Increased Chaos of Beat-to-Beat QT Interval Variability in Patients with Congestive Cardiac Failure: Decreased Chaos of QT with Clinical Improvement

Abstract

There is increasing evidence supporting the additional utility of nonlinear techniques such as chaos analysis of heart rate (HR) time series as indictors of prognosis in cardiac patients. Prolongation of rate-corrected QT interval, QTc, is strongly associated with sudden cardiac death. Recent studies of beat-to-beat QT interval variability (QTV) have shown that an increase in QTV is associated with increased sympathetic activity. QTvi, a QT variability index (QT variance corrected for mean QT divided by HR variance corrected for mean HR), is also a predictor of sudden cardiac death. We studied the utility of a measure of chaos, the Largest Lyapunov Exponent (LLE) of beat-to-beat HR and QT time series at baseline and after treatment in patients with congestive heart failure (CHF). We compared beat-to-beat HR and QT interval data in 23 patients with CHF and 19 age-matched normal controls. The ECG data were acquired in lead II configuration at a sampling rate of 1000 Hz. CHF patients had a significantly lower LLE of HR and a significantly higher LLE of QT time series than did normal controls. Clinical improvement in a subgroup of these patients was associated with a decrease in QT-LLE and a decrease in LLEqthr (LLE-QT/LLE-HR). These results demonstrate a decreased chaos of HR and increased chaos of QT time series in patients with CHF suggesting decreased cardiac vagal and increased sympathetic function in patients with CHF. Nonlinear measures may prove to be of additional utility to linear measures as surrogate end points to evaluate treatment effect in these patients.