Nonstationary time series analysis of heart rate variability
  An analysis of the RR-interval time series, $t_i$, is presented for the case in which the average time, $\bar{t}$, changes slowly. In particular, $\bar{t}$ and a short-time scale variability parameter, $V$, are simultaneously measured while $\bar{t}$ decreases for subjects in the reclined position. The initial decrease in $\bar{t}$ is usually linear with $V$ yielding parameters that can be related to physiological quantities. 
P.B. Siegel, J. Sperber, W. Kindermann, and A. Urhausen