Effect of aortic constriction on the functional border zone.

To evaluate how aortic constriction affects nonischemic myocardium adjacent to the perfusion boundary (the "functional border zone"), we measured systolic wall thickening (dWT) with sonomicrometers in eight anesthetized, open-chest dogs. The locations of the wall thickening measurements relative to the perfusion boundary (PB) were determined with myocardial blood flow (microspheres) maps constructed from multiple, small tissue samples. In nonischemic myocardium more than 10 mm from the PB produced by circumflex coronary occlusion, dWT increased significantly from 2.57 +/- 0.62 (mean +/- SD) to 3.24 +/- 0.73 mm (P less than 0.01). Within 10 mm of the PB, however, dWT did not change significantly (2.48 +/- 0.79 to 2.38 +/- 0.66 mm, NS). When the aorta was mechanically constricted, peak systolic pressure increased approximately 50%. Wall thickening decreased to the same relative degree in nonischemic muscle less than 10 mm and more than 10 mm from the perfusion boundary. By fitting sigmoid curves to the data, we estimated the extent of nonischemic dysfunction. It averaged 26 +/- 6 degrees (6-8 mm of endocardial circumference) during coronary occlusion alone and it was not significantly different (29 +/- 11 degrees) after aortic constriction. Thus elevated afterload affects nonischemic myocardium uniformly and does not increase the size or relative severity of the functional border zone.