Effect of beta-blockers atenolol and metoprolol on heart rate variability depends on heart rate before the treatment

In acute coronary syndromes greater heart rate variability and lower heart rate were shown to be associated with better prognosis. The study was undertaken to elucidate the relationship between heart rate variability and heart rate changes caused by the beta-blockers. In 75 patients with acute unstable angina frequency-domain measures of heart rate variability and scatter plot area were obtained from short-term electrocardiogram recordings (512 R-R intervals) at rest in supine position at admission and after I week of regular use of atenolol or metoprolol. Dose of beta-blockers was titrated to suppress symptoms of myocardial ischemia and to decrease heart rate below 60 bpm. Dose of metoprolol (n=35) was 282,1+/-136,9 mg/day, atenolol (n=40) - 148,1+/-70,3 mg/day. The degree of heart rate reduction and especially the level of heart rate before treatment were important. For increasing of heart rate variability by beta-blockers. In patients with relatively low heart rate before treatment (less than or equal to 67 bpm) reduction of heart rate was not accompanied with rise of heart rate variability. For potentially beneficial changes of heart rate variability by beta-blockers it is necessary to reduce resting heart rate below 60 bpm.