Calcification modelling in artificial heart valves.

This study has examined a range of methods of studying the calcification process in bovine pericardial and polyurethane biomaterials. The calcification methods include static and dynamic, in vitro and in vivo tests. The analytical methods include measurement of depletion rates of calcium and phosphate from in vitro calcifying solutions, analysis of tissue contents of calcium, histological staining of tissue sections for calcium, X-ray elemental analysis, by scanning electron microscopy, of calcium and phosphorus distributions over valve leaflets calcified in vitro under dynamic conditions. Bovine pericardium, in all test settings, calcified to a much greater degree than polyurethane biomaterials. Polyurethane extracts calcified to a greater degree than bulk polyurethanes. The test protocol used allows progress through increasingly demanding calcification tests, with the possibility of eliminating unsuitable materials with tests of limited complexity and expense.     

Issues surrounding the preservation of viable allograft heart valves.

Allograft heart valves have been used for over 30 years. During the first decades of use, the research and clinical objectives were to find a means for long-term storage of tissue. Methods such as irradiation, glutaraldehyde fixation, long-term antibiotic storage at 4 degrees C and other methods were common. These methods, however, were found to give reduced long-term clinical performance when compared with viable fresh tissue or tissue which had been cryopreserved. Recognizing this fact, more recent emphasis has been to address issues surrounding means by which allografts can be cryopreserved and thawed to retain maximum viability. An additional concern was to find a means to maximize donor retrieval by salvaging tissue which normally would be discarded because of bacterial contamination. This study demonstrates that when a proper cryopreservation technique is used, with stringent antibiotic treatments, biomechanical parameters remain normal with only a slight decrease in cell viability.     

Late complications of prosthetic heart valves

Iatrogenic lesions induced during prosthesis insertion are likely to be encountered by a pathologist examining a patient who died of late complications of heart-valve prosthesis. The lesions are classified as those that may occur in any patient who has had a heart-valve prosthesis inserted, including the iatrogenic type and those that are uniquely associated with a particular prosthesis because of its structure or design.     

Biopsy findings in cases of rejection of liver allograft.

Features of rejection were found in 21 needle biopsies obtained from seven patients after liver transplantation. Wedge biopsies taken peroperatively were used as a baseline for comparison. Rejection was diagnosed by excluding other known causes of graft dysfunction using appropriate methods. In cases in which these criteria were fulfilled a consistent picture of rejection was seen, and this was useful in clinical management. Two features constantly present in cases of acute rejection were: a dense mixed portal inflammatory infiltrate; and polymorphonuclear infiltration of biliary epithelium.     

The fine structure of the heart valves in the chicken. I. Mitral valve

The fine structure of the mitral valve in the chicken heart is described. The chicken has no distinct muscular component within the mitral valve; on the other hand, the tricuspid valve is a muscular fold. The mitral valve is a thin, membranous, fibrous structure and is composed of three layers: (a) an auricular and ventricular endothelial layer, (b) a zona spongiosa, and (c) a zona fibrosa. The endothelium of the heart valve resembles closely that of vascular channels, but the shape and size of the valvular endothelial cells vary considerably from those of other vascular channels. From the valvular endothelial cells thin villus-like projections extend into the cardiac lumen. Their cellular junctions frequently have a complicated pattern in the form of interdigitating folds, although some show a long linerar zone of simple overlapping between adjoining cells. A fine filamentous plexus is a feature of the cytoplasm but is less evident than in other vascular channels. The basement membrane varies with its location in the valve; it is distinct on the atrial side but is often scanty and obscure on the ventricular surface. Some elastic tissue is found in the subendothelial tissue. The zona spongiosa and zona fibrosa are quite similar to those of other species, but the amorphous ground substance in the zona spongiosa is more abundant. Both layes contain elastic fibers and collagen in both fibrils and bundles. Their amount, composition and configuration vary in different locations within the valve. Avian valvular endothelium is compared with that of lining vascular channels and valves of other species. There are structural differences, some of which may reflect physical conditions and function, such as mobility, extensibility and state of contraction.     

Sex-dependence of the relative number of elastic fibres in human heart valves.

The aim of the current study was to find out whether there are sex-dependent differences in the relative number of elastic fibres in human heart valves. Twenty-six aortic valves, 26 mitral valves, 33 pulmonary valves and 28 tricuspid valves of both sexes were obtained at autopsy from newborn to 89-year-old patients who died of noncardiac diseases. The quantitatively morphometric investigations were carried out on conventionally stained (Resorcinfuchsin) histological sections. The results were qualitatively examined with immuno-histochemically marked (anti-elastin antibodies) histological sections. Earlier examinations by Leutert [1976. Z. Gesamte Inn. Med. 31, 97-104] showed that the atrioventricular valves have the following layers: endothelium, atrial fibroelastic tissue (S1), fibrous tissue, ventricular fibroelastic tissue (S2) and endothelium. In our study, the ventricular side of the semilunar valves corresponds to side S1, whereas the vessel side corresponds to side S2. Three regions of interest were examined on each side of the valves: base, mid and tip. The number of elastic fibres per measuring area for all four human heart valves was significantly higher (p < 0,001) in fibroelastic tissue of side S1 than in fibroelastic tissue of side S2. Neither on side S1 nor on side S2 were there significant gender-related differences in the relative number of elastic fibres per measuring area. The results suggest a characteristic distribution of the elastic fibre system which is not sex-dependent but closely related to the function of the heart valves.     

Calcification and stress distribution in bovine pericardial heart valves.

There is a strong relationship between mechanical stress and calcification in biological prosthetic heart valves. A dynamic in vitro calcification test has been used to study the relationship between stress distributions in the leaflets of bovine pericardial valves and the deposition of calcium over the leaflet surfaces. Intuitive stress regions have been defined over the leaflet surfaces. Calcium uptake by the leaflets has been assayed directly by ashing of leaflet material and analysis of the ash by atomic absorption spectrophotometry. Calcium and phosphorus distribution over the leaflet surface has been analyzed using energy-dispersive x-ray analysis by scanning electron microscope and data points assigned to the appropriate stress region. The uptake of calcium is assessed by comparing stress regions, surfaces, and the degree of calcification of the valve. Differences between stress regions and surfaces are significant. Uptake of calcium in these valves appears to be strongly related to the degree and type of stress present in the valve leaflets.     

Prosthetic heart valves: catering for the few

Prosthetic heart valves epitomize both the triumphant advance of cardiac surgery in its early days and its stagnation into a retrospective, exclusive first world discipline of late. Fifty-two years after the first diseased heart valve was replaced in a patient, prostheses largely represent the concepts of the 1960s with many of their design-inherent complications. While the sophisticated medical systems of the developed world may be able to cope with sub-optimal replacements, these valves are poorly suited to the developing world (where the overwhelming majority of potential valve recipients reside), due to differences in age profiles and socio-economic circumstances. Therefore, it is the latter group which suffered most from the sluggish pace of developments. While it previously took less than 7 years for mechanical heart valves to develop from the first commercially available ball-in-cage valve to the tilting pyrolytic-carbon disc valve, and another 10 years to arrive at the all-carbon bi-leaflet design, only small incremental improvements have been achieved since 1977. Similarly, bioprosthetic valves saw their last major break-through development in the late 1960s when formalin fixation was replaced by glutaraldehyde cross linking. Since then, poorly understood so-called 'anti-calcification' treatments were added and the homograft concept rediscovered under the catch-phrase 'stentless'. Still, tissue valves continue to degenerate fast in younger patients, making them unsuitable for developing countries. Yet, catheter-delivered prostheses almost exclusively use bioprosthetic tissue, thereby reducing one of the most promising developments for patients of the developing world into a fringe product for the few first world recipients. With tissue-engineered valves aiming at the narrow niche of congenital malformations and synthetic flexible leaflet valves being in their fifth decade of low-key development, heart valve prostheses seem to be destined to remain an unsatisfying and exclusive first world solution for a long time to come.