Changes in contractility fail to alter the size of the functional border zone in anesthetized dogs

The functional border zone is nonischemic myocardium that exhibits reduced function adjacent to an ischemic area. To determine if the functional border zone can be modified by pharmacologic interventions that alter contractility, we infused isoproterenol (0.04-0.10 micrograms/kg/min) or administered propranolol (2 mg/kg) during circumflex coronary occlusion in nine anesthetized, open-chest dogs. We measured systolic wall thickening on both sides of the perfusion boundary, which was delineated with myocardial blood flow (microsphere) maps constructed from small tissue samples. By fitting sigmoid curves to the composite systolic wall thickening data after coronary occlusion, we modeled the distribution of functional impairment across the perfusion boundary. Defined as the distance from the perfusion boundary to 97.5% of the nonischemic asymptote of the sigmoid fits, the functional border zone was 31 degrees of circumference after coronary occlusion alone. Isoproterenol increased +dP/dt by 58% and augmented nonischemic systolic wall thickening without changing the lateral extent of the functional border zone (32 degrees). Propranolol reduced +dP/dt by 24% and depressed nonischemic systolic wall thickening, but the size of the functional border zone remained limited to 28 degrees. Within the functional border zone, wall thickening was significantly but only moderately reduced (-28%) compared with thickening in nonischemic myocardium more than 10 mm away from the perfusion boundary. The ratio of nonischemic border zone to central nonischemic area wall thickening remained the same with each intervention. We conclude that the dimensions of the functional border zone are fixed early after coronary occlusion and that inotropic interventions do not modify the extent or relative severity of nonischemic regional dysfunction.