ABSTRACT
We have discussed the mechanical interaction between ischemic and nonischemic areas of the left ventricle and the changes in local mechanics when the heart is stimulated from various sites of the left ventricular epicardium. We discussed how local mechanics is related to global left ventricular function and the way in which the latter relates in this relation to oxygen demands and whether we can interfere in this relation with drugs to increase the efficiency of left ventricular performance. We barely glanced at transmural differences, if any, in mechanical function and the reason is obvious. To my knowledge, at present nobody has a technique at hand to measure in vivo stress in the endo- and epicardial layers of the left ventricle. The measurement of fiber shortening in these layers has been chosen as an alternative, but this is not really the parameter of interest. Fiber shortening is interesting to describe local mechanical activity, but in terms of oxygen consumption, myocardial, metabolic, and so forth, we are more interested in stress. Information about distribution of, for example, stress and sarcomere length across the left ventricular wall have been mainly derived from model studies. It will be obvious that the results obtained depend on the characteristics of the model chosen. But we only looked at the ejection phase, for obvious reasons. During the ejection phase, the changes in geometry are relatively easy to describe, but during the isovolumic and relaxation phases the changes in geometry are rather complicated. It is still a matter of debate whether the stress is homogeneously distributed across the left ventricular wall. I do not know whether we are right, but if you consider the gradients in metabolism, flow, and oxygen consumption across the left ventricular wall, one may conclude that the differences are small. It has been suggested that flow is higher in the endo- than in the epicardial layers. This idea is based upon measurements with radioactive microspheres, but when molecular markers are used to measure flow, the flow distribution seems to be close to unity. So, I don’t say that the stress distribution across the left ventricular wall is homogeneous, but if there are differences, they are small to my opinion. I am anxious to hear what Rafi has to contribute.
