Abstract
With the use of an epicardial Doppler probe, systolic wall thickening was selectively measured in the inner, mid, and outer layers of the left ventricular (LV) wall in 16 conscious dogs undergoing a 15-min left anterior descending artery (LAD) occlusion followed by 7 days of reperfusion (REP). Under control conditions, percent thickening fraction (ThF) was significantly greater (P < 0.01) in the inner layer [36.0 ± 2.3% (mean ± SE)] than in the mid (28.6 ± 2.1%) or outer (21.3 ± 2.2%) layers. During LAD occlusion, 11 dogs exhibited transmural dyskinesis (group 1), whereas 5 had transmural hypokinesis (group 2). In group 1, all layers exhibited comparable degrees of paradoxical systolic thinning during LAD occlusion. After REP, however, recovery was delayed in the inner compared with the mid and outer layers. At 2 h, ThF averaged 34.2 ± 11.9% of base line in the endocardium vs. 61.7 ± 16.2% in the midmyocardium and 51.0 ± 12.3% in the epicardium (F = 4.29, P < 0.002); similar differences were noted at 3 and 4 h. In the mid and outer layers, ThF returned to base-line values by 24 h, whereas in the inner layer it was still significantly depressed (P < 0.05) at 24 h (77.3 ± 5.1% of base line) and recovered by 48 h. The inner-to-outer ThF ratio was decreased (P < 0.01) for 24 h after REP, indicating maldistribution of thickening in the 'stunned' myocardium. In group 2, all layers exhibited hypokinesis during LAD occlusion. Again, recovery of function after REP was delayed in the endocardium compared with the other layers. This study demonstrates that after both severe ischemia resulting in dyskinesis and mild ischemia resulting in hypokinesis, REP is associated with slower recovery of function in the inner than in the outer layers. Thus myocardial 'stunning' is a nonuniform phenomenon with maximal severity in the subendocardium.