Ejection- and isovolumic contraction-phase wall thickening in nonischemic myocardium during coronary occlusion

During acute regional ischemia in open-chest anesthetized dogs, contraction is augmented in nonischemic myocardium. The effectiveness of enhanced contraction, however, has been questioned based on recent studies (using midwall segment length measurements) that reported that most of the increase occurred only during the isovolumic contraction phase (ICP) and not the ejection phase (EP) of systole. We evaluated this issue retrospectively using a different means of measuring regional myocardial function, systolic wall thickening (measured with sonomicrometers). In open-chest anesthetized dogs we observed a significant increase in both ICP and EP wall thickening. With circumflex occlusion, ICP and EP thickening each contributed an average of 50% to the augmentation in total thickening. With left anterior descending occlusion, ICP thickening contributed a significantly greater proportion (71%) to the total increase, but the change in EP thickening was also significant (29%). Therefore, in open-chest dogs we observed evidence of intraventricular unloading (in that ICP thickening increased), but we also observed an increase in effective (EP) thickening. In a series of conscious dogs, however, we observed no significant change in total nonischemic thickening after circumflex coronary occlusion. There was a statistically significant increase in the ICP component acutely after coronary occlusion, but its contribution to total wall thickening was small. In a group of dogs studied at intervals for 2 wk after circumflex occlusion, there was no evidence of intraventricular unloading beyond 3-5 days postocclusion. Therefore, we conclude that significant intraventricular unloading is restricted to the acute phases of myocardial infarction, and we suggest that its functional importance is smaller in conscious closed-chest chest dogs than in open-chest dogs.