Ambulatory assessment of the QT interval in patients with hypertrophic cardiomyopathy: risk stratification and effect of low dose amiodarone.

This study aims to assess the dynamics of the QT interval in patients with hypertrophic cardiomyopathy (HCM). Three consecutive QT intervals and the preceding RR intervals were measured on 24 hour ambulatory electrocardiograms at 30-minute intervals in ten high risk patients with HCM (sudden cardiac death [SCD] and/or documented ventricular fibrillation), aged 29 +/- 17 years, compared with ten age and sex matched low risk patients with HCM (no syncope, no adverse family history, and no ventricular tachycardia on Holter monitoring), and ten normal subjects. Another ten patients who were on amiodarone therapy (200-mg daily) were also studied. Patients with intraventricular conduction defects were excluded. There were 4,424 pairs of QT intervals and their preceding RR intervals were measured in this study. A nonsignificant prolongation in the QT interval and a significant prolongation in QTc values (Bazett's and Fridericia's formulas) were demonstrated in patients with HCM compared with normals. There were no significant differences in the QT and QTc between high and low risk patients. The slope of regression line for the QT against RR interval was significantly different between normals and HCM (0.1583 +/- 0.040 vs. 0.2017 +/- 0.043, P < 0.05), but not between high and low risk patients. Amiodarone significantly prolonged the QT and QTc without significantly altering the slope of the regression line (0.2017 +/- 0.043 vs 0.2099 +/- 0.037, NS). Our findings support the observations that there is a prolonged QT interval in patients with HCM and that there is no significant use dependent effect of amiodarone on ventricular repolarization.(ABSTRACT TRUNCATED AT 250 WORDS)