Is the linear VO2 PVA relation of the heart a fortuitous empiricism?

Content Partner: The University of Auckland Library. We aimed to understand the much observed, but ill understood, linear relation between oxygen consumption (VO2) and pressure volume area (PVA) of the intact heart, first observed by Suga and colleagues. PVA consists of two components: mechanical work (W) and ‘potential energy’ (‘U’). To say whether the linear VO2 PVA relation is the physiological reality or merely a fortuitous empiricism, it is necessary to grasp the meaning of ‘U’. To that end, we constructed a work loop calorimeter. In this unique device, cardiac trabeculae were required to perform work loop contractions to mimic the PV loops of the intact heart. Their stress (force per cross sectional area) production, length change and heat output were simultaneously measured. We retrospectively calculated the equivalent VO2 (W + heat) and the 1 D equivalent of PVA [i.e., stress length area (SLA)] of the trabeculae. We observed that the equivalent VO2 of trabeculae was a linear function of their generated SLA, consistent with the linear VO2 PVA of the heart. We found that the ‘U’ component of SLA was a linear function of the heat component of VO2. We further discovered that ‘U’ was proportional to the area under the twitch stress time profile. We consider that ‘U’ is the unused energy that is liberated as heat. It should not be appeared in the numerator of any index of cardiac efficiency.