QT variability strongly predicts sudden cardiac death in asymptomatic subjects with mild or moderate left ventricular systolic dysfunction: a prospective study

The most widely accepted marker for stratifying the risk of sudden cardiac death (SCD) in post myocardial infarction patients is a depressed left ventricular function. Left ventricular ejection fractions (EF) of 35% or less increase the risk of sudden death but values between 35 and 40% raise concern. The underlying pathophysiological mechanism is sustained ventricular tachycardia or fibrillation, both associated with increased cardiac repolarization variability. We assessed whether the indices of QT variability from a short-term electrocardiographic (ECG) recording predict sudden death. A total of 396 subjects with chronic heart failure (CHF) due to post-ischaemic cardiomyopathy, with an EF between 35 and 40% and in NYHA class I, underwent a 5 min ECG recording to calculate the following variables: QT variance (QTv), QT normalized for the square of the mean QT (QTVN), and QT variability index (QTVI). Corrected QT (QTc) was calculated from a 12-lead ECG recording. All participants were followed for 5 years. A multivariable survival model indicated that a QTVI greater than or equal to the 80th percentile indicated a high risk of SCD hazards ratio (HR) 4.6, 95% confidence interval (CI) 1.5–13.4, P = 0.006 and, though to a lesser extent, a high risk of total mortality (HR 2.4, 95% CI 1.2–4.9, P = 0.017). The model including QTVI as a continuous variable confirmed a similar high risk for SCD (HR 2.9, 95% CI 1.3–6.5, P = 0.01) and for total mortality (HR 2.6, 95% CI 1.3–5.2, P = 0.008). Although asymptomatic patients with CHF who have a slightly depressed EF are at low risk of sudden death, the category is extraordinarily numerous. The QTVI could be helpful in stratifying the risk of sudden death in this otherwise undertreated population.