How Far has Your Heart Travelled? Examining Heart System Trajectories in State Space.

This paper introduces a new measure to evaluate heart output from a dynamical systems approach. The measure is based on the time delay technique for two-dimensional state space reconstruction from time series of interbeat intervals. The system's trajectories within this space are depicted and the mean distance, as well as the total and maximum distances travelled by the system, are calculated in pixels. Preliminary data from adolescents with highly positive emotional regulation (HPER) style (n=10) and adolescents with highly negative (HNER) style (n=10) who underwent a protocol of stress induction show the usefulness of the new metrics to distinguish the dynamical behavior of the heart systems from these groups. Repeated measures ANOVAs revealed that changes in all three distances across conditions (baseline, anticipation of stress, exposure to stress, and recovery) were significant in the HPER group but not in the HNER group. As to the physiological meaning of the new measure a correlational analysis revealed that associations with time-domain HRV measures were stronger than associations with frequency-domain HRV measures in both groups. Because of the small sample size, bootstrap resampling was used to obtain confidence intervals. Distances calculated with the new measure are sensitive to the ER-related cardiac flexibility under acute stress conditions. However, the physiological meaning of the new indices remains unclear.