Characteristics of the Circadian Rhythm of Heart Rate Variability in Patients with Sudden Cardiac Death after Myocardial Infarction

Background: Sudden cardiac death (SCD) after myocardial infarction, most commonly caused by ventricular fibrillation, is closely associated with autonomic nervous tone. However, the mechanism of SCD and its association with autonomic nervous tone and the circadian rhythm of this tone remain obscure. We examined the characteristics of autonomic nervous tone in patients with SCD using heart rate variability (HRV) analysis. Methods: HRV was analyzed by spectral and nonspectral methods in 46 postinfarction patients, including 6 patients who experienced SCD (SCD[+]) and 40 survivors (SCD[—]). The circadian rhythm of HRV was assessed by a nonlinear least squares method combined with the maximum entropy method (MEM) using data from 24‐hour ambulatory ECGs. The mean follow‐up period was 27 ± 12 months. Results: Midline estimating statistic of rhythm and the amplitude of the high frequency (HF) component in the SCD(+) group were lower than those in the SCD(—) group, and differences of the circadian rhythm of the HF component between the SCD(+) group and the SCD(—) group were observed. Conclusions: A decreased HF component indicating diminished parasympathetic nervous activity was the strongest independent predictor of SCD. These results suggest that postinfarction patients whose parasympathetic nervous activity is low and whose circadian changes are small may be susceptible to SCD.