Characterizing short-term linear and nonlinear heart rate variability in typically developing children and...

Linear heart rate variability (HRV) quantifies heart rate (HR) changes due to stochastic processes while nonlinear HRV captures the complexity of the HR structure. Although linear HRV in childhood and adolescence has been explored, little is known about nonlinear HRV. The present study characterized short-term linear and nonlinear HRV in typically-developing children (7-12 years, n = 35) and adolescents (13-17 years, n = 29). HRV during quiet rest was assessed using a three-lead electrocardiogram for 5-min and analyzed using linear (time and frequency domain) and nonlinear (Poincáre analysis, approximate [ApEn] and sample entropy [Sampen], and detrended fluctuation analysis [DFA]) metrics. A greater mean RR time-interval was observed in adolescents (p = 0.007). Time-domain HRV was not different between children and adolescents, although sex differences were observed, such that females had lower linear HRV (p < 0.05, $\hat{\theta }$  > 62); namely, standard deviation of the normal sinus interbeat intervals (SDNN) and root mean square of the successive differences between the normal interbeat intervals (RMSSD). ApEn was lower in adolescent males (p < 0.001, $\hat{\theta }$  = 86). Short-term scaling DFA was also lower in adolescents (p = 0.023, $\hat{\theta }$  = 64). Poincaré analysis revealed higher nonlinear short-term (p < 0.003, $\hat{\theta }$  = 68) and long-term variability (p < 0.001, $\hat{\theta }$  = 83) in males. Nonlinear metrics correlated with linear indices of parasympathetic tone. Collectively, age and sex differences were observed in nonlinear HRV in children and adolescents. Future research is needed to elucidate the mechanisms underlying nonlinear HRV during development and how these processes are altered in childhood disorders.