CONTRACTILE STRENGTH AND MECHANICAL EFFICIENCY OF LEFT-VENTRICLE ARE ENHANCED BY PHYSIOLOGICAL AFTERLOAD

Recent studies have shown that at the same end-systolic volume, ejecting beats can achieve a higher end-systolic pressure than isovolumic beats. The purpose of this study was to assess the metabolic cost, in terms of oxygen consumption (MVo2), and efficiency, in terms of the relation between MVo2 and pressure-volume area (PVA), of this increase in strength during ejection. The slope of the end-systolic pressure-volume relation (ESPVR) (E(es)) was greater during ejecting than isovolumic contractions when ejection fraction (EF) was greater than approximately 30%, indicating an increase in contractile strength. The difference in E(es) between the two modes of contraction was as much as 30% at EFs of 60%. In contrast, the slope of the MVO2-PVA relation was less during ejecting than isovolumic contractions, indicating a decrease in MVO2 at any given PVA. The difference in slope was as much as 20% at EFs of 60%. Thus afterload conditions, allowing substantial fiber shortening, shift the ESPVR toward greater contractile strength and increase the metabolic efficiency when viewed in terms of the relation between MVO2 and total mechanical energy generation (PVA) by the ventricle. This may reflect an energetically favorable effect of shortening on muscle force-generating capability.