Comparative Evaluation of Myocardial Performance Factors

Abstract

The manner in which the heart functions as a pump is of vital importance to maintaining an adequate flow of blood to the tissues of the body. Evaluation of its performance as a pump continues to be a serious problem in diseased states where that performance is compromised. Numerous variables have been proposed to evaluate pumping ability, hopefully related to inotropic changes such as can be described for the isolated papillary muscle fibre. These relate to the effect of a change in contractility on the relationship between velocity of contraction and force or pressure produced (afterload), velocity and initial length (preload) and the length-force relationship (work) (Figure 1). Many variables have been based on one plane of that diagram, with a shift to a different curve expected with a change in the inotropic state. Such variables, however, are often of limited value in the intact individual because of difficulty in making the measurements or because they are correlated with changes in preload, heart rate, or afterload. The desired variable would be independent of these factors but sensitive to changes in the inotropic state whether they are produced from extrinsic or intrinsic sources. The sensitivity of the various measurements utilizing pressure measurements alone have previously been described (1). This paper is an extension of those observations to include flow and impedance variables.