Theoretical analysis of the relationship between regional and global measures of myocardial contractility.

Contractile function of the left ventricle is often measured regionally, particularly in intact animals. In this paper I dispute the validity of the common assumption that regional shortening accurately reflects whole-heart mechanics, even when contractility is homogeneous, by using an ellipsoidal model of the left ventricle to examine the relationship between measures of regional function and whole-heart function. Segment shortening and wall thickening are the most common measures of regional function, while the end-systolic pressure-volume relationship, or Emax, is a well-validated index of contractility in the whole heart. Assuming that the arterial end-systolic pressure vs stroke volume relationship is constant, segment shortening shows a monotonically increasing, convex relation to Emax. If, in addition, myocardium is considered an incompressible material, then wall thickening also shows a monotonically increasing, convex relation to Emax. Thus, segment shortening and wall thickening are quite insensitive when measuring high contractility and overly sensitive when measuring low-contractility. Finally, I consider the non-linear relationship between regional measures of function and global contractility, and thereby resolve apparently contradictory findings regarding the inotropic responsiveness of stunned myocardium.