Defining the mechanical border zone: a study in the pig heart.

Wall motion abnormalities can occur in nonischemic areas contiguous with ischemic myocardium. The extent of the mechanical border zone contiguous with an acute myocardial ischemic region was mapped in eight open-chest anesthetized pigs using sonomicrometer crystals implanted parallel with the visible ischemic border in the subepicardium of ischemic (IZ), proximal border (BZ1), distal border (BZ2), and remote normal (NZ) zones. Regional systolic shortening fraction was near 15%, and epicardial blood flow was approximately 1.5 ml X min-1 X g-1 in all locations before ischemia was induced. Blood flow fell to less than 0.05 ml X min-1 X g-1 and the left ventricular free wall supplied by the distal one-third of the left anterior descending artery exhibited holosystolic lengthening when the vessel was occluded. After occlusion, systolic shortening fraction was depressed by 45% in the BZ1 located 3.1 +/- 1.1 mm from the ischemic margin and by 25% in the BZ2, which was measured at 9.3 +/- 1.5 mm from the ischemic margin without significant flow reduction. The NZ, 22.3 +/- 4.6 mm from ischemic margin, was unaffected by occlusion. Computer analysis of the data shows that wall motion is depressed as far as 12 mm from the ischemic margin. This abnormal wall motion surrounding focal transmural myocardial ischemia is presumed to result from mechanical tethering.