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Twenty-five human cryopreserved valves with harvest-related warm ischemic times (WITs) ranging from 0 to 20 hours were studied using transmission electron microscopy to characterize the effects of harvesting and preservation on leaflet matrix cells. The valves were divided into seven groups on the basis of WIT and processed using standard transmission electron microscopic methods. Each cell (528 micrographs) was graded for reversible and irreversible cellular injury and subjected to a Cochran-Mantel-Haenszel trend analysis. Our results demonstrated a progression in cellular injury with increasing WIT. During the first 12 hours of warm ischemia, reversible cellular injury predominated (0.0%, 30.0%, 51.2%, 31.3%, 35.1%, 45.1%, and 40.0% at WITs of 0, 1, 2, 8, 12, 16, and 20 hours, respectively). A positive correlation (p < 0.0001) between increasing WIT and reversible cellular injury through the first 12 hours was observed. Minimal morphologic evidence of irreversible injury was noted in valves harvested with less than 12 hours of warm ischemia; however, after 12 hours there was a marked increase (0.0%, 0.0%, 4.7%, 2.4%, 2.7%, 31.4%, and 40.0% at WITs of 0, 1, 2, 8, 12, 16, and 20 hours, respectively) in irreversible cellular injury (p < 0.001 between 12 and 20 hours WIT). These data demonstrate a progression in cellular injury with increasing WIT. There was virtually no morphologic injury in valves with harvest-related WITs less than 2 hours and minimal irreversible cellular injury observed in valves exposed to 12 hours or less of warm ischemia. If cellular viability is critical to homograft durability then harvest-related warm ischemia may need to be restricted to 12 hours.

Cryopreserved allograft valves are increasingly being used as valvular replacements. Leaflet fibroblast viability has been suggested to influence clinical durability. The warm ischemic time is thought to be a critical determinant of this cell viability. The purpose of this study was to apply quantitative morphometric methods to characterize, by transmission electron microscopy, valvular cellular injury resulting from progressive warm ischemic time. Porcine aortic valves were harvested with a spectrum of warm ischemic times (40 minutes and 2, 6, 12, 24, and 36 hours; five valves per warm ischemic time; n = 30) and processed by standard electron microscopic methods. To ensure randomized tissue selection within each warm ischemic time interval, we randomly selected one thin section from each leaflet. The first ten cells in each thin section were photographed and cellular injury was assessed (cell disruption, dilation of endoplasmic reticulum, cytoplasmic edema, nuclear and mitochondrial changes). Nine hundred micrographs have been analyzed by Cochran-Mantel-Haenszel statistics to determine if a significant association between warm ischemic time and cellular injury exists. Our findings indicate a significant association between reversible cell injury through 24 hours of warm ischemic injury (p less than 0.0001). Furthermore, a significant association between irreversible cell injury and progressive warm ischemia through 36 hours was also found. These findings indicate that the ischemic interval after donor death is associated with progressive leaflet cell injury. Cellular damage begins shortly after donor death and continues incrementally throughout 36 hours. After 2 hours of warm ischemic injury 37% of the cells had morphologic evidence of injury. After 6 hours of warm ischemic injury the number of injured cells increased to 73%. By 36 hours 22% of the cells appeared normal. Irreversible cell injury increases with prolonged ischemia and becomes quantitatively impressive at 24 hours, by which time 26% of cells are so affected. Conversely, some cells are resistant to irreversible injury for a prolonged ischemic interval.

Criteria and evaluation procedures have been established to assess the measurement accuracy of Automated Microwave Oven Leakage Survey Systems. These criteria and procedures are based upon analytical characterizations of the oven leakage fields, measurement data and statistical analysis and experience gained from evaluation of hand-held oven survey instruments. Results of our evaluation of a typical survey system are presented.