Allograft Aortic Valve Replacement in the Adult: A Review

Learning From Controversy: Contemporary Surgical Management of Aortic Valve Endocarditis

Aortic valve replacement is the commonest cardiac surgical operation performed worldwide for infective endocarditis (IE). Long-term durability and avoidance of infection relapse are goals of the procedure. However, no detailed guidelines on prosthesis selection and surgical strategies guided by the comprehensive evaluation of the extension of the infection and its microbiological characteristics, clinical profile of the patient, and risk of infection recurrence are currently available. Conventional mechanical or stented xenografts are the preferred choice for localized aortic infection. However, in cases of complex IE with the involvement of the root or the aortomitral continuity, the use of homograft is suggested according to the surgeon and center experience. Homograft use should be counterbalanced against the risk of structural degeneration. Prosthetic bioroot or prosthetic valved conduit (mechanical and bioprosthetic) are also potentially suitable alternatives. Further development of preservation techniques enabling longer durability of allogenic substitutes is required. We evaluate the current evidence for the use of valve substitutes in aortic valve endocarditis and propose an evidence-based algorithm to guide the choice of therapy. We performed a systemic review to clarify the contemporary surgical management of aortic valve endocarditis.

Lanthanum carbonate, a phosphate binder, inhibits calcification of implanted aortic allografts in a rat model

OBJECTIVES

Calcification is one of the major postoperative problems after aortic allograft implantation. We hypothesized that phosphate binders, lanthanum carbonate and calcium carbonate inhibit calcification of implanted aortic allografts and verified this hypothesis using a rat model.

METHODS

Aortas were harvested from 4-week-old Brown Norway rats and implanted into the subdermal space of 4-week-old Lewis rats. Twenty-seven recipient Lewis rats were divided into Group N, Group L, and Group C (9 rats per group), which were fed a normal diet, a normal diet containing 3% lanthanum carbonate, and a normal diet containing 3% calcium carbonate, respectively. Implanted aortic allografts were explanted 2 weeks later. Calcification of aortic allografts was evaluated using von Kossa staining and calcium content assay. Calcification score was defined in von Kossa staining as 0 (none), 1 (mild), 2 (moderate), and 3 (severe). Serum calcium and phosphorus levels at euthanasia were measured.

RESULTS

Calcification scores were 2.6, 1.2, and 0.8, and calcium content was 48.9, 15.8, and 8.9 mg/dry·g, in Groups N, L, and C, respectively. Calcification was significantly reduced in Groups L and C. Serum calcium level was 11.5, 12.2, and 13.5 mg/dl, and serum phosphorus level was 15.4, 12.5, and 11.7 mg/dl, in Groups N, L, and C, respectively. Serum calcium level in Group C was significantly higher than in the other two groups.

CONCLUSIONS

Lanthanum carbonate and calcium carbonate significantly reduced calcification of implanted aortic allografts in young rats. Although calcium carbonate induced hypercalcemia, lanthanum carbonate has significant potential to inhibit calcification of implanted aortic allografts.

Iranian homograft heart valves: assessment of durability and late outcome

Durability and the rate of complications of homograft heart valves, adjusted for patient-related contributors and surgical techniques, rely mainly on the quality of allografts which in turn are mirrored in the donor characteristics and most importantly recovery and processing procedures. Aimed to assess the quality, a study was conducted to figure out the durability and late outcome following homograft replacement with valved conduits procured by the Iranian Tissue Bank. Retrospectively, the pre-implantation, perioperative and follow-up data of 400 non-consecutive recipients of cryopreserved heart valves (222 pulmonary and 178 aortic) from 2006 to 2015 were collected and analyzed in terms of variables reflecting late outcome including adverse events and durability. In the context of durability, the event of interest was defined as the need for homograft replacement and homograft-related death. The mean follow-up time (SD) of study entrants (male/female ratio, 1.4) was 49.8 (36.3) months. Median age at the time of implantation was 11 years. Total 10-years mortality was 21 % (84/400), including 66.7 % early (30-days mortality: 56/84) and 33.3 % late (28/84). Overall late complication rate was 2 %. Median survival time was 120 months (95 % CI 83.3-156.6). The pulmonary valves appeared to be more durable (P value <0.001) and survival probabilities in small sized grafts were lower (P value 0.008). One-, five-, and ten-year graft survival was 82, 76 and 73 %, respectively. The evidences suggest that the homografts function satisfactory with low rate of late complications; nevertheless, more emphasis should be given to make long-term durability comparable.

Iranian homograft tissue processing

Tissue transplantation is a life-enhancing therapeutic modality for damaged or non-functioning tissues. In most cases, there is no alternative other than human tissue as replacement, and taking into account the ever-increasing demand for tissue grafts, it makes sense to set up an establishment in charge of human tissue procurement to meet local needs. A quality assurance system, clearly defined standards, and regular audits complement the infrastructure which make this activity feasible. The process of tissue procurement consists of donor identification, consent, tissue recovery, donor screening and testing, tissue processing, preservation, packaging, labeling, terminal sterilization, storage and distribution. The transplantation of homograft heart valves remains controversial, due to the availability of prosthetic and bioprosthetic alternatives. The limited durability of homografts has not yet outweighed the advantages which this graft offers. Adherence to regulations and regularly revised guidelines improve long-term efficacy and minimizes complications or malfunction. Furthermore, the lower price of homograft heart valves and the removal of the need for a lifetime of anticoagulation therapy are noteworthy advantages of this replacement. In our practice, the proportion of homograft heart valves meeting release criteria and successfully implanted grafts were 83% and 95%, respectively.

Resistance to infection of long-term cryopreserved human aortic valve allografts

OBJECTIVE

To analyze the in vitro antimicrobial activity of 3 antibiotic regimens (group A, gentamicin-piperacillin-vancomycin-metronidazole-amphotericin B; group B, gentamicin-piperacillin-flucloxacillin-metronidazole-amphotericin B; and group C, meropenem-vancomycin-tobramycin-colistin-amphotericin B) used in the processing of cryopreserved human ascending aortic tissue and aortic valves against Staphylococcus epidermidis and Staphylococcus aureus. The results were additionally compared with the infection resistance of cryopreserved ascending aortic tissue against Escherichia coli and Pseudomonas aeruginosa.

MATERIALS

Each of 10 cryopreserved human allografts (CHAs) was divided into 25 pieces (separating aortic wall and valve). Eighteen segments were microbiologically tested, and 7 pieces underwent scanning electron microscopy. A bacterial solution (4 mL; optical density, 0.20 ± 0.02) was used for contamination. After incubation, the optical density of the solution was measured. CHAs underwent sonication to release viable adherent bacteria. The number of attached bacteria was quantified by the colony forming units per square centimeter of CHA surface.

RESULTS

Antibiotic regimen groups B and C were more efficient than group A in eradicating gram-positive organisms adherent to the aortic wall (P < .001). Group C showed enhanced resistance against E coli compared with group A or B (P < .001), whereas group B appeared to be more effective against P aeruginosa (P < .001). With reference to each antibiotic regimen, ascending aortic tissue showed significantly less bacterial contamination with staphylococcal bacteria than valve grafts (P ≤ .01).

CONCLUSIONS

CHAs possess antibacterial activity despite long-term storage over 5 years. Antibiotic combinations applied during CHA processing have a significant influence on their infection resistance. Ascending aortic tissue shows a significantly enhanced bacterial resistance against staphylococcal bacteria compared with aortic valves.

Xenograft Failure of Pulmonary Valved Conduit Cross-linked with Glutaraldehyde or Not Cross-linked in a Pig to Goat Implantation Model

Background

Biologic valved grafts are important in cardiac surgery, and although several types of graft are currently available, most commercial xenografts tend to cause early disfiguration due to intimal proliferation and calcification. We studied the graft failure patterns on non-fixed and glutaraldehyde-fixed pulmonary xenograft in vivo animal experiment.

Materials and Methods

Pulmonary valved conduits were obtained from the right ventricular outflow tract of eleven miniature pigs. The grafts were subjected to 2 different preservation methods; with or without glutaraldehyde fixation: glutaraldehyde fixation (n=7) and non-glutaraldehyde fixation (n=4). The processed explanted pulmonary valved grafts of miniature pig were then transplanted into eleven goats. Calcium quantization was achieved in all of the explanted xenograft, hemodynamic, histopathologic and radiologic evaluations were performed in the graft which the transplantation period was over 300 days (n=7).

Results

Grafts treated with glutaraldehyde fixation had more calcification and conduit obstruction in mid-term period. Calcium deposition also appeared much higher in the glutaraldehyde treated graft compared to the non-glutaraldehyde treated graft (p<0.05).

Conclusion

The present study suggests that xenografts prepared using glutaraldehyde fixation alone appeared to have severe calcification compared to the findings of non-glutaraldehyde treated xenografts and to be managed with proper anticalcification treatment and novel preservation methods. This experiment gives the useful basic chemical, histologic data of xenograft failure model with calcification for further animal study.

Heterotopic implantation of a porcine bioprosthetic heart valve in a dog with aortic valve endocarditis

CASE DESCRIPTION

A 5-year-old male German Shepherd Dog was evaluated because of a 5-month history of progressive lethargy, weight loss, and heart failure.

CLINICAL FINDINGS

On physical examination, bounding femoral pulses and systolic and diastolic murmurs were detected. Echocardiography revealed severe aortic valve insufficiency (AVI) and a large vegetative lesion on the aortic valve consistent with aortic valve endocarditis. The AVI velocity profile half-time was 130 milliseconds; the calculated peak systolic pressure gradient across the aortic valve was 64 mm Hg. Left ventricular diameter during diastole was 63.6 mm (predicted range, 40.2 to 42 mm) and during systole was 42.9 mm (predicted range, 25.4 to 27 mm). Systolic, diastolic, and mean arterial blood pressures were 120, 43, and 65 mm Hg, respectively.

TREATMENT AND OUTCOME

To palliate severe AVI, the descending aorta was occluded (duration, 16.75 minutes) and heterotopic implantation of a porcine bioprosthetic heart valve in that vessel was performed. After surgery, systolic, diastolic, and mean arterial blood pressures were 115, 30, and 61 mm Hg, respectively, in the forelimb and 110, 62, and 77 mm Hg, respectively, in the hind limb. Within 6 months, the AVI velocity profile half-time had increased to 210 milliseconds, indicating diminished severity of AVI. After 24 months, the dog was able to engage in vigorous exercise; no pulmonary edema had developed since surgery.

CLINICAL RELEVANCE

Heterotopic bioprosthetic heart valve implantation into the descending aorta during brief aortic occlusion appears feasible in dogs and may provide substantial palliation for dogs with severe AVI.