BIAXIAL MECHANICS OF EXCISED CANINE PULMONARY-ARTERIES

A new method has been developed for measuring the stress-strain relati onship in excised canine pulmonary arteries. Segments of dog main righ t pulmonary arteries were isolated by making two transverse cuts at ea ch end of a segment near the bifurcations, yielding short cylinders, w hich were then cut radially, relieving the residual stress, causing th e cylindrical shells to spring open to approximately flat rectangular slabs with dimensions similar to 1.0 x 3.0 x 0.1 cm. The specimens wer e then tested using a biaxial tensile testing machine. The resulting d ata show an approximately linear relationship between Kirchhoff stress and Lagrangian strain with very little hysteresis. The following pseu dostrain energy function serves as a practical approximation for pulmo nary arteries subjected to physiological levels of stress and strain: rho(o)W((2)) = 1/2(alpha(1)E(xx)(2) + alpha(2)E(yy)(2) + 2 alpha(4)E(x x)E(yy)), where rho(o) is the density of the wall (mass per unit volum e), W is the energy per unit mass [superscript ''(2)'' indicates this is a 2-dimensional strain energy function], E is strain, alpha(1), alp ha(2), and alpha(4) are material constants with units of stress, and t he subscripts x and y refer to the circumferential and axial axes, res pectively, of the artery. To assess the physiological level of strain in the main right pulmonary artery, vessels were perfused in situ at p hysiological pressure (26 cmH(2)O) with silicone elastomer. The arteri es were then excised and marked with small ink spots. Photographs of t he spots on four tangent planes of the excised artery indicate a maxim um circumferential strain of 21.5% and a maximum axial strain of 36.5% relative to the zero-stress state. These values are within the range of strain used in the biaxial tests. The relationship between Kirchhof f stress and Green's strain is approximately linear within the physiol ogical range. The stress levels required to cause tissue failure are a t least 10 times greater than the estimated normal physiological level .