Effects of beta blockers (atenolol or metroprolol) on heart rate variability after acute myocardial infarction

This study analyzed, with spectral techniques, the effects of atenolol or metroprolol on RR interval variability in 20 patients 4 weeks after the first uncomplicated myocardial infarction. Beta blocker-induced bradycardia was associated with a significant increase in the average 24 hour values of RR variance (from 13,886 ± 1,479 to 16,728 ± 1,891 ms2) and of the normalized power of the high-frequency component (from 22 ± 1 to 28 ± 2 normalized units), whereas the low-frequency component was greatly reduced (from 60 ± 3 to 50 ± 3 normalized units). When considering day and nighttime separately, the effects of both drugs were more pronounced in the daytime. In addition, a marked attenuation was observed in the circadian variation of the low-frequency component after β blockade. As a result, the early morning increase of the spectral index of sympathetic modulation was no longer detectable. These results indicate that β-blocker administration has important effects on RR interval variability and on its spectral components. The observed reduction in signs of sympathetic activation and the increase in vagal tone after β blockade help to explain the beneficial effects of these drugs after myocardial infarction. However, the potential clinical relevance of the increase in RR variance remains to be established.