Correlation of Heart Rate and Cardiac Dysfunction in Duchenne Muscular Dystrophy

Sinus tachycardia is common in cases of Duchenne muscular dystrophy (DMD). The authors hypothesized that an elevated heart rate would herald cardiomyopathy onset. A retrospective case-control study was performed with 55 DMD boys and 150 age-matched control boys. The variables were age, heart rate, shortening fraction, and left ventricular end-diastolic dimension. Cardiomyopathy was defined as a shortening fraction less than 28%. The DMD boys had a higher initial heart rate with no baseline echocardiographic evidence of cardiomyopathy. The control subjects showed a statistically significant age-related decline in heart rate (p = 0.001) but not the DMD boys. Cardiomyopathy developed in 17 of the 55 DMD boys over a period of 4.6 +/- A 1.6 years. The DMD upper and lower heart rate groups were similar in age, follow-up time, and initial shortening fraction, yet cardiomyopathy developed in 14 (42%) of 33 upper quartile boys but only 3 (14%) of 22 lower quartile DMD boys (odds ratio, 6.5 (95% confidence interval, 1.15-18.92; p < 0.05). Compared with the control subjects, the DMD boys had a higher resting heart rate and a lack of age-related heart rate decline. The DMD boys in the upper heart rate quartile were more likely to progress to cardiomyopathy than those in the lower quartiles. This study establishes heart rate elevation as a statistically significant risk factor for cardiomyopathy. Further studies may define heart rate cutoffs for early pharmacologic intervention for incipient cardiomyopathy.