SUBEPICARDIAL FIBER STRAIN AND STRESS AS RELATED TO LEFT-VENTRICULAR PRESSURE AND VOLUME

In a mathematical model of the mechanics of the left ventricle (LV) by Arts et al. (1), assuming uniformity of fiber stress (sigma(f)) and f iber strain (DELTAepsilon(f)) in the wall during the ejection phase, f iber stress and fiber strain were related to LV cavity pressure (P(lv) ), LV cavity volume (V(lv)) and wall volume (V(w)) by the following pa ir of equations: sigma(f) = P(lv) (1 + 3 V(lv)/V(w)) and DELTAepsilon( f) = 1/3 DELTAln(1 + 3 V(lv)/V(w)). The ratio of V(lv) to V(w) appeare d to be the most important geometric parameter, whereas the actual LV shape was of minor importance. The relationships on fiber strain and s tress were evaluated experimentally in six anesthetized open-chest dog s during normal and elevated (volume loading) end-diastolic LV pressur e. Subepicardial fiber strain was measured simultaneously in 16 adjace nt regions of the LV anterior wall, using optical markers that were at tached to the epicardial surface and recorded on video. Changes in V(l v) were measured by use of four inductive coils sutured to the LV in a tetrahedric configuration. V(w) was measured postmortem. During contr ol as well as hypervolemia the following results were found. At the an terior free wall of the LV, the slope of the estimated linear relation ship between measured and calculated fiber strain was 1.017 +/- 0.168 (means +/- SD), which is not significantly different from unity. Calcu lated fiber stress corresponded qualitatively and quantitatively with experimental results reported on isolated cardiac muscle. Calculated s ubepicardial contractile work per unit of tissue volume was not signif icantly different from global pump work as normalized to V(w). These f indings support the assumption of homogeneity of muscle fiber strain a nd stress in the left ventricular wall during the ejection phase. Furt hermore, average values of fiber stress and strain can be estimated on the basis of measured left ventricular pressure and volume.