EFFECTS OF METOPROLOL ON EARLY INFARCT EXPANSION AFTER ACUTE MYOCARDIAL-INFARCTION

The effects of metoprolol on early infarct expansion after acute myoca rdial infarction were studied in rats (n = 54) that underwent either l eft coronary artery ligation (MI) or sham operation. Immediately after surgery, the rats received either metoprolol (M) by mouth, which had been dissolved in drinking water, for 72 hours supplemented with three intraperitoneal doses over the first 24 hours or no treatment (H2O). Three days after the initial surgery, hemodynamic measurements were ma de before and after volume loading. The rats were killed, the hearts w ere removed, and passive pressure-volume curves were obtained. The hea rts were then fixed at a constant pressure and analyzed morphometrical ly. Infarct sire was nonsignificantly lower in the metoprolol-treated group compared with the untreated group (38% +/- 5% MI-M vs 48% +/- 3% MI-H2O, p = 0.10) Compared with infarcted untreated rats, infarcted m etoprolol-treated rats had a lower heart rate (322 +/- 13 beats/min MI -M vs 452 +/- 19 beats/min MI-H2O, p < 0.001), lower left ventricular systolic pressure (63 +/- 4 mm Hg MI-M vs 90 +/- 6 mm Hg MI-H2O, p = 0 .004), and lower +dp/dt (1340 +/- 169 mm Hg/sec MI-M vs 2872 +/- 273 m m Hg/sec MI-H2O, p < 0.001), but left ventricular end-diastolic pressu re and cardiac index did not differ between the two groups. Left ventr icular weight corrected for body weight was higher in infarcted rats t reated with metoprolol compared with infarcted untreated rats (2.76 +/ - 0.07 gm/kg MI-M vs 2.41 +/- 0.09 gm/kg MI-H2O, p < 0.05). The initia l slope of the pressure-volume relationship K-i, an index of operative volume stiffness, was lower in infarcted rats treated with metoprolol compared with infarcted untreated rats (p = 0.03). There were, howeve r, no significant differences in the expansion index, thinning ratio, or left ventricular volume between the two infarcted groups. Thus meto prolol therapy begun in the immediate postinfarction period promotes a n increase in left ventricular weight and reduces operative volume sti ffness but has no significant effect on indexes of early infarct expan sion.