Viscoelastic properties of the diastolic left ventricle in the conscious dog.

The mechanical properties of the normal left ventricular wall during diastole were studied in 15 chronically instrumented, conscious dogs. Left ventricular minor and major axis diameters and equatorial wall thickness were measured with implanted pulse-transit ultrasonic dimension transducers. Left ventricular and pleural pressures were measured with high fidelity micromanometers. Circumferential mural stress was calculated by using an ellipsoidal shell theory; circumferential strain was calculated by using a natural strain definition. The static elastic properties of the myocardium were estimated by fitting the stress-strain values at the points of diastasis during a vena caval occlusion to an exponential function. A modified creep test was used to evaluate the series viscous properties of the myocardium. Acute increases in systolic and diastolic loading were produced by inflating implanted aortic occluders for 15 minutes in five dogs. In these dogs, the static stress-strain curves were not altered significantly after this period of pressure loading, indicating tbat short-term series viscous properties are negligible. Parallel viscous properties were evaluated in 10 dogs by means of the variable rate stretch test of dynamic diastolic filling. A viscoelastic model incorporating a parallel viscous element fit the dynamic stress-strain data better and predicted the static elastic properties more accurately than a simple exponential model. Thus, the mechanical characteristics of the diastolic left ventricle can be represented most precisely by a viscoelastic model that includes a parallel viscous element.