MYOCARDIAL STIFFNESS DERIVED FROM END-SYSTOLIC WALL STRESS AND LOGARITHM OF RECIPROCAL OF WALL THICKNESS - CONTRACTILITY INDEX INDEPENDENT OF VENTRICULAR SIZE

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 (σ) and the natural logarithm of the reciprocal of wall thickness [ln(1/H)], σ = Ce(kln(1/H)), corresponds to the stiffness constant of the myocardium (k(SM)), a contractile index that should be independent of ventricular size and geometry. To examine the size independence of k(SM), we studied left ventricular k(SM) during β-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 k(SM) 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 β-blockade, was closely and negatively correlated to end-diastolic volume (r = 0.92). To test the ability of k(SM) to measure changes in contractile state, we altered contractile state pharmacologically. The k(SM) increased from 3.7 ± 0.5 to 4.8 ± 0.8 (p < 0.01) with infusion of dobutamine (after reversal of β-blockade) and decreased to 3.1 ± 0.3 (p < 0.05) with inhalation of isoflurane, a negative inotrope, during β-blockade (p < 0.05). We conclude that k(SM) 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.