Experimental evaluation of the elastic determinants of myocardial function in vivo.

Shortening of myocardial fibers occurs following force development in those fibers. The extent, speed and timing of shortening are determined by kinetics and extent of force. However, shortening is also influenced by the elastance/viscosity of the muscle tissue, because that determines the coupling between force and shortening. In the in vivo dog heart, we estimated that coupling by measuring local myocardial force and fiber shortening independently under various conditions. We determined the effect of positive and negative inotropy (by intracoronary injection of dobutamine and acetylcholine, respectively), and of dysfunctional contraction produced by local ischemia/reperfusion and BDM. Under baseline and both positive and negative intropy, most shortening occurred during systole, and dobutamine increased the proportion of total shortening in early systole from 45.8 +/- 8.5% to 74.9 +/- 9.6%. During reperfusion following ischemia, shortening in early systole was markedly reduced to 16.5 +/- 2.9; BDM caused a similar reduction to 16.5 +/- 8.1. Most of the shortening occurred during early diastole (53.0 +/- 6.8 for reperfusion, and 54.0 +/- 10.3 for BDM). These effects were all reversible. It is concluded that energetic efficiency is greatly affected by the elastic properties coupling force and shortening. Thus appropriate analysis of muscle function must take into account the changeable elastic properties of the tissue-both force and shortening, and their interaction must be considered.