Effect of afterload and beta-adrenergic blockade on nonischemic myocardial contraction pattern

We studied how changes in afterload affect regional contraction in the anterior wall of the left ventricular after circumflex coronary arterial (CFX) occlusion and subsequent beta-adrenergic blockade in pentobarbital sodium-anesthetized cats. Regional function was determined by orthogonal sonomicrometry. CFX occlusion produced nonuniform hyperkinesis in the nonischemic anterior wall; shortening of circumferential segments increased from 10.1 to 14.1% (P < 0.001), whereas shortening of longitudinal segments increased from 3.0 to 9.6% (P < 0.001). Hyperkinesis of longitudinal segments was influenced neither by changes in afterload over a pressure range of +/- 30 mmHg nor by beta-adrenergic blockade, indicating that hyperkinesis of longitudinal segments does not rely on increased inotropic state or resistance to ventricular emptying. Hyperkinesis of longitudinal segments occurred at end-diastolic lengths equal to preocclusion conditions, whereas hyperkinesis of circumferential segments was dependent on activation of the Frank-Starling mechanism. Furthermore, shortening of circumferential segments decreased with increments in afterload, particularly after CFX occlusion and subsequent beta-adrenergic blockade. In conclusion, CFX occlusion alters the contraction pattern of the nonischemic anterior wall. The postocclusion contraction is sensitive to increased afterload in the cardiac minor axis direction. These initial alterations may well direct the following remodeling process in infarcted hearts.