Valvular xenograft and valvular xenobioprosthesis: Past, present, and future

Porcine bioprosthetic heart valves: The next generation

There have been significant advances in organ xenotransplantation (cross-species transplantation), especially in the development of genetically engineered pigs, but clinical trials of solid organ transplants are still a time away. However, there is a form of pig-to-human xenotransplantation that has been taking place since the 1960s-bioprosthetic heart valve (BHV) replacement. Recently, there has been increasing evidence that, despite glutaraldehyde fixation of BHVs, there is a significant immune reaction to the valves, leading to calcification, rapid structural deterioration, and failure, particularly in young patients who have a more vigorous immune system and metabolism than the elderly. However, it is the young patients who would most benefit from such BHVs because these avoid the complications associated with the lifelong anticoagulation required with mechanical valves. In this review, we examine pathologic and immunohistochemical reports of failed BHVs that suggest that there is an immune response to these valves. Small animal studies that link the development of calcification and BHV failure to the immune response are reviewed. We draw parallels between the problems of glutaraldehyde-fixed tissue xenotransplantation and those currently being faced in live organ xenotransplantation. Finally, we discuss the advances being made in the production of genetically modified pigs and the evidence that these pigs may become a source of BHVs that can be used worldwide to treat valvular heart disease in children and young adults (for whom there is no ideal valve replacement in existence today). The design of a BHV that is resistant to the host's immune response would be a major step forward in cardiac surgery.

A computerized system for video analysis of the aortic valve

A novel technique was developed to study the dynamic behavior of the porcine aortic valve in an isolated heart preparation. Under the control of a personal computer, a video frame grabber board continuously acquired and digitized images of the aortic valve, and an analog-to-digital (A/D) converter read four channels of physiological data (flow rate, aortic and ventricular pressure, and aortic root diameter). The valve was illuminated with a strobe light synchronized to fire at the field acquisition rate of the CCD video camera. Using the overlay bits in the video board, the measured parameters were super-imposed over the live video as graphical tracing, and the resultant composite images were recorded on-line to video tape. The overlaying of the valve images with the graphical tracings of acquired data enabled the data tracings to be precisely synchronized with the video images of the aortic valve. This technique enabled us to observe the relationship between aortic root expansion and valve function.

Effects of dynamic fixation on shear behaviour of porcine xenograft valves

To evaluate an alternative valve fixation technique, we measured the ability of glutaraldehyde-fixed valve tissue to undergo internal shearing during bending. Porcine aortic valves were fixed statically using conventional means, and dynamically while opening and closing repeatedly in a pulse tank. Using a polarized light microscopy technique developed previously, we measured shear deformation angles in thin sections of bent leaflet tissue and calculated shear strains. Statically-fixed leaflet tissue sheared only 1.2% +/- 2.29% (Mean +/- SD) when bent to curvatures of 2.0 mm-1, while dynamically-fixed tissue sheared 5.1% +/- 2.63% (significant at P less than 0.05). It is likely that dynamic fixation increases the ability of prosthetic valve leaflets to shear during bending by reducing the number of interfibre cross-links that would otherwise impede such deformations. Because shear strains reduce internal fibre strains and protect the leaflets against fatigue, prosthetic valves constructed from dynamically fixed tissue should experience lower stresses and hence last longer.