Left ventricular rupture threshold during the healing phase after myocardial infarction in the dog

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Abstract
The mechanical resistance of the infarcted left ventricle to rupture, or rupture threshold, was measured by the balloon technique 1-42 days after left anterior descending coronary artery ligation in 70 dogs: 26 without infarction (18 sham, 8 with ligation) and 44 with infarction. Rupture threshold in noninfarcted hearts was higher than in infarcted hearts (1168 .+-. 165 (SD) vs. 754 .+-. 223 mmHg (1 mmHg = 133.32 Pa), p < 0.001) and did not change over 6 weeks. In contrast, rupture threshold in infarcted hearts decreased (p .ltoreq. 0.05) after 14 days, the average value for 21-42 days being less than that for 1-14 days: 577 .+-. 140 vs. 867 .+-. 191 mmHg, p < 0.001. Passive left ventricular stiffness in infarcted hearts was higher than for noninfarcted hearts throughout the 6 weeks during early filling (11.1 .+-. 3.9 vs. 7.1 .+-. 1.4 mmHg/mL, p < 0.001) but decreased (p .ltoreq. 0.05) after 14 days during the prerupture phase (11.3 .+-. 5.3 vs. 6.2 .+-. 3.0 mmHg/mL, p < 0.005). Between 7 and 42 days, the infarct zone showed marked increase in hydroxyproline (10.0 .+-. 2.0 vs. 48.8 .+-. 19.7 mg/g dry weight, p < 0.001), shrinkage (infarct size, 25 .+-. 9 vs. 9 .+-. 5% of the left ventricle, p < 0.005), and thinning (infarct to normal wall thickness ratio, 0.83 7U 0.11 vs. 0.51 .+-. 0.09, p < 0.001) but little further stretching (expansion index or the ratio of lengths of infarcted and noninfarcted segments, 1.14 .+-. 0.10 vs. 1.28 .+-. 0.17, p < 0.2). A mild decrease (p < 0.05) in left atrial pressure and increase (p < 0.05) in diastolic area and fractional change in area (two-dimensional echocardiography) were detected at 6 weeks. The late decrease in rupture threshold and prerupture stiffness of the infarcted left ventricle and thinning of the scar suggest a late decrease in mechanical strength and resistance of the infarcted left ventricle to distension.