Myocardial Stiffness Derived From End-Systolic Wall Stress and Logarithm of Reciprocal of Wall Thickness

The slope of the end-systolic pressure-volume relation (ESPVR) is useful in assessing acute changes in contractile state. However, a limitation of ESPVR is that its slope decreases progressively as ventricular size increases without this change necessarily indicating a change in contractile state. In this respect, an index of contractile function that is independent of ventricular size would have an obvious advantage. The exponential constant (k) of the end-systolic relation between wall stress (cr) and the natural logarithm of the reciprocal of wall thickness [1n(1/H)], cr=Ce'D(lfH), corresponds to the stiffness constant of the myocardium (ksM), a contractile index that should be independent of ventricular size and geometry. To examine the size independence of ksM, we studied left ventricular ksM during P-blockade (to stabilize inotropic state) in 25 normal dogs with greatly differing ventricular sizes whose end-diastolic volumes ranged from 14 to 82 ml. The ksM was nearly constant (3.6±0.4) over this wide range of end-diastolic volumes and thus was independent of end-diastolic volume. Conversely, ESPVR, also obtained during P-blockade, was closely and negatively correlated to end-diastolic volume (r=0.92). To test the ability of ksM to measure changes in contractile state, we altered contractile state pharmacologically. The ksM increased from 3.7±0.5 to 4.8±0.8 (p<0.01) with infusion of dobutamine (after reversal of 13-blockade) and decreased to 3.1+0.3 (p <0.05) with inhalation of isoflurane, a negative inotrope, during j/-blockade (p<0.05). We conclude that ksM is independent of ventricular size and is sensitive to changes in inotropic state. As such, it should be useful as an index of contractile function. (Circulation 1990;82:1352-1361)