Relations between the ejection fraction (EF), parameters describing the end-systolic pressure-volume relation (ESPVR) and the areas under the ESPVR are derived for a linear model of the ESPVR. The active pressure generated by the myocardium during an ejecting contraction (also called isovolumic pressure Piso by physiologists) is included in the mathematical formalism describing the ESPVR. The various areas under the ESPVR are sensitive indexes that reflect the state of the myocardium. Applications to clinical data published in the medical literature show the consistency of the mathematical formalism. Results indicate that using bivariate (or multivariate) analysis of data is superior to using one index (like EF) for the purpose of segregation between different clinical groups. When ratios of pressures are used, calculations can be carried out on data measured in a non-invasive way (ratio of pressures can be calculated by using the mathematical formalism introduced).