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Ann Thorac Surg 1998;65:1032-1038
© 1998 The Society of Thoracic Surgeons

Allograft Heart Valve Viability and Valve-Processing Variables

^a Department of Cardiac Surgery, The Prince Charles Hospital, Chermside, Brisbane, Australia

Accepted for publication November 3, 1997.

Address reprint requests to Dr O’Brien, Department of Cardiac Surgery, The Prince Charles Hospital, Rode Rd, Chermside, Brisbane 4032, Queensland, Australia

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Background. The impact of allograft valve viability on valve durability remains controversial. Analyses of our clinical results have demonstrated the superiority of the cryopreserved valve viable at the time of implantation over the 4°C stored valve nonviable at the time of implantation. In this study, we quantitatively assessed the effects on viability of current and past valve-processing protocols at The Prince Charles Hospital.

Methods. The viability of pulmonary valves was quantitatively analyzed by thin-layer autoradiography to assess the effects of donor type, antibiotics, and valve storage.

Results. Control valve segments obtained from beating-heart donor valves had a higher initial viability (0.92 ± 0.02) than nonbeating-heart donor valves (0.66 ± 0.03). Cryopreservation after low-dose antibiotic sterilization significantly reduced viability to 50% to 60% of the control, and in the presence of amphotericin B, viability dropped further to 10% to 36% of the control. After 7 days’ storage at 4°C, viability was reduced to 2% of control and to 0% viability after 21 days.

Conclusions. For maximal preimplantation viability, valves should be procured as soon as possible after cessation of heart beat and should be cryopreserved if they are not to be clinically implanted within 1 to 2 days. Amphotericin B should not be used in conjunction with cryopreservation if viability is to be maximized.

	Introduction

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
The clinical results for 4°C stored and cryopreserved allograft aortic valves implanted at The Prince Charles Hospital (TPCH) continue to show after many analyses over time, the superiority of allografts viable at the time of implantation [1–3]. Many factors, including viability, can influence the durability of an allograft as assessed by freedom from structural deterioration.

The importance of viability is disputed by many. There is no strong substantial evidence that viable cells persist and produce collagen and repair damaged matrix. Their presence (before implantation) indicates optimal tissue preservation overall. This at least may be the strongest argument for aiming for and maintaining allograft valve viability in valves for clinical implantation.

The clinical outcome at TPCH for viable allograft aortic valves with respect to structural deterioration has been clearly defined [1] and compares more than favorably with other allograft valve series [4]. The results obtained at TPCH are of particular importance because of the level of control this institution has had over all aspects of collection, processing, and storage. The protocol for valve collection from nonbeating-heart donors has remained virtually the same for more than 27 years, and its most important aspect is an insistence on early collection, ie, within 24 hours after death (mean time, 15 hours). The antibiotic sterilization protocol has been altered only twice: to respond to the potential deleterious effects of amphotericin B and to reduce antibiotic incubation time from 24 hours to 6 hours (Table 1). A recent retrospective analysis of the microbiology of tissue samples obtained at collection, at processing, after antibiotic sterilization, and at implantation has shown the effectiveness of the current TPCH protocol of only 6 hours’ antibiotic sterilization [5].

The aim of this study was to assess quantitatively the effects on valve viability of the current TPCH protocol of valve procurement, antibiotic exposure, cryopreservation, and cryoprotectant removal at valve thaw. Previous TPCH protocols (see Table 1), including the use of the antifungal agent amphotericin B, 24-hour incubation, and refrigerated storage, were investigated to link the historical results of nonviable 4°C stored valves at TPCH. Gentamicin sulfate, although never used clinically at TPCH, was studied as a possible alternative to streptomycin sulfate.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Valves
Pulmonary allograft valves were used in this investigation in accordance with TPCH Ethics Committee (approval No. EC9104). Valves were obtained from two sources: beating-heart donors (multiorgan donors and heart transplant recipients) and nonbeating-heart donors where valves were procured within 24 hours after cessation of heart beat (autopsy source). The leaflets of the pulmonary valves were divided into two pieces. Thus each pulmonary valve used in this investigation provided one control segment and five segments for experimental use.

Experimental series
Four series of experiments were performed to assess both the current and past methods of valve processing and storage. The experiments were performed on a minimum of 5 valves for each series.

Series 1 used beating-heart donor valves (n = 5) and investigated the effects of short exposure to antibiotics at room temperature, cryopreservation, and method of cryoprotectant removal (Table 2). Gentamicin, a possible substitute for streptomycin, was also studied.

Cryopreservation
For CRC, valve segments were packaged in 100 mL of M199 containing 10% dimethyl sulfoxide (DMSO) (Merck, Darmstadt, Germany) and frozen at a rate of approximately 1°C per minute in a computer-controlled freezer (model 1010A; Cryomed, Mount Clemens, MI). Segment 6 in series 4 was similarly packaged and underwent a non-CRC process. The package was wrapped in cotton wool, placed inside a Styrofoam container, and suspended in the liquid nitrogen vapor immediately above the liquid nitrogen. The cryopreserved segments were stored in the vapor phase of a liquid nitrogen freezer at lower than -135°C for a minimum of 7 days before thawing. The non-CRC process was used to mimic the clinical protocol before the introduction of CRC when valves were cryopreserved by suspension in the liquid nitrogen vapor phase.

Thawing and DMSO removal
Segments were thawed by the current TPCH protocol of package immersion into a warm (42°C) normal saline solution bath. The temperature of the saline solution was maintained at 37°C during thawing. As soon as each segment was thawed, it was taken from the freezing solution. The DMSO was removed from the thawed segments by a single-step dilution or stepwise dilution method. The single-step dilution method involved four rinses of 2 minutes each in M199. The stepwise dilution method involved four rinses of 2 minutes each in M199 containing 5% DMSO, 2.5% DMSO, 0% DMSO, and 0% DMSO, respectively. All rinse solutions were maintained at lower than 10°C.

Autoradiography
Proline uptake
Segments were rinsed in 20 mL of HAMS F-10 medium (ICN Pharmaceuticals Australasia Pty Ltd, Sydney, Australia) before being placed in vials containing 7.5 mL of HAMS F-10 medium, 0.75 mL of fetal bovine serum (CSL Limited), and 0.1 mL of L-[5-³H]proline (37.0 MBq/mL) (Amersham Australia, Sydney, Australia). The open vials were placed in a 5.0% carbon dioxide incubator at 37°C for 6 hours.

Histologic processing and sectioning
After proline uptake, segments were placed in vials of cold (4°C) phosphate-buffered saline solution for two rinses of 5 minutes’ duration to remove unincorporated proline. The leaflets were then put in vials containing 20 mL of 4% phosphate-buffered formaldehyde for 30 minutes before being transferred to an automatic tissue processor for routine histologic fixation, dehydration, wax embedding, and sectioning.

Nuclear emulsion
Slides were placed in slide racks, hydrated, and dipped into K2 nuclear emulsion (Ilford Imaging Australia, Mount Waverley, Victoria, Australia) to provide a thin film over the sections under darkroom conditions. The slides were dried in a 37°C oven and placed in lightproof boxes with silica gel for 21 days at 4°C.

Developing, staining, and mounting
Slides were developed (D-10; Kodak, Australia) and fixed (Na₂S₂O₃, 15.6 g/L, and K₂S₂O₅, 1.6 g/L). They were then stained with a routine hematoxylin and eosin method and mounted with glass coverslips.

Counting
The slides were randomly labeled with a sequential number to blind the observer performing the count. A single observer was used to ensure that the same criteria for viability were applied to all sections and that other cell types, eg, endothelial cells and white blood cells, were not included in the count. Fibroblast viability was indicated by a greater concentration of silver grains around the cell nucleus than in the background (Fig 1). Nonviable fibroblasts were easily identified by the absence of the silver grains.

View larger version (101K): [in this window] [in a new window]   Fig 1. Segment of pulmonary leaflet after autoradiography. Silver grains clustered around a cell nucleus identify a viable fibroblast. Nonviable fibroblasts are without silver grains. (Hematoxylin and eosin; x400 before 50% reduction.)

Statistical analysis
The mean percentage of the initial viability (control) was used for statistical comparisons between treatments within the same series and between series. A two-way analysis of variance without replication was used for this analysis with a p value of less than 0.05 considered significant. A regression analysis was performed on the initial viability (control) at processing over time from cessation of heart beat to collection with a p value of less than 0.05 being significant.

	Results

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
A graphic representation of the initial viability (controls) of all valves examined plotted against time from asystole to processing for beating-heart donors and nonbeating-heart donors is depicted in Figure 2. A regression analysis of valve viability from all sources produced a correlation coefficient (r) of -0.692 and a test for correlation (p) of 0.0004. The beating-heart group had an r of -0.994 and a p of 0.0006 and the autopsy group, an r of -0.268 and a p of 0.298. The results for series 1 through 4 are shown at the end of Tables 2 through 5, respectively. The viability for each segment exposed to a particular treatment is expressed as the mean viability ± the standard error and as the mean percentage of the initial (control) viability.

View larger version (14K): [in this window] [in a new window]   Fig 2. Viability of pulmonary valves at time of processing: a comparison between beating-heart and nonbeating-heart donors. This represents the control viability for each donor.

Pulmonary valve leaflet segments from the series 2 group of nonbeating-heart donors (see Table 3) showed the lower initial (control) proportion of viable cells of 0.58 ± 0.06 encountered with nonbeating-heart donors. This did not change significantly after 60 minutes’ exposure to antibiotics (penicillin and streptomycin) at room temperature. The current clinical protocol for valves from this source resulted in a viability level of 54% of control after cryopreservation and DMSO washout. There was no significant difference between the two antibiotic cocktails used or the method of DMSO washout.

The initial (control) proportion of viable cells from leaflet segments obtained from the group of nonbeating-heart donor pulmonary valves used in series 3 (see Table 4) was 0.62 ± 0.05. Viability decreased to 85% of control after 24 hours’ incubation with penicillin, streptomycin, and amphotericin B, but this was not significant. Subsequent cryopreservation followed by nonsequential washout of DMSO as in the processing protocol used at TPCH from 1975 to 1988 resulted in a significant reduction in viability to 17% (p < 0.05) of control. No significant difference between the two methods of DMSO removal was noted. Storage of valves at 4°C for 7 days significantly reduced viability to 2% of control, and after 21 days, no viable cells were observed in any section.

Pulmonary valve leaflet segments from the series 4 group of nonbeating-heart donors (see Table 5) demonstrated an initial (control) proportion of viable cells of 0.74 ± 0.04. Viability decreased to 85% of control after 60 minutes’ exposure to antibiotics (penicillin, streptomycin, and amphotericin B) at room temperature. However, this was not significant. A further 6-hour incubation in these antibiotics at 37°C did not reduce viability. However, after cryopreservation, viability was significantly reduced to 36% of control (p < 0.05) after 60 minutes’ exposure at room temperature prior to cryopreservation and to 34% of control (p < 0.05) after 6 hours’ incubation at 37°C prior to cryopreservation. The use of a non-CRC technique produced a lower level of viability, 26% of control but this was not significant.

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
A number of methods for determining valve viability have been proposed. Methods using the uptake of radiolabeled thymidine, which relies on the division of cells as a marker of viability, are unsatisfactory for fibroblasts, which by nature are slow dividing [11]. A variety of other methods including assessment of collagen synthesis, protein synthesis, dye uptake, and dye exclusion are available. The method selected must apply to the cell type investigated [12]. Autoradiography has the advantage of assessment of cell function over a period of 6 hours and also examination of the cells in situ in their natural environment. Repeat counting and determination of overall valve cellularity can also be performed with this technique. For these reasons, autoradiography after tissue uptake of radioactive proline was selected as the quantitative method of determining viability.

Because of the paucity of aortic valves for research use, the viability of pulmonary valves was examined in this study. Although the correlation between pulmonary and aortic valve viability has not been assessed, there is no obvious reason to suspect that there would be a significant difference. The initial viability of beating-heart valve donors, 0.92 ± 0.02, compared with that of nonbeating–heart donors, 0.66 ± 0.03, indicates the superior viability of this valve source. Figure 2 demonstrates the reduction in viability with time from cessation of heart beat. In the beating-heart donor group, there is a significant correlation between time for processing and viability but not in the autopsy group. A longer time interval was not investigated, as all retrievals occur within the strict 24-hour period. Other factors such as donor weight, cooling, and initial leaflet cellularity may be important in determining the viability at the time of collection. For instance, a small donor with high cellularity may cool rapidly and provide a high level of viability at the time of valve procurement. The shortage of pediatric donors in our experience may warrant extending the 24 hours from death to collection interval in this particular group. However, the impact of extending this interval on the risk of increasing microbial contamination would require assessment before clinical protocol modification.

The antibacterial effectiveness of the current TPCH antibiotic sterilization protocol of incubation in low-dose penicillin and streptomycin for the short time of only 6 hours at 37°C has been reported [5]. Now the gentleness of this protocol has been further demonstrated by the evidence of the maintenance of viability after incubation. The use of amphotericin B was discontinued at TPCH in 1988 in response to its potential deleterious effects on cell membranes. The results of this investigation confirm the reported toxicity of amphotericin B [13, 14]. Whereas the use of amphotericin B at 37°C for 6 and 24 hours had no significant effect on viability before cryopreservation, the combination of exposure to amphotericin B and cryopreservation was very deleterious to viability. An exposure time of 24 hours followed by cryopreservation produced a significantly lower level of viability (10%) compared with the level of viability obtained when the exposure was limited to 60 minutes (36%) and 6 hours (34%) followed by cryopreservation.

The presence of the cryoprotectant DMSO increases the osmolarity of the freezing medium. Reports suggest that the DMSO should be removed gradually from the tissue to reduce the risk of rapid water movement into the cell, as this may lead to cellular disruption [15, 16]. In this study, a simple single-step method of DMSO removal produced a slightly lower level of viability than the stepwise dilution method of cryoprotectant removal, but the difference was not significant.

The use of refrigerated storage of leaflet segments for 7 days resulted in a significantly lower viability than that obtained for all incubation and cryopreservation methods investigated. We do not recommend the practice of refrigerated storage for a few days or weeks followed by cryopreservation. Even if viability is not a consideration, the possibility of deterioration of the noncellular matrix requires attention.

Controversy has surrounded and continues to surround the importance of the presence of viable fibroblasts within the allograft leaflet matrix at the time of implantation. At TPCH, there is evidence that cryopreserved valves, viable at the time of cryopreservation, have a much lower level of structural deterioration than nonviable valves, and this is supported by other published valve series [4]. In addition, from the work by Yacoub and colleagues [17] on homovital valves (from beating-heart donors, unprocessed, and implanted within 3 days), the inference can be made that valves with some or major fibroblast viability produced improved clinical long-term durability. The clinical experience at TPCH [3] and the pathologic analyses strongly support this theory. At explantation, TPCH cryopreserved valves are more likely to have leaflets that are thicker and lack perforation compared with refrigerated storage valves, nonviable at implantation. Tissue culture of explanted cryopreserved valves viable at the time of implantation has demonstrated both the presence of viable fibroblasts, although in markedly reduced numbers, and acellular leaflet segments. On these occasions when sufficient cells for chromosomal analysis have been cultured, the cells have always been shown to be of donor origin [8, 9]. Only on one occasion have fibroblasts been cultured from an explanted 4°C stored valve. This valve had been implanted within 7 days of procurement, and this result is consistent with a finding of this study that valves stored at 4°C contain viable cells up to around 7 days. Mochtar and associates [11] have also demonstrated this decrease in leaflet viability with time when valves are stored at 4°C.

In summary, the results from this investigation into preimplantation viability support the current protocol for antibiotic sterilization and cryopreservation. Beating-heart donors show a much higher level of initial viability than nonbeating-heart donors and are therefore the preferred source of valves. Retrieval within 24 hours after death provides valves with an initial viability level of around 50% of the total fibroblast population. Although it is not within the TPCH protocol, if valves are to be stored longer than 3 days, we recommend that they be cryopreserved immediately after antibiotic sterilization. Amphotericin B should not be used in conjunction with cryopreservation.

The simplicity of the rigid TPCH protocol has stood the test of time over 27 years. Valves retrieved under semisterile conditions within 24 hours require only a low-dose, short-term antibiotic exposure for sterilization. The clinical success of the allograft for aortic valve replacement gives credence to the importance of such a protocol of valve preparation, sterilization, and storage [5]. This coupled with the improved implantation technique of root replacement [18, 19], which minimizes valvular distortion, and a better understanding of the immune response in the young and elderly [20, 21] augurs well for further improvements in the durability of the allograft valve. The clinical results with valves that have been collected and cryopreserved soon after death are similar to those obtained with homovital valves implanted within 3 days [17]. This fact probably highlights the importance of minimal cellular and matrix disturbance before implantation. This may also help explain why the long-term durability of implanted allograft valves, including those stored under refrigerated conditions, cryopreserved and homovital, still cannot mimic that of the aortic valve within the immunosuppressed transplanted heart.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
This research project was supported by a grant from The Prince Charles Hospital Foundation.

We thank Ms Anne Brosnan for her assistance during the development of this research project and Mr Ian Smith, MApplSc (Med Phys), for his analysis of the data.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

1. O’Brien M.F., Stafford E.G., Gardner M., et al. The viable cryopreserved allograft aortic valve. J Cardiac Surg 1987;1(Suppl):153-167.
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15. Heacox A.E., McNally R.T., Brockbank K.G.M. Factors affecting the viability of cryopreserved allograft heart valves. In: Yankah A.C., Hetzer R., Miller D.C., Ross D.N., Sommerville J., Yacoub M.H., eds. Cardiac valve allografts 1962–1987. Darmstadt: Steinkopff Verlag, 1988:37-42.
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19. Jones E.L., Shah V.B., Shanewise J.S., et al. Should the freehand allograft be abandoned as a reliable alternative for aortic valve replacement?. Ann Thorac Surg 1995;59:1397-1404.[Abstract/Free Full Text]
20. Zhao X.-M., Green M., Frazer I.H., Hogan P., O’Brien M.F. Donor-specific immune response after aortic valve allografting in the rat. Ann Thorac Surg 1994;57:1158-1163.[Abstract]
21. O’Brien M.F. Homografts and autografts. In: Baue A.E., ed. Glenn’s textbook on thoracic and cardiovascular surgery; vol 2, 6th ed. Stamford, CT: Appleton and Lange, 1996:2005-2042.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Susan E. Smith Mark F. O’Brien Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Gall, K. L. Articles by O’Brien, M. F. Search for Related Content PubMed PubMed Citation Articles by Gall, K. L. Articles by O’Brien, M. F.