Effects of fixation pressure on the biaxial mechanical behavior of porcine bioprosthetic heart valves with long-term cyclic loading

Zero transvalvular pressure fixation is thought to improve porcine bioprosthetic heart valve (BHV) durability by preserving the collagen fiber architecture of the native tissue, and thereby native mechanical properties. However, it is not known if the native mechanical properties are stable during long-term valve operation and thus provide additional durability. To address this question, we examined the biaxial mechanical properties of porcine BHV fixed at 0 and 4 mmHg transvalvular pressure following 0, 1×106, 50×106, and 200×106 in vitro accelerated test cycles. At 0 cycles, the extensibility and degree of axial cross-coupling of the zero-pressure-fixed cusps were higher than those of the low-pressure-fixed cusps. Furthermore, extensibility of the zero-pressure-fixed tissue decreased between 1×106 and 50×106 cycles, approaching that of the low-pressure-fixed tissue, whose extensibility was unchanged over 0–200×106 cycles. The decrease in extensibility of the zero-pressure-fixed tissue between 1×106 and 50×106 cycles may be attributable to the ability of its collagen fibers to undergo larger changes in orientation and crimp with cyclic loading. These observations suggest that the collagen fiber architecture of the 0-mmHg-fixed porcine BHV, although locked in place by chemical fixation, may not be maintained over a sufficient number of cycles to be clinically beneficial. This study further underscores that chemically treated collagen fibers can undergo conformational changes under long-term cyclic loading not associated with damage.