INFLUENCE OF METOPROLOL ON HEART-RATE-VARIABILITY IN SURVIVORS OF REMOTE MYOCARDIAL-INFARCTION

We assessed the influence of metoprolol on heart rate variability in s urvivors of remote myocardial infarction. In 43 survivors of myocardia l 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 metoprol ol for the previous year, the drug was discontinued for 2 weeks, after which the first recording was done. The second recording was done 2 w eeks after metoprotol was resumed. In patients in group B (n = 15), wh o had not taken metoprolol for the previous year, it continued to be w ithheld, and two 24-hour recordings were done 2 weeks apart. In group A, metoprolol increased the time domain variables indicative of enhanc ed vagal tone: root-mean-square successive difference in normal RR (NN ) intervals was 20 +/- 11 ms (mean +/- SD) without and 24 +/- 9 ms wit h 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 parasym pathetic activity) were increased (5.12 +/- 1.03 and 4.48 +/- 1.5l, re spectively, without metoprolol; 5.32 +/- 0.99 and 4.83 +/- 1.24, respe ctively, with metoprolol, p < 0.05 for both). Thus, in survivors of re mote myocardial infarction, metoprolol enhances parasympathetic cardia c activity in the time and frequency domain measures of heart rate var iability.