Nonlinearity changes of 24 hour heart rate variability in children with tetralogy of Fallot

It has been reported that sudden cardiac death and ventricular tachycardia can occur after treatment of tetralogy of Fallot (TOF). It is likely that TOF has effect on the heart rate variability because of the ventricular arrhythmia. We evaluated how complex and periodic dynamics of heart rate occurred in the normal children (group A, n=13) and TOF children (group B, n=13) throughout 24 hours. We examined each ECG data on 24-hour Holter ECG segmented into 1-hour length. We analyzed each HR time series, and quantified the overall complexity of each HR time series by its correlation dimension. We also calculated the power spectrum of HR, and obtained low-frequency component (0.08-0.15 Hz) and high-frequency component (0.15-0.4 Hz). We compared the results between group A and B. Group B had lower correlation dimension (4.055±0.4134 vs. 4.9310±0.2054, p<0.05) than group A, even though there were no significant differences in the low-(0.9864±0.5598 vs. 1.5560±0.8325, p<0,05) and high-(1.1168±0.1.1448 vs. 0.9271±0.6528, p<0.05) frequency components. It can be concluded that HR time series of group B are more regular than those of group A, and that correlation dimension reveals a nonlinear characteristics of HR time series which is not determined in the frequency domain