Fractal clustering of ventricular ectopy correlates with sympathetic tone preceding ectopic beats

Fractal geometric analysis of ventricular ectopy yields a fractal dimension, which can range from zero to one and is inversely related to clustering of ventricular premature contractions (VPCs). Low values of this fractal dimension, which reflect significantly nonuniform distributions of ventricular ectopy, are found in patients with life-threatening ventricular arrhythmias and predict adverse outcomes in selected patients with congestive heart failure and with mitral regurgitation. However, the physiological mechanism and correlates of the fractal dimension are unknown.

To explore the physiological correlates of clustered ventricular ectopy, we studied 30 patients with a history of sustained ventricular tachycardia or ventricular fibrillation who had inducible sustained monomorphic ventricular tachycardia during electrophysiological study and also underwent drug-free 24-hour ambulatory ECG. In addition to fractal dimension (determined by use of our previously described algorithm), we measured the mean RR interval (+/- SD) for all sinus beats preceding a sinus beat and for all sinus beats preceding a single VPC and the mean root-mean-square difference (RMSSD) of all windows of 15 successive RR intervals (excluding ectopic beats) preceding a sinus beat and preceding a single VPC. Based on the directional changes of mean RR (a measure of both sympathetic and parasympathetic tone) and of RMSSD (a measure of parasympathetic tone), each patient's inferred relative sympathetic tone preceding ventricular ectopy was classified as increased, unchanged, or decreased. If these values changed concordantly, relative sympathetic tone was indeterminate. Fractal dimension did not correlate with the mean RR interval, SD of the RR interval, or RMSSD preceding sinus beats or preceding VPCs (all P > .10). In 20 patients, fractal dimension was significantly lower among those with increased relative sympathetic tone (n = 14) than those with unchanged or decreased sympathetic tone (n = 6, P = .008). Ten patients had indeterminate relative sympathetic tone.

Clustering of ventricular ectopy, as measured by the fractal dimension, is observed in patients at increased risk of sudden cardiac death. A low fractal dimension (clustered ventricular ectopy) is related to changes in heart rate and heart rate variability that are consistent with transient increases in cardiac sympathetic tone.