Serial changes in left ventricular relaxation and chamber stiffness after large myocardial infarction in rats

To determine the time course of changes in left ventricular diastolic properties after a large myocardial infarction, we serially measured left ventricular relaxation, chamber stiffness, and the ratio of left ventricular cavity to wall volume (V/V(w)) after coronary artery ligation in rats. Left ventricular relaxation was measured during the occlusion and then both relaxation and chamber stiffness were measured at 3 hr, 24 hr, and 3, 5, and more than 22 days after infarction. Left ventricular pressures and left ventricular dP/dt were recorded with micromanometer-tipped catheters. Left ventricular relaxation was measured by computer digitization of the left ventricular pressure tracings and averaged over 100 to 150 cardiac cycles. Five chamber stiffness constants were calculated from pressure-volume curves that were obtained ex vivo. We found that ventricular relaxation prolonged for the first hour after coronary occlusion; relaxation was maximally prolonged at 10 to 15 min after onset of occlusion. After 1 hr relaxation returned to normal. However, by 5 days ventricular relaxation was again prolonged. Left ventricular stiffness constants were increased at 3 and 24 hr, resulting in a shift of the left ventricular pressure-volume relation to the left. At 3 days after coronary artery ligation, all stiffness constants and the pressure-volume relation returned to normal. At more than 22 days the pressure-volume relation was shifted to the right and the stiffness constant for low filling pressures was decreased. V/V(w) was significantly decreased from 0.603 ± 0.021 at 3 and 24 hr to 0.379 ± 0.024 and 0.362 ± 0.032, respectively. V/V(w) was significantly increased at more than 22 days (0.921 ± 0.094). We conclude that left ventricular diastolic function in rats is dynamic during the acute and healing phases of myocardial infarction. Left ventricular relaxation is prolonged at 10 to 15 min after coronary occlusion, then returns to normal, and by 5 days is again prolonged. Changes in left ventricular chamber stiffness are biphasic, first increasing then decreasing. These observed changes in chamber stiffness are related in part to changes in V/V(w).