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 (MV̇O₂), and efficiency, in terms of the relation between MV̇O₂ 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 ~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 MV̇O₂-PVA relation was less during ejecting than isovolumic contractions, indicating a decrease in MV̇O₂ 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 MV̇O₂ and total mechanical energy generation (PVA) by the ventricle. This may reflect an energetically favorable effect of shortening on muscle force-generating capability.