Cardiac function and myocardial contractility: a perspective

An experimental study was designed to validate postextrasystolic potentiation assessment of myocardial viability or functional reserve of cardiac segments after acute coronary occlusion. Segmental systolic fractional area changes and wall thickening in pacing-induced postextrasystolic beats were mapped in 12 closed chest dogs by two-dimensional echocardiography during a control period and from 20 minutes to 3 hours after occlusion of the left anterior descending coronary artery. The extent of myocardial ischemic and necrotic zones was evaluated in left ventricular slices and subsegements corresponding to echographic cross sections. During two-dimensional echocardiography, left ventricular segments that were found to be neither ischemic nor necrotic always exhibited a significant augmentation of both fractional area change and wall thickening during the postextrasystolic beat that followed an induced premature contraction with a 42.4% coupling interval. In segments without necrosis but with varying degrees of ischemia, significant postextrasystolic potentiation was also demonstrated, even after 3 hours of occlusion. In contrast, segments that developed more than 80% necrosis failed to potentiate systolic fractional area change after 2 hours, and systolic wall thickening, even after 20 minutes of coronary occlusion. Statistical evaluation revealed a characteristic threshold at 41 to 60% necrosis, beyond which no potentiation of function could be elicited 3 hours after occlusion. Extrapolation from the experimental data suggests that when two-dimensional echographic studies in myocardial ischemia indicate postextrasystolic augmentation of segmental left ventricular function, the latter segments may be assumed to contain only small infarcts or to consist of reversibly ischemic and normal myocardium. Conversely, segments that fail to exhibit postextrasystolic potentiation can be assumed to be more than 60% necrotic.