Impaired exercise capacity predicts sudden cardiac death in a low-risk population: enhanced specificity with heightened T-wave alternans

Because sudden cardiac death (SCD) is due to cardiac electrical instability, we postulated that prediction of this mode of death by exercise capacity will be enhanced by combined assessment with T-wave alternans (TWA), an index of repolarization abnormality.The Finnish Cardiovascular Study enrolled consecutive patients (n=2,044) with a routine clinically indicated exercise test. Exercise capacity was measured in metabolic equivalents (METs) and TWA by time-domain modified moving average method.During 47.2+/-12.8-month follow-up (mean+/-SD) 120 patients died; 58 were cardiovascular deaths, and 29 were SCD. In multivariate analysis after adjustment for sex, age, smoking, use of beta-blockers, as well as other common coronary risk factors, the relative risk of patients whose exercise capacity was depressed (MET8) was 8.8 (95% CI 2.0-38.9, P=0.004) for SCD. The combination of low exercise capacity (MET8) and elevated TWA (or =65 microV) yielded relative risks for SCD of 36.1 (6.3-206.0, P0.001), for cardiovascular mortality of 21.1 (6.7-66.2, P0.001), and for all-cause mortality of 7.8 (3.5-17.4, P0.001) over patients with neither factor.Reduced exercise capacity, particularly in combination with heightened TWA, indicating enhanced cardiac electrical instability, powerfully predicts risk for SCD in patients referred for exercise testing.