Balance and imbalance of regional myocardial contractile function and blood flow.

Abstract

During normoperfusion, both contractile function and myocardial blood flow are heterogeneously distributed throughout the left ventricle. Midwall segment shortening is greater at the apex than at the base of the left ventricle, and it is greater in the anterior than in the posterior wall. Also, transmural heterogeneity of myocardial deformation exists, with greater segment shortening and wall thickening in inner than in outer myocardial layers. Myocardial blood flow is greater in inner than in outer myocardial layers. Apart from transmural heterogeneities, there are adjacent regions with largely different resting flow in the same heart. While an increase in myocardial contractile function will lead to a metabolically mediated increase in myocardial blood flow, an increase in regional coronary perfusion within or above the autoregulatory range does not, in turn, increase regional myocardial contractile function. During hypoperfusion induced by a proximal coronary stenosis, the reduction in subendocardial blood flow is more pronounced than that in subepicardial blood flow, and contractile function in the inner myocardial layers ceases more rapidly than that in outer myocardial layers. The reduced regional myocardial contractile function is closely matched to the reduced regional myocardial blood flow; however, such coupling between reduced flow and function is lost when ischemia is prolonged for several hours, i.e., function for a given flow is further reduced. Acute embolization of the coronary microcirculation induces a progressive loss of regional myocardial function at unchanged regional myocardial blood flow, i.e., perfusion-contraction mismatch. During reperfusion, regional myocardial contractile function remains depressed for a prolonged period of time, depending on the severity, duration and location of the preceding ischemic episode, while regional myocardial blood flow is restored.