MYOCARDAL ISCHEMIA Dissociation between epicardial and transmural function during acute myocardial ischemia

The relationship between epicardial and transmural function (measured with sonomicrometers) was examined in 13 anesthetized open-chest dogs. Systolic wall thickening was used as a standard of integrated transmural function to compare with epicardial function measured as segment shortening parallel to surface fibers. Three levels of coronary inflow restriction were produced by using decrements in systolic wall thickening as an index of changes in the transmural distribution of myocardial blood flow (microspheres) in myocardium perfused by the left anterior descending artery (anterior-apical group, n = 7) or circumflex artery (posterior-basal group, n = 6). Levels 1 and 2 were characterized by reductions in systolic wall thickening of 35% and 80%, respectively, and marked decreases in deep myocardial blood flow. In the subepicardium, myocardial blood flow was minimally affected at levels 1 and 2 and there was no change in posterior-basal epicardial segment shortening, but anterior segment shortening decreased significantly (by 21% and 37%, respectively). At level 3 myocardial blood flow was reduced transmurally, producing systolic wall thinning and marked epicardial dysfunction in both groups. Parallel epicardial segment shortening underestimated the extent of transmural dysfunction in both groups at levels 1 and 2 but the degree of underestimation was greatest in the posterior-basal group. Anterior-apical segment shortening was impaired at levels l and 2, whereas posterior-basal segment shortening was unaffected, suggesting that significant regional variability exists in the epicardial response to nontransmural ischemia. Circulation 71, No. 6, 1279-1291, 1985. EPICARDIAL contractile function has been used extensively to assess regional myocardial performance during experimental myocardial ischemia. Important findings demonstrating the dependence of regional myocardial function on graded reductions in total coronary blood flow were obtained with mercury-in-silicone rubber length gauges or Walton-Brodie strain gauges sutured to the epicardium.' More recent studies have focused on the functional consequences of changes in myocardial blood flow distribution produced by coronary stenosis. With tracer-labeled microspheres or hydrogen desaturation curves to docuFrom the Thoracic Surgery Research Laboratory, Departments of Surgery (Section of Thoracic Surgery) and Physiology, The University of Michigan Medical School, Ann Arbor. Supported in part by the American Heart Association of Michigan and American Heart Association grant-in-aid 81-1161. Dr. Gallagher was a recipient of NIH New Investigator Award HL 30067 when this study was performed. Address for correspondence: Kim P. Gallagher, Ph.D., Thoracic Surgery Research Laboratory, R3484 Kresge I, Box 056, The University of Michigan, Ann Arbor, MI 48109. Received Nov. 5, 1984; revision accepted Feb. 21, 1985. Presented in part at the Federation of American Societies for Experimental Biology meeting (April 1984) and American Federation for Clinical Research National Meeting (May 1984). Vol. 71, No. 6, June 1985 ment blood flow distribution within the myocardial wall and sonomicrometers to measure segment length shortening or wall thickening during systole, close coupling between myocardial perfusion and contractile performance in the same area has been well established.6-4 Because coronary stenosis produces a nonuniform pattern of flow distribution, characterized by more severe subendocardial than subepicardial flow restriction, investigators have begun addressing whether or not corresponding differences also develop in epicardial and subendocardial contractile function."-2' Of particular interest is epicardial shortening when subepicardial perfusion is normal (or near normal) and only the deeper myocardial layers are ischemic. The findings and conclusions on this issue are contradictory. Weintraub et al.'6 and Hattori et al.'8 suggested that epicardial shortening is constrained or tethered in these circumstances, despite apparently adequate outer myocardial perfusion. However, we presented data showing that epicardial shortening parallel to local fiber orientation is not altered as long as subepicardial perfusion remains normal. 17 This suggested that parallel epi1279 by gest on Jne 1, 2017 http://ciajournals.org/ D ow nladed from