Heart rate dynamics during acute pain in newborns.

Autonomic nervous system modulation of heart rate is significantly altered during painful procedures in newborns. Most studies investigating pain employed only linear-based analysis methods, thus ignoring the complex, non-linear nature of heart rate control mechanisms. The emergences of dynamic, nonlinear analysis methods enable us to uncover information embedded in the fluctuations of heart rate not otherwise noticeable. Our objective was to examine how cardiac dynamics change in newborns who undergo heel lancing by analyzing linear and nonlinear characteristics of heart rate fluctuations. We used dynamic nonlinear analyses methods to reveal heart rate variability and complexity alterations during painful stimulus in newborns. Poincaré plots were applied to examine the dynamics of the system, sample entropy to investigate the complexity of the system, and detrended fluctuation analysis, to reveal the fractal properties of the system. Heart rate significantly increased (165 vs.123 beats per minute, p < 0.001) while variability decreased. Sample entropy and the quantitative measures of the Poincaré plots (SD1 and SD2) significantly decreased during heel lancing (0.75 vs. 1.0, p < 0.01; 6.4 vs. 12.8, p < 0.001; and 30.4 vs. 50.5, p < 0.01, respectively). Detrended fluctuation analysis showed a significant decrease in the short-term scaling exponent α1 (1.06 vs. 1.3, p < 0.001), and an increase in the long-term scaling exponent α2 (1.5 vs. 1.1, p < 0.001). Our results indicate altered complexity of heart rate variability during painful stimulus in newborns and disruption of the mechanisms that regularly control it. Such alterations resemble certain pathological conditions and may represent stress reaction.