Subepicardial Segmental Function during Coronary Stenosis and the Role of Myocardial Fiber Orientation

SUMMARY. 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 coronary 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 paralle