Structural basis of regional dysfunction in acutely ischemic myocardium

Objective: Impaired systolic function in the normally per fused myocardium adjacent to an ischemic region - the functional border zone - is thought to result from mechanical interactions across the perfusion boundary. We inve stigated how segment orientation and vessel involved affect regional strain s in the functional border zone and whether altered stresses associated wit h a step transition in contractility can explain the functional border zone , Methods and results: Regional epicardial strain distributions were obtain ed from measured displacements of radiopaque markers in open-chest anesthet ized canines, and related to local myofiber angles and blood flows. The fun ctional border zone for fiber strain was significantly narrower than that f or cross-fiber strain and significantly wider for left anterior descending (LAD) than left circumflex (LCx) coronary occlusion (1.23 vs. 0.45 cm). A d etailed three-dimensional computational model with a one-to-one relation be tween perfusion and myofilament activation and no transitional zone of inte rmediate contractility showed close agreement with these observations and s ignificantly elevated stresses in the border zone. Differences between LAD and LCx occlusions in the model were due to differences in left ventricular systolic pressure and not to differences in perfusion boundary or muscle f iber orientation. The border zone was narrower for fiber strain than cross- fiber strain because systolic stiffness is greatest along the muscle fiber direction. Conclusion: Abnormal regional mechanics in the acute ischemic bo rder arise from increased wall stresses without a transitional zone of inte rmediate contractility. Perfusion is more tightly coupled to fiber than cro ss-fiber strain, and a wider functional border zone of fiber strain during LAD than LCx occlusion is primarily due to higher regional wall stresses ra ther than anatomic variations. (C) 2000 Elsevier Science B.V. All rights re served.