Poincare plot indices as measures of sympathetic cardiac regulation: Responses to psychological stress and associations with pre-ejection period

The Poincare plot is a visual representation of the R-R time series within a Cartesian plane constructed by plotting each R-R interval as a function of the previous one. The plot can be characterized by parameters that quantify the standard deviation (SD) of short- and long-term R-R interval variability (SD1 and SD2, respectively). Claims regarding the use of Poincare plot analysis as an alternative to conventional time-domain and spectral analytic measures of parasympathetic contributions to heart rate variability (HRV) have been evaluated somewhat extensively. However, its proposed utility for characterizing sympathetic influences on HRV during periods of acute mental stress, using SD2 and the SD1/SD2 ratio, has received minimal attention. In the present study, the electrocardiogram was used to record R-R interval variability in healthy participants during a resting baseline period, while they performed a challenging reaction time task, and then during a recovery period. Simultaneously, the thoracic impedance cardiogram was used to derive the pre-ejection period (PEP), which reflects sympathetic cardiac control. HRV was quantified using time-domain, spectral analytic, and Poincare plot indices. Task performance elicited significant reductions in HRV and PEP parameters, indicating withdrawal of parasympathetic tone with concomitant sympathetic activation. Following cessation of task performance, all HRV and impedance measures returned to baseline levels, signaling resumption of parasympathetic dominance. Bivariate associations linking SD2 and the SD1/SD2 ratio to PEP were observed; however, these were not large in magnitude, were exceeded in size by associations between the Poincare metrics and conventional HRV markers of parasympathetic cardiac influences, and were eliminated when the latter were controlled statistically. These observations challenge the proposal that the Poincare SD2 and SD1/SD2 metrics are suitable markers of sympathetic cardiac regulation in research concerned with mental and physical health.