Cryopreserved Allograft Heart Valves

Allograft valves are removed at either autopsy or heart transplantation from one person and transplanted to another. Most such valves are cryopreserved (i.e., stored at -170° in dimethyl sulfoxide) until needed (Fig. 7). Aortic valve allografts have good hemodynamic profiles, low thromboembolic rates despite the absence of anticoagulation, low rates of infection, and low rates of degeneration.66,67 There has been ongoing controversy regarding modes of failure, cellular viability, durability of the extracellular matrix, and whether immune responses contribute to failure.

We examined 20 explanted cryopreserved valves in place for several hours to 9 years, obtained by virtue of the author's role as core pathologist for a large clinical trial conducted under an investigational device exemption held by a consortium of five tissue banks. The original surgeries and explants were done in more than 100 hospitals. Arrangements were made by the consortium to have removed valves sent directly to the core pathology laboratory, accompanied by clinical data. We also examined thawed but unimplanted allografts (obtained via the same route) and 16 donor aortic valves obtained from heart transplants of recipients who later died.

Our studies indicated that removed cryopreserved allograft heart valves undergo progressively severe loss of the normal layered structure and stain-able deep connective tissue cells with minimal inflammation.68 Following either short-term or extended function, cryopreserved allograft heart valves have minimal, if any, viable cells but largely retain the original collagen network; it is this preservation of the autolysis-resistant collagenous skeleton that likely provides the structural basis for their generally favorable function. Because inflammation in these valves is minimal in almost all cases, we have found no evidence that immune responsiveness can impact late allograft function or degeneration. Interestingly, and in contrast, aortic valves of transplanted whole hearts maintained near-normal overall architecture, and cells.

Our studies of retrieved allograft valves emphasize that (1) not only failure modes but also mechanisms of successful function can be elucidated by careful study, (2) correlation of results from multiple types of implants can be exceedingly important in understanding structure-function correlations, and (3) a well-organized network can pool relatively unusual specimens obtained from many centers at a core pathology laboratory, lead to insights not readily obtainable from clinical material that accrues and thereby at a single institution.

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