Autonomic Activity, Cardiac Chaos and Circadian Rhythm in Asymptomatic Children with Postoperative Tetralogy of Fallot

ABSTRACT

PURPOSE: This study aimed to characterize the autonomic and chaotic control of heart rate and circadian rhythm in asymptomatic patients with postoperative tetralogy of Fallot (pTOF). METHODS: Twenty-four-hour electrocardiogram recordings were obtained in 30 asymptomatic pTOF patients and in 30 age-and sex-matched controls, aged between 6 and 11 years. The data was digitized and partitioned into sections of 30- minute'durations. For each section, time-domain and frequency-domain measures (low- and high- frequency component) of heart rate variability and three measures based on chaotic dynamics- approximate entropy, correlation dimension and Lyapunov exponent-were calculated. RESULTS: In pTOF patients, 24-hour mean values of the time domain measures, high-frequency component, and all chaotic measures were significantly lower, while 24-hour mean value and all 6-hour mean values of the low-frequency component were significantly higher; all 6- hour mean values of high-frequency component, except from 6am to midday, were significantly lower. In pTOF patients, all 6-hour mean values of all three chaotic measures were significantly lower. In pTOF patients, the day- night circadian variation seen in controls was diminished (time- domain measures) or absent (low- and high- frequency component). CONCLUSION: Even in asymptomatic patients with pTOF, who are thought to be at minimal risk of fatal arrhythmia, a sustained increase in sympathetic activity and decrease in vagal activity, abnormal circadian rhythm of the autonomic activity, and decreased cardiac chaos were found. When other arrhythminogenic risk factors are superimposed, these abnormalities may contribute to the development of fatal arrhythmia and sudden death.