Servo-controlled indenter for determining the transverse stiffness of ventricular muscle

A dynamic indentation system has been developed that can determine transverse stiffness of ventricular muscle in as little as 10 ms, allowing estimation of wall stress over a single contraction cycle. The apparatus consists of an indentation probe coupled to a linear motor. This indentation system was tested on two beating canine ventricular septa that were mounted in a biaxial system that could apply strains in the plane of the septa and measure the resulting in-plane stresses. The probe indented the septa with peak displacements of 0.1-0.5 mm at frequencies of 20 and 50 Hz. The transverse stiffness was calculated as the slope of the relation between the indentation stress and the indentation strain during each high-frequency indentation. Consistent with earlier studies, the transverse stiffness was related to the in-plane stress. In contrast to earlier studies, however, these dynamic transverse stiffness determinations could be made during a single contraction. Thus, dynamic transverse stiffness determinations allow estimation of wall stress in the isolated septa by minimal surface contact, and may lead to methods for estimating wall stress in the intact heart.