The mechanical properties of ventricular muscle are a critical determinant of cardiac function. The mechanical properties can be quantified in terms of the muscle's stress-strain relationships, where the stresses and strains are the normalized forces and displacements. The objective of this project is to determine how the mechanical properties of reperfused myocardium are related to the level of flow, to mechanical interventions, and to administered pharmacologic agents. The investigators will first study the mechanical properties of reperfused myocardium in an isolated ventricular septal preparation, where the stresses, strains, and level of flow can be directly measured and controlled. The investigators will first study the mechanical properties of reperfused myocardium in an isolated ventricular septal preparation, where the stresses, strains, and level of flow can be directly measured and controlled. The investigators then intent to obtain an index of wall stress in intact hearts so that they can study the mechanical properties of reperfused muscle in intact hearts. These studies in intact hearts can provide information that is not available from the isolated septum, such as data about ischemic border zones, the influence of ischemic and reperfused areas on nonischemic areas, and the effect of geometry. These studies will require accurate estimates of regional ventricular wall stress, but with currently available methods, it is not possible to accurately quantify wall stress in the intact heart. Our recent studies have shown, however, that the mechanical properties in the plane of the tissue are related to those in the direction perpendicular to the plane of the tissue. They are, therefore, developing a probe for measuring these perpendicular (or transverse) properties. They will verify the close correlation between the in-plane wall stress and the transverse (across the wall) stiffness. The transverse stiffness is determined by transversely indenting the muscle, and is defined as the ratio of the indentation stress to indentation strain. Since the transverse stiffness can be measured in both isolated muscle and intact hearts, once it is validated in the isolated septum that the close correlation between in-plane stresses and transverse stiffness pertains during reperfusion and its attendant interventions, transverse stiffness can be used in the intact heart to estimate in-plane properties.
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