Effects of Preimplantation Processing on Allograft Valves

Abstract

An increase in the actuarial freedom from primary tissue failure and reoperation has been reported following the use of cryopreserved allograft heart valves. It has been hypothesized that the increased durability of cryopreserved heart valves is related to the retention of viable cuspal cells; however, other investigators have suggested that this observation is the consequence of tissue processing methods which do not necessarily preserve cell viability but minimally damage the valvular extracellular matrix. If the retention of viable donor cuspal fibroblasts is responsible for the noted increase in allograft valve durability, then every effort should be made to optimally harvest and process allograft heart valve tissue in a manner that ensures minimal loss of viable cuspal cells. Currently, in most clinical centers, allograft heart valves are harvested, disinfected and cryopreserved in the following manner: 1) warm ischemic time (see discussion to follow) is generally restricted to 24 hours or less; 2) cold temperatures (4°C) are used for dissection to procure the allograft and for subsequent transportation; 3) disinfection typically involves the immersion of the allograft for 24 hours (4°C) in solutions containing antibiotics (e.g., cefoxitin, lincomycin, polymyxin B, vancomycin); 4) the use of dimethlysulfoxide (10%) as a cryoprotectant, and 5) controlled-rate freezing (1 degree per minute to -70°C) of the allograft valve and subsequent storage in liquid nitrogen vapor (-170°C). Independent of the processing method selected, there is an obligatory interval of time, referred to as the warm ischemic time, which elapses from the cessation of the donor’s heartbeat to the initial cooling of the allograft in transport media. The duration of warm ischemia represents a period of potential cellular injury and cell death, which significantly alters the number of viable cells present in the cusp at the time of implantation. Detailed ultrastructural studies evaluating the extent of cellular injury induced by increasing the duration of the warm ischemic intervals have been conducted on porcine, and human aortic and pulmonary valves. Morphologic indicators of reversible cellular injury (dilatation of endoplasmic reticulum, cytoplasmic edema, mitochondrial swelling) are observed early (e.g., 40 minutes) after the death of the donor and gradually progress to irreversible injury (mitochondrial flocculent densities, karyolysis, cell disruption) through 12 hours of warm ischemia. A significant increase (i.e., 10% to 25%) in the number of irreversibly injured cells occurs between 12 and 24 hours of warm ischemia. Approximately 40% of the cuspal cells in human cryopreserved allograft valves demonstrate ultrastructural evidence of irreversible injury following 16 to 20 hours of warm ischemia. As the warm ischemic time increases, a progressive loss of endothelial cells, fibroblasts and myofibroblasts occurs. Endothelial cell loss was most notable initially, while the 24 Effects of Preimplantation Processing on Allograft Valves