Observations on Factors Affecting Local Forces in the Left Ventricular Wall during Acute Myocardial Ischemia

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Abstract
The factors affecting local myocardial function in the presence of ischemic myocardial damage may considerably influence cardiac function and the extent of tissue damage resulting from the initial ischemic injury. However, previous attempts to study local wall forces in the ischemic myocardium have produced variable results. In the present study, a technique was developed for recording isometric left ventricular wall tension that minimized artifacts due to external forces acting on the ischemic area. Miniature isometric force gauges were implanted in 23 dogs in the left ventricular wall within a zone buffered by two rows of transmural prongs attached to a ring. During a test occlusion of a branch of the left anterior descending coronary artery, the ischemic area was defined by the presence of epicardial ST segment elevation, and force gauges were implanted in marginal or ischemic areas of the myocardium. During a subsequent coronary occlusion wall force in the ischemic area fell immediately by an average of 31 ± 8%. When the occlusion was repeated during isoproterenol infusion, wall tension decreased in previously unaffected areas of the myocardium, although in ischemic areas it remained above previous control ischemic levels. Arterial counterpulsation improved the development of wall tension in the ischemic area by 27% and decreased the extent and amount of ST segment elevation. Thus the present study clearly indicates a significant reduction but not elimination of active myocardial wall force in ischemic zones following experimental coronary occlusion. Moreover, force could be augmented by either isoproterenol or counterpulsation, and counterpulsation can produce a significant improvement in function of the ischemic myocardium while reducing the area of ischemic injury.