In the second approach, we focused on the average correlation time of the phase differences of two processes as a measure of their time delay stability. The first method takes into account only the linear interaction of the processes while the second method represents more sophisticated mechanisms of their interaction. Our results indicate that moderate correlation between the stability coefficients obtained by two different approaches can be obtained. We have also shown that the stability coefficients differ significantly in healthy volunteers and in diabetic patients. We suggest that these stability measures might appear useful as complementary indicators of the efficacy of short-term blood pressure regulation.