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.
Department of Tracer Kinetics, Osaka University Medical School, Japan.
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Kusuoka
D008955:Models, Cardiovascular; D009200:Myocardial Contraction / Q000502:physiology*