Wall stress measurements in non-beating canine hearts: application of a method tested in rubber models

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Abstract
A method was devised for measuring the forces in the left ventricular wall in more detail than was previously possible, making use of 3 force transducer elements in a delta-rosette arrangement. The readings from the 3 elements were combined to obtain the measured stresses. This procedure took into account the fact that a force transducer inserted in a material is sensitive to forces in 2 perpendicular directions instead of only the intended one. Tests in rubber ellipsoids showed that with this method the principal stresses and the angles that they make with the transducer can be measured accurately. Using this method stresses in the circumferential and apex-base directions were measured in 11 passively distended canine hearts and wall stresses were calculated from pressure and geometry, using a simple ellipsoidal model. In 3 experiments measured circumferential stresses were higher than calculated stresses; in 3 experiments measured and calculated circumferential stresses were approximately equal; and in 5 experiments measured stress was lower. In the apex-base direction the results were similar. Calculated stress did not consistently under- or overestimate measured stress, although such a consistency did exist within each experiment. In 10 of the 11 hearts, negative shear stress values suggested that the principal directions of stress in the heart do not correspond exactly to the circumferential and apex-base direction but are shifted slightly counterclockwise. The consistent difference between the calibration constants in directions making angles of -30.degree. and +30.degree. with the equator is evidence for considerable anisotropy in the left ventricular wall.

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