Subepicardial segmental function during coronary stenosis and the role of myocardial fiber orientation

Abstract

In six open-chest, anesthetized dogs, segmental shortening was measured with one pair of ultrasonic dimension gauges placed in the subendocardium (used as a marker for regional ischemia) and two pairs in the epicardium. One of the epicardial crystal pairs was aligned parallel with the surface fibers and the other was oriented circumferentially, parallel with the short axis. During control conditions, with uniform transmural perfusion measured by microspheres (endo/epi ratio 1.17 ± 0.10), epicardial systolic shortening measured by parallel and circumferential segment pairs was comparable but significantly different (9.2 ± 2.4% and 7.3 ± 1.0%, respectively). Partial circumflex coronary artery narrowing produced subendocardial ischemia (blood flow reduced from 0.99 ± 0.24 to 0.29 ± 0.07 ml/min per g, P <0.01 from control), characterized by elimination of subendocardial shortening (-1.3 ± 1.7% P <0.01). Outer myocardial perfusion remained unchanged [0.86 ± 0.17 to 1.00 ± 0.37 ml/min per g, not significant (NS)] and parallel epicardial shortening was not significantly reduced (8.4 ± 3.7%, NS); however, circumferential epicardial shortening was eliminated (-0.8 ± 2.2%, P <0.01). With complete occlusion, severe transmural ischemia was produced (including the subepicardial layer), and systolic lengthening was observed in the subendocardial as well as both epicardial segments. We conclude that circumferential and parallel epicardial shortening are comparable under conditions of uniform transmural perfusion (normal conditions and complete coronary occlusion), but that considerable nonuniformity of transmural contraction occurs during partial coronary narrowing associated with nonuniform transmural blood flow. Thus, unidimensional measurements of epicardial motion without regard to fiber orientation can lead to variable results under the latter conditions. Despite sustained epicardial blood flow, circumferential epicardial function correlated well with impaired subendocardial shortening during ischemia, indicating that a portion of the myocardium in the outer third of the wall is constrained from shortening during partial ischemia.