Transmural differences in energy metabolism of the left ventricular myocardium: fact or fiction.

It is a matter of continuous debate whether mechanical and metabolic activities are differently distributed across the left ventricular wall. It has been suggested that under normal circumstances the subendocardial layers have a higher workload and, hence, higher energy requirements than the subepicardial layers. Direct assessment of the transmural distribution of workload in the left ventricular wall is hampered by technical difficulties. Recent attempts to estimate the transmural heterogeneity in workload by mathematical models indicate that major differences between subendocardial and subepicardial layers are not very likely. Flow determinations with adequately sized microspheres or molecular flow markers indicate that transmural flow distribution is close to unity. The observation in some studies that oxygen pressure and venous hemoglobin oxygen saturation is lower in subendocardial than in subepicardial layers suggests a higher metabolic activity in the former layers. However, other biochemical parameters, such as metabolic fluxes, enzyme activities, or concentrations of substrates, cofactors and high-energy phosphates and related compounds, fail to reveal a consistent and substantial transmural difference in energy metabolism. It cannot be excluded that under certain circumstances energy requirements are unevenly distributed across the left ventricular wall. For example, the size of the heart, the awake or anesthetized state of the animal, the level of overall cardiac workload and the efficiency of metabolic-mechanical conversion might influence to some extent the distribution of energy metabolism across the left ventricular wall. Although at present no definite conclusions can be drawn, it is likely that the transmural differences in energy requirements of normally functioning, intermediate-size hearts are limited. Small differences in the order of 10-20% between the subendocardial and subepicardial layers cannot be excluded.