Cardiac energetics and the Fenn effect

Summary

The energy output of a cardiac contraction can be divided into several phenomenologically measured components, although it must be emphasized that such subdivisions are often thermodynamically misleading. There is an activation term that relates to Ca ^{+ +} release and retrieval, a work term and a stress or load-dependent heat term. The work and load-dependent energy terms presumably have their origin in the actin-activated myosin ATPase. It can be shown that the enthalpy: load relationship has a similar format across both mammalian and amphibian hearts: the scaling of both the energy and load axes is however altered by changes in contractility. The fact that enthalpy production is so clearly load-dependent indicates that there is a Fenn effect in cardiac muscle, although the discovery that energy output is greatest in an isometric contraction clearly contradicts one of the two central findings of Fenn’s skeletal muscle investigations. Cardiac oxygen consumption per beat can be linearly correlated with ventricular systolic pressure — volume area (PVA) which is defined in terms of stroke work and potential energy components. If the basal and activition components are subtracted out cardiac muscle can be shown to operate at a constant PVA efficiency. The existing myothermic and polarographic data can be reconciled with the PVA concept.