Effects of Beta‐Blockers on the Relation Between QT Interval and Heart Rate in Survivors of Acute Myocardial Infarction

Background β‐blockade has been found to reduce the heart rate dependency of the QT interval in healthy individuals, but the effect of β‐blockade on the QT/RR relationship in patients suffering from myocardial infarction (Ml) is unknown. Methods Patients with a first Ml were enrolled in a double‐blind, placebo‐controlled study. A 24‐hour ambulatory ECG was performed at discharge before study medication was initiated, and repeated after 4 weeks of therapy. QT and RR intervals were assessed beat to beat for all sinus beats by a computer program. For each patient, the mean QT interval duration for every RR value was determined on an hourly basis and the QT/RR relation was described as the best fit linear regression line. Results Before randomization, no differences between the groups could be demonstrated. After 4 weeks of drug study (37 patients receiving metoprolol 100 mg per day and 34 patients receiving placebo), the day, night, and 24‐hour regression lines differed significantly between the groups. Metoprolol reduced the QT/RR slope, increased the intercept, and blunted the day‐night difference of the QT/RR slope. Conclusion Long‐term β‐blockade in Ml patients results in a reduction of the QT/RR slope with a prolongation of the QT interval at higher heart rates. These effects, which are similar to observations found in healthy individuals, may be related to the decreased susceptibility to life‐threatening arrhythmias in patients treated with beta‐blockers, possibly suggesting the QT/RR relationship as a marker of ventricular autonomic tone.