Relationship between cardiac autonomic function and sustained ventricular tachyarrhythmias in patients with an implantable cardioverter defibrillators

AIMS: Low left ventricular ejection fraction (LVEF) is the main indication of implantable cardioverter defibrillators (ICD) in patients with dilated cardiomyopathy (DCM) for the primary prevention of sudden cardiac death, but ICD therapy at follow-up occurs in a minority of patients. We investigated whether heart rate variability (HRV) may improve risk stratification in DCM patients. METHODS AND RESULTS: We studied 42 patients (age 67.3 ± 3.5; 37 males) who had undergone ICD implant for either idiopathic or ischaemic DCM (LVEF <40%) 34.6 ± 19.7 months prior to the study (range 6-84). Patients underwent 24 h electrocardiographic Holter monitoring, and HRV was assessed over 2 hours in the afternoon showing stable sinus rhythm. Left ventricular ejection fraction was measured by two-dimensional echocardiography. The serum levels of C-reactive protein and N-terminal pro-B-type natriuretic peptide (NT-proBNP) were also obtained. The primary endpoint was the occurrence of appropriate ICD shocks in the 6 months preceding the study. The occurrence of appropriate ICD discharge from ICD implant was considered as a secondary endpoint. In the last 6 months, appropriate ICD shocks had occurred in seven patients (17%). There were no differences between patients with and without ICD shocks in clinical variables, as well as in LVEF and in C-reactive protein and NT-proBNP serum levels. In contrast, most HRV parameters were significantly depressed in patients with, compared with those without, ICD shocks; the most significant difference was shown for the average of the standard deviations of RR intervals in all consecutive 5 min segments (n ¼ 12) within the 2 h (26.7 ± 9 vs. 39.7 ± 14 ms; P = 0.02) in the time domain and for LF amplitude (8.4 ± 3 vs. 14.8 ± 7 ms; P = 0.02) in the frequency domain. Implantable cardioverter defibrillator discharge had occurred in 11 patients (26%) since ICD implant (average 35 months). No clinical or laboratory variable showed significant differences between patients with or without ICD discharge, except very low-frequency (VLF) amplitude (23.8 ± 7 vs. 30.8 ± 10.6 ms, respectively; P = 0.049). CONCLUSION: In ICD patients with reduced LVEF, several depressed HRV indices were significantly associated with appropriate ICD shocks in the previous 6 months, and VLF amplitude was the only variable significantly associated with ICD shocks recorded since ICD implant. These data suggest that full HRV analysis might be helpful for improving risk stratification for life-threatening ventricular arrhythmias and ICD indication in patients with DCM.