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J Thorac Cardiovasc Surg 1997;114:513
© 1997 Mosby, Inc.
LETTERS TO THE EDITOR |
John P. Robarts Research Institutea
Department of Chemical and Biochemical Engineeringb
Department of Medical Biophysicsc
University of Western Ontario
Division of Cardiologyd
Department of Medicine
London Health Sciences Center
University Campus
London, Ontario, Canada
Reply to the Editor:
I would like to respond to the issues addressed to me by Dr. Chanda concerning extent of glutaraldehyde fixation and calcification of bioprosthetic tissue. Our hydrothermal isometric temperature or denaturation temperature data indicate that dynamic fixation stabilized porcine aortic heart valve tissue within 3 hours 15 minutes and 24 hours when the tissue was fixed at room temperature in 0.5% and 0.05% glutaraldehyde, respectively. Previous authors have suggested that at concentrations from 0.625% down to 0.025%, glutaraldehyde cross-linking is a rapid and the degree of cross-linking reaches a plateau within 24 hours, consistent with our study.
1–4 In contrast, Chachques and associates 5 suggested that adequate fixation of pericardium could be achieved with a brief 10-minute immersion in standard glutaraldehyde solution (0.625%). Complete saturation remains elusive. Schoen, Tsao, and Levy 2 have shown that only 70% of the reactive sites of collagen react. This is due to limitations in the accessibility of the reactive sites and also resistance to diffusion of the glutaraldehyde reagent, a problem likely caused by glutaraldehyde polymer film formation (e.g., steric hinderance factors). 6, 7 We believe some of these limitations may be overcome by means of our dynamic fixation technique. The gradual depletion of glutaraldehyde in the preservative solution during long periods (exceeding 4 to 8 weeks) is less likely caused by completion of saturation than by rapid incorporation of glutaraldehyde monomers recruited during polymer formation. Polymer growth (i.e., alpha- or beta-unsaturated aldehyde polymers) can be catalyzed by amino acids (e.g., lysine, hydroxylysine), the rate of growth increasing rapidly with time and exponentially with temperature and pH. 3,8 Thus the observation, by Chanda and associates, that shorter periods of time are required to achieve complete saturation at higher temperatures (i.e., more than 8 weeks at 4° or 10°C and less than 4 weeks at 37° C) is consistent with the hypothesis of glutaraldehyde consumption by polymer formation. The fact that glutaraldehyde incorporation finally reaches a limit in their study (i.e., after more than 8 weeks at 0.25%) may be associated with simple limitations in the extent of glutaraldehyde polymer growth.
With respect to glutaraldehyde-mediated calcification, although the mechanism is yet unclear, ample evidence indicates a direct relationship between the extent of tissue calcification and the amounts of glutaraldehyde used or incorporated during tissue fixation, 1,2,9,10 along with the duration of tissue exposure to the glutaraldehyde solution. 11
Thus the use of a technique (e.g., dynamic fixation) that can both reduce glutaraldehyde levels and concomitantly achieve similar or more effective cross-linking can only be beneficial in potentially limiting the adverse side effects of glutaraldehyde used, such as calcification or cytotoxicity (not discussed here). We recognize, however, that further calcification studies are required to fully confirm this hypothesis.
We do not exclude the possible advantage of combining such a technique (i.e., dynamic fixation) with anticalcification treatments to optimally reduce the risks of longterm calcification of bioprosthetic tissue.
12/8/83228
References
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