CHANGES IN MYOCARDIAL-CONTRACTION PATTERN DURING INITIAL REPERFUSION

The characteristics of hypercontraction during initial reperfusion wer e studied in 10 pentobarbitone-anesthetized cats. The left anterior de scending coronary artery was occluded for 10 min followed by 1 h of re perfusion, and regional function was assessed by two cross-oriented pa irs of sonomicrometers placed in the left ventricular anterior wall. A t 1 min of reperfusion (hyperfunctional phase) there was an uniform co ntraction pattern with 90% recovery of ejection shortening in both cir cumferential and longitudinal segments. During initial hypercontractio n, end-diastolic segment lengths remained unchanged, whereas end-systo lic segment lengths decreased transiently. Inotropic stimulation durin g reperfusion in four additional animals also affected end-systolic le ngths more than end-diastolic lengths. This suggests that the initial hyperfunctional phase is due to an inotropic stimulation of the stunne d myocardium, most probably caused by intracellular Ca2+ overload. At 5 min of reperfusion a nonuniform contraction pattern had developed wi th 68% recovery of shortening in circumferential segments vs. 25% in l ongitudinal segments. The decreased performance in longitudinal segmen ts was paralleled by a delayed start of contraction as well as a decre ased velocity of contraction. Because longitudinal segment shortening is a sensitive parameter of subendocardial performance, our results in dicate a brief transmural hypercontraction followed by increasing dysf unction (stunning) in the subendocardial layer.