Influence of metoprolol on heart rate variability in survivors of remote myocardial infarction.

We assessed the influence of metoprolol on heart rate variability in survivors of remote myocardial infarction. In 43 survivors of myocardial infarction 12 to 18 months previously (26 men and 17 women, aged 38 to 69 years), two 24-hour ambulatory electrocardiograms were recorded 2 weeks apart. In patients in group A (n=28), who had taken metoprolol for the previous year, the drug was discontinued for 2 weeks, after which the first recording was done. The second recording was done 2 weeks after metoprolol was resumed. In patients in group B (n=15), who had not taken metoprolol for the previous year, it continued to be withheld, and two 24-hour recordings were done 2 weeks apart. In group A, metoprolol increased the time domain variables indicative of enhanced vagal tone; root-mean-square successive difference in normal RR (NN) intervals was 20 +/- 11 ms (mean +/- SD) without and 24 +/- 9 ms with metoprolol (p<0.05), and the proportion of NN that differ by >50 ms (pNN50%) was 3.6 +/- 6.0 without and 5.5 +/- 6.0 with metoprolol (p<0.05). In the frequency domain, the logarithms of the 24-hour very low frequency and the 24-hour high-frequency power (reflecting parasympathetic activity) were increased (5.12 +/- 1.03 and 4.48 +/- 1.51, respectively, without metoprolol; 5.32 +/- 0.99 and 4.83 +/- 1.24, respectively, with metoprolol, p <0.05 for both). Thus, in survivors of remote myocardial infarction, metoprolol enhances parasympathetic cardiac activity in the time and frequency domain measures of heart rate variability.