An alternate formulation of blood vessel mechanics and the meaning of the in vivo property.

In an effort to bridge the gap between theoretically based vascular mechanics and simpler, clinically relevant compliance models, an alternative complementary energy formulation as well as an improved experimental method are proposed. This formulation generalizes the uniaxial compliance models to multiaxial stretch and twist and clarifies the role of the total in vivo force (measured in biaxial vascular testing) in vessel stability. The measurement of wall thickness when incompressibility is assumed, and the assumption of the existence of an unloaded state are unnecessary. Scattered results from both clinical and mechanical literature are reinterpreted in view of this new understanding of vascular mechanics. Canine saphenous veins (4-5 mm in diameter) conform to the behavior expected from the analysis. Their in vivo property is shown to be a passive property, independent of smooth muscle tone. Redundant particle tracking, control of force and pressure values and the use of papaverine, result, respectively, in strain measurements accurate to within 0.002, experimental control accurate to 0.5% of full range, and repeatable experiments over a few days. The validated system and the new formulation show great potential for exploiting the benefits of innovative experimental design and statistically reliable data analysis.