Scientists studied how heart muscle in rats reacts after a simulated heart attack, focusing on how the size of scar tissue affects the muscle's ability to contract stronger after a brief rest. They found that small scars (less than about 37% of the heart wall) didn’t change this response, while large scars (more than about 44%) greatly weakened it.

We studied the dependence of post-rest positive inotropic response of isolated rat papillary muscles subjected to rhythmic stimulation on severity of postinfarction cardiosclerosis developed during 6 weeks after occlusion of the left descending coronary artery. The isolated papillary muscles were perfused with oxygenated Krebs-Henseleit solution and electrically stimulated at a rate of 0.5 Hz. In all rats, coronary occlusion provoked postinfarction cardiosclerosis with the formation of a scar occupying 20-50% (min-max of the sample) of the left ventricular wall. Despite the presence of large postinfarction scar in all rats, the positive post-rest inotropic responses greatly varied. The post-rest response in rats with scar occupying <37% left ventricular wall was similar to that in intact animals, but rats with scar area >44% demonstrated dramatically decreased inotropic response to rest periods.