The linear relation between oxygen consumption and pressure-volume area can be reconciled with the Fenn effect in dog...

We studied the Fenn effect in 12 excised cross-circulated dog left ventricles in control contractility and in a contractility enhanced by dobutamine or depressed by propranolol. The additional oxygen consumption (Vo2) in an ejecting contraction compared with that found in an isovolumic contraction at a comparable end-systolic pressure was considered to constitute the cardiac Fenn effect. We examined whether this load-dependent Vo2 could be reconciled with the linear relation between Vo2 and pressure-volume area (PVA) common for both ejecting and isovolumic contractions that has so far been consistently observed and was presently confirmed. PVA is a specific area in the pressure-volume diagram, represents the total mechanical energy generated by each contraction, and consists of external mechanical work (EW) and mechanical potential energy. Because potential energy is common in the isovolumic and ejecting contractions producing the same end-systolic pressure, PVA of the ejecting contraction is greater by EW than that of the isovolumic contraction. Despite this difference in PVA by EW, the Vo2-PVA relation was always linear and load independent regardless of the isovolumic and ejecting contractions in a given heart in any given contractile state. By contrast, the upward convex Vo2-end-systolic pressure relation was higher for ejecting contractions than the downward convex Vo2-end-systolic pressure relation for isovolumic contractions in each contractile state. The difference of Vo2 between the ejecting and isovolumic contractions was proportional to EW at comparable end-systolic pressure. The slope of the additional Vo2 of ejecting contractions plotted against their EW had a slope close to the slope of the Vo2-PVA relation. Thus, the load-independent linear Vo2-PVA relation can be reconciled with the cardiac Fenn effect.