[Who may place the first donor heart? Conflict surrounding the first heart transplant in the Netherlands]. / De strijd rond het primaat van de eerste harttransplantatie in Nederland.

On Friday 22 June 1984 a patient was waiting in Rotterdam to be moved to Brussels for a heart transplant. The Dutch heart surgery centres tried to mutually agree on the place where the first heart transplant should be performed. When this proved impossible, the decision was made by the cardiac surgeons from Rotterdam and Leiden. In this article we will reconstruct the events that took place at that time, partly on the basis of a recent 'witness seminar'.

Prime solutions for cardiopulmonary bypass in neonates: antioxidant capacity of prime based on albumin or fresh frozen plasma.

OBJECTIVE: Oxidative damage and inflammation are believed to play an important role in postoperative complications after cardiopulmonary bypass. During bypass, a prime solution with a high antioxidant capacity may reduce the oxidative damage and inflammation. We investigated total antioxidant capacity and individual scavengers during the preparation of 2 different prime solutions. METHODS: The prime solutions were prepared with either pasteurized human albumin or fresh frozen plasma. The total antioxidant capacity was measured with the total radical antioxidant parameter assay and with the ferric-reducing ability of plasma assay. The individual scavengers vitamin C, sulfhydryl groups, uric acid, and total protein were measured before, during, and after the prime preparation. Malondialdehyde was measured as a parameter for lipid peroxidation. RESULTS: Neither prime solution showed a total radical antioxidant parameter value. The ferric-reducing ability of plasma value of prime solutions was lower than that of undiluted human albumin or fresh frozen plasma. Addition of mannitol did not increase the ferric-reducing ability of plasma value. Vitamin C was only found in the fresh frozen plasma prime. Both prime solutions contained sulfhydryl groups and uric acid in low concentrations. During ultrafiltration, low-molecular-weight antioxidants were lost into the ultrafiltrate. CONCLUSIONS: We showed that prime solutions based on either albumin or fresh frozen plasma had very low antioxidant capacity and that ultrafiltration of the prime solution further lowers this capacity. A prime solution with a low antioxidant capacity may increase oxidative stress in neonates undergoing cardiopulmonary bypass.

Outcome and follow-up of aortic valve replacement with the freestyle stentless bioprosthesis.

BACKGROUND: The aim of this study was to determine the morbidity, mortality, and hemodynamics after implantation of the Freestyle stentless bioprosthesis in the aortic position. METHODS: A total of 280 patients were operated on from June 1993 to July 1999 as part of a multicenter investigation. Factors influencing hospital mortality and long-term survival were assessed by logistic regression and Cox proportional hazards analysis. Patients were evaluated postoperatively at discharge, at 3 to 6 months, and yearly by clinical examination and color flow Doppler echocardiography. RESULTS: Hospital mortality in this group was relatively high (9.6%). Logistic regression analysis showed that cross-clamp time, age, myocardial infarction, diabetes, left ventricular hypertrophy, coronary artery disease, New York Heart Association class III or IV and female gender were the independent predictive factors. According to the Kaplan-Meier method, the 4-year survival for hospital survivors was 94%. In the multivariate Cox proportional hazard analysis, only coronary artery disease proved to be prognostic. During follow-up, 11 patients developed paravalvular leakage due to prosthetic dehiscence at the side of the noncoronary cusp. Performance of the prosthesis as assessed by echocardiography was excellent. Mean gradient decreased significantly between discharge and follow-up at 3 to 6 months. At 1-year follow-up trivial regurgitation was found in 6 patients (3%) and mild regurgitation in 4 (2%). Regurgitation did not increase with time. The effective orifice area increased significantly from discharge to follow-up at 3 to 6 months. CONCLUSIONS: Hospital mortality after implantation of a stentless bioprosthesis was higher compared to conventional prosthesis. A high incidence of prosthesis dehiscence at the proximal suture line was found, which was probably due to technique. Hemodynamic performance up to 3 years showed low transvalvular gradients. There is echocardiographic evidence for reduction of left ventricular hypertrophy and improvement of left ventricular function.

Hemodynamics of the Freestyle bioprosthesis in young patients.

The objective of this study was to analyze hemodynamics of the Freestyle stentless bioprosthesis in young patients compared with the older age group. The Freestyle aortic bioprosthesis is a stentless aortic xenograft. Hemodynamics are proven to be good in older patients for a long period. Experience is limited in the younger age group. Therefore the hemodynamics of 71 patients younger than 60 years (14-59 years) were compared with the data of 471 patients older than 60 years (60-86 years) during a follow-up period of 6 years. In the study, 542 consecutive patients with a Freestyle aortic bioprosthesis underwent an echocardiographic and Doppler examination according to a common protocol. Investigations were done within 4 weeks after operation, after 3 to 6 months, and then every year. Mean gradients of the young patients were 8.7 +/- 5.3 mm Hg at discharge and remained between 5.2 +/- 3.9 mm Hg and 11 +/- 5.6 mm Hg during the complete follow-up period. Gradients of the older patients were 8.3 +/- 5.4 mm Hg at discharge and remained between 5.1 +/- 3.7 mm Hg and 6.5 +/- 7.9 mm Hg. Cardiac index during the follow-up period was equal in both groups. Doppler values were evaluated for the Freestyle stentless porcine bioprostheses in the aortic root both for patients younger than 60 years and patients over 60 years of age. Midterm hemodynamic performance is equal in both groups.

Echocardiographic imaging of stentless aortic valve prostheses.

UNLABELLED: Homografts and stentless xenografts are increasingly used in aortic valve surgery. Echocardiography technicians and cardiologists have to know what they will find when performing an echo-Doppler examination in patients who received a stentless valve. We therefore evaluated echocardiographic images of 74 patients who received a Freestyle stentless bioprosthesis with three techniques and a follow-up of 2 years in two high-volume hospitals. Of the patients studied, 81% were operated using the subcoronary technique, 12% using the root-inclusion technique, and 7% using the full-root technique. RESULTS: Transvalvular gradients across the stentless valves were low: 8.0 mmHg when implanted with the subcoronary technique, 8.2 +/- 5.1 mmHg using the root-inclusion technique, and 6.5 mmHg using the full-root technique. Trivial aortic insufficiency (grade 1) was observed in 10.7% of the patients (8.9% for the subcoronary technique, 13% for the root-inclusion technique, and 0% for the full-root technique). When the bioprosthesis was implanted using the subcoronary technique or the root-inclusion technique, the prosthesis was placed inside the recipient aortic root. Using these techniques, a lumen between the double layer of the xenograft and the aortic wall could be observed. With the root-replacement technique, the porcine root became the most proximal part of the ascending aorta. As the native aortic wall was removed, in most cases, no double lumen could be observed with imaging of the ascending aorta.

Remodeling of the porcine pulmonary autograft wall in the aortic position.

OBJECTIVE: Dilatation and valve regurgitation are disturbing sequelae of the pulmonary root functioning at systemic pressures. We tried to characterize the histologic mode of adaptation of the neoaortic wall. METHODS: We compared routine histologic studies, immunohistochemical staining, and computer-assisted morphometric analysis of aortic, pulmonary autograft, and native pulmonary wall specimens from pigs in which, as a newborn, a valveless pulmonary autograft had been implanted in the aorta. RESULTS: Histologic examination of the pulmonary autograft revealed a viable, normally revascularized wall without degenerative phenomena. Smooth muscle cells were enlarged and rearranged. The characteristic "pulmonary" medial elastin lamellar structure was retained, which was confirmed by morphometry. Immunohistochemistry of the autograft revealed relatively strong staining of type III collagen and alpha smooth muscle actin, exclusive staining of basic fibroblast growth factor, and no staining of proliferation markers proliferating cell nuclear antigen and Ki67. CONCLUSION: The developing pulmonary autograft in the aortic position becomes normally revascularized, lacks major degenerative phenomena, and retains its own typical pulmonary morphologic features. Remodeling is accomplished by increased extracellular matrix deposition with collagen as an important constituent. The marked expression of growth factors in the autograft suggests the persistence of increased metabolic activity.

Stentless bioprostheses have ideal haemodynamics, even in the small aortic root.

OBJECTIVE: To determine normal Doppler and 2D gradients and flow characteristics of the Freestyle stentless aortic bioprosthesis related to valve size. BACKGROUND: The Freestyle stentless aortic bioprosthesis is one of the newer aortic xenografts. Only limited data are available of the echocardiographic flow characteristics during a mid-term follow-up period of this valve. Therefore valve performance related to valve size was measured during a follow-up period of two years. METHODS: 175 consecutive patients with a Freestyle aortic bioprosthesis underwent an echocardiographic and Doppler examination according to a common protocol. Investigations were done within 4 weeks after operation, after 3 to 6 months, and after 1 and 2 years. RESULTS: With a valve size from 19 to 27 mm mean gradients decreased from 8.0 +/- 5.1 mmHg at discharge to 5.8 +/- 3.8 mmHg after 3-6 months (p < 0.001). Thereafter gradients remained stable. The performance index, the ratio of the measured effective orifice area in the patient divided by the effective orifice area measured in vitro increased from 69 +/- 20% at discharge to 79 +/- 29% after one, two and three years. Performance index was especially very high in the smaller sized valves with a performance index of 85 +/- 17% for the 21 mm valve. During follow-up mean gradients remained below 10 mmHg even in the 21 mm valve. CONCLUSION: Stentless xenografts have ideal haemodynamics, even in the small aortic root.

Dynamic obstruction, an unusual complication after aortic valve replacement with a stentless porcine valve.

In the early nineties, the stentless porcine aortic bioprosthesis has been reintroduced successfully. Because of the limited experience, knowledge of clinical complications is limited. Therefore, we describe an unusual complication of dynamic obstruction after aortic valve replacement with a stentless porcine valve in a 70 year old man 18 months after implantation. We discuss the complications of stentless aortic prostheses known so far, describe operative techniques used and their characteristic two dimensional echocardiographic images.

Echocardiographic parameters of the freestyle stentless bioprosthesis in aortic position: the European experience.

The objective of this study was to determine normal Doppler and 2-dimensional characteristics of the Freestyle stentless aortic bioprosthesis. The Freestyle aortic bioprosthesis is a new type of aortic xenograft, and experience is limited. We therefore determined the normal range of echocardiographic and Doppler examinations of this valve. Three hundred thirty-nine consecutive patients with a Freestyle aortic bioprosthesis underwent an echocardiographic and Doppler examination according to a common protocol. Investigations were done within 4 weeks after operation, after 3 to 6 months, and after 1, 2, and 3 years. With a valve size from 19 to 27 mm, mean gradients decreased from 7.9 +/- 5.1 mm Hg at discharge to 5.5 +/- 3. 8 mm Hg after 3 to 6 months (P <.001). Thereafter, gradients remained stable. Effective orifice area 1 year after implantation was 1.59 +/- 0.58 cm(2) for the 21-mm valves, 1.92 +/- 0.74 cm(2) for the 23-mm valves, 2.03 +/- 0.64 cm(2) for the 25-mm valves, and 2.52 +/- 0.72 cm(2) for the 27-mm valves (P <.001). The performance index, the ratio of the measured effective orifice area in the patient divided by the effective orifice area measured in vitro, increased from 67% +/- 20% at discharge to 82% +/- 29% after 1, 2, and 3 years. Performance index was especially very high in the smaller-sized valves. After implantation with the subcoronary technique or root-inclusion technique, small cavities could be seen between the native aortic root and the Freestyle valve. Doppler values were evaluated for the Freestyle stentless porcine bioprostheses in the aortic root. Gradients appear to be close to those measured in native valves over a time period of 3 years.

Brom's three-patch technique for repair of supravalvular aortic stenosis.

OBJECTIVE: Case histories of all patients (n = 29) operated on for supravalvular aortic stenosis from 1962 to the present were reviewed to study different techniques and outcomes. The technique of symmetric aortoplasty with 3 patches (1 in each sinus) is described and compared with other methods. METHODS: Case reports were reviewed and follow-up was completed by contacting the patient's (pediatric) cardiologist. We aimed for a last follow-up visit, including Doppler echocardiographic studies, in a period no more than 12 months earlier than December 1997. Supravalvular aortic stenosis was discrete in 25 and diffuse with involvement of the aortic arch and arch vessels in 4 patients. Additional anomalies were bicuspid aortic valve (n = 5), coarctation (n = 3), ascending aortic aneurysm (n = 1), mitral valve insufficiency (n = 2), pulmonary valvular stenosis (n = 1), and peripheral pulmonary artery stenosis (n = 2). Eleven patients had Williams syndrome and 1 patient had Noonan syndrome. Symmetric aortoplasty with 3 patches (1 in each sinus) was used in 13 patients, whereas other nonsymmetric methods (1, 2, or Y-shaped patches) were used in 16 patients. Mean follow-up was 10.5 years (range: 4 months-36 years). RESULTS: All techniques adequately decreased the pressure gradient. Progression of preoperative aortic valve insufficiency or de novo regurgitation was not observed except in 1 patient in whom the patches inserted were too large. CONCLUSIONS: No difference could be demonstrated in outcome for any surgical technique; however, reconstruction of the aortic root with autologous pericardial patches in each sinus after transection of the aorta has the advantage of symmetry while restoring the normal aortic root anatomy.

Aortic valve replacement with Medtronic Freestyle bioprosthesis: 5-year results.

The Medtronic Freestyle aortic root bioprosthesis has been implanted in patients since August 1992. This study reviews clinical and echocardiographic results at midterm (5 years) after implantation. The Freestyle bioprosthesis was implanted in 1,100 patients in a 21-center Food and Drug Administration clinical trial from August 1992 to October 1998. The device was implanted (1) as a subcoronary valve replacement, (2) as a complete aortic root replacement (full-root), or (3) as a root inclusion. Patients were followed annually by clinical examination and echocardiography. There were 47 deaths early after operation (7.1%). There were 2,478 patient-years of follow-up during which there were 99 deaths or 4.0/pt.-yr. Before implantation, 73% of patients were in New York Heart Association functional class III or IV. After operation, 95% were in class I or II. Transvalvular gradient 4 years after operation was low (7.5+/-5.3 mm Hg) for all valve sizes (subcoronary implant). Small valves (19 and 21 mm) had mean gradients (10.2+/-3.0, 9.1+/-4.4 mm Hg). There was no or mild valve insufficiency in 98% of patients. Actuarial analysis at 5 years showed the rate for freedom from thromboembolism of 86% in subcoronary implant and 93% in full root replacement. Freedom from endocarditis was 98%. Freedom from reoperation for explant of the valve highest in patients having full root replacement (98%) and lower with root inclusion (94%). There were 20 bioprostheses explanted; 10 for endocarditis, 8 for technical reasons, and 2 for structural deterioration. The Medtronic Freestyle bioprosthesis (1) has excellent hemodynamic performance, (2) techniques for insertion that result in a competent valve, (3) low rates of thromboembolism and endocarditis, and (4) a rare rate of structural deterioration at 5 years.

Dehiscence of the Freestyle stentless bioprosthesis.

This study aimed to identify and characterize patients who developed prosthetic dehiscence after aortic valve replacement with the Freestyle Stentless bioprosthesis. Review of patients' records and preoperative echocardiogram was performed. Prosthetic dehiscence developed in 10 patients after 2.5 to 49 months (mean 19). Most patients were symptomatic at presentation. Nine patients were reoperated. Dehiscence was typically located at the inflow suture line beneath the noncoronary cusp. Four patients died. The cause of dehiscence is assumed to be related primarily to surgical technique.

Short-term follow up of the Ross operation in children.

BACKGROUND AND AIMS OF THE STUDY: Aortic valve disease in the pediatric population poses special problems to surgeons and cardiologists. The pulmonary autograft has proven to be a good alternative for aortic valve replacement and left ventricular outflow tract (LVOT) reconstruction in this special group. METHODS: Forty-one children (mean age 10.0 +/- 4.8 (SD) years; range: 35 days to 18.8 years) underwent aortic root replacement with a pulmonary autograft between February 1994 and April 1998. Twenty-one patients (51%) had previous cardiac surgery; seven (17%) had balloon valvulotomy. Aortic root replacement was combined with other techniques for various disorders, including tunnel LVOT obstruction, ventricular septal defect (VSD)-aortic insufficiency complex, neoaortic insufficiency following arterial switch procedure, and subvalvular stenosis following correction of type B interruption of the aortic arch (IAA) with VSD (IAA-B/VSD). RESULTS: The mean follow up was 1.7 +/- 1.0 years (range 44 days to 4.1 years). Total follow up time was 67.8 patient-years. Two patients, both after repair of interrupted aortic arch, died intraoperatively (4.9%). There was no late mortality. Two patients were reoperated on (5.1%), one for autograft insufficiency due to cuspal perforation and one for right ventricular outflow tract stenosis at the distal anastomosis. Thirty-eight patients (97%) are currently in NYHA class I; one child with a preoperatively poor left ventricular function did not improve and is in class II. At the latest echocardiographic follow up, neoaortic regurgitation was absent in 19% of patients, trivial in 69% and mild in 11%. Homograft insufficiency was absent in 64%, trivial in 31% and mild in 6%. All mean gradients for both autograft and homograft were < 15 mmHg. CONCLUSIONS: The Ross operation can be performed with good results in infants and children with different forms of LVOT obstruction and aortic insufficiency, though aortic stenosis following IAA-B/VSD repair poses a surgically difficult problem.

Autologous pericardium for ventricular septal defect closure.

BACKGROUND AND AIMS OF THE STUDY: The use of living, untreated autologous pericardium for patch repair in the left ventricular outflow tract was considered attractive in children. METHODS: Ventricular septal defect (VSD) closure with an untreated autologous pericardial patch was performed in 102 children of mean age 13.4 months (range: 1 to 73 months). Postoperative transthoracic Doppler echocardiography was performed in all children at a mean of nine weeks (range: one day to 50 weeks) after surgery. One pericardial patch, which was explanted at autopsy two months after surgery, was studied microscopically. RESULTS: At short-term follow up, no or only minor residual VSD was found in 97 patients, moderate VSD in two and severe VSD in one patient. One patient was reoperated for residual VSD and an aneurysmic patch first diagnosed seven days after surgery. Two more patients showed ballooning of the patch without VSD after five and seven days respectively. All aneurysmic patches were attributed to intraoperative patch oversizing. Patch integrity was confirmed in all other patients. No inflammatory or degenerative changes were observed at microscopy, rather a remodeling response had caused the patch to thicken, indicating an adaptation of the living tissue. CONCLUSIONS: The untreated autologous pericardial patch has shown to be a safe alternative for VSD closure, provided that the patch is properly sized.

Pulmonary root replacement with the Freestyle stentless aortic xenograft in growing pigs.

BACKGROUND: The stentless xenograft with its favorable hemodynamic performance on the left side of the heart seems an attractive, readily available alternative for the reconstruction of the right ventricular outflow tract in children. METHODS: To assess its function in a preclinical animal investigation, we replaced the pulmonary root with a Freestyle stentless aortic xenograft in 18 piglets of 26.6 +/- 3.2 kg weight. The animals were allowed to grow as much as possible and slaughtered when symptoms of heart failure developed or body weight reached more than 160 kg. All valve explants were analyzed by gross examination and photography and, in 4 representative pigs, by histologic examination. RESULTS: Fourteen animals died prematurely after 2 weeks to 11 months. Twelve xenograft explants showed thick, immobilized, large nodular structures as cuspal remnants causing significant stenosis. At microscopy, large cuspal masses of degenerating collagen and fibrin and various inflammatory cells were frequently found. In the growing pig, most of the xenografts implanted in the pulmonary position showed early degeneration causing severe stenosis. CONCLUSIONS: Use of this valve for right ventricular outflow tract reconstruction in children cannot be recommended.

Disproportionate enlargement of the pulmonary autograft in the aortic position in the growing pig.

PURPOSE: This study was aimed to demonstrate growth in the pulmonary autograft after transplantation to the aortic position. METHODS AND MATERIALS: In 20 piglets (weight 25.4 +/- 3.5 kg) (mean +/- standard deviation) a Ross operation was performed and in five piglets (weight 9.3 +/- 0.7 kg) (mean +/- standard deviation) the ascending aorta was replaced with a valveless pulmonary autograft. Animals were allowed to grow as much as possible. Postmortem explanted autografts were studied by direct measurements of the valve cusps in the Ross group and of the wall segments in the valveless autograft group. Measurements of the first group were compared with the values of a separate control group, and values of the second group were compared with values of samples taken at operation. RESULTS: In the Ross group, cuspal weight, height, and width increased significantly by comparison with body weight (p < or = 0.003). The rate of increase did not differ significantly from that of the control group with a native pulmonary valve. However, there was a rapid adaptation of the autograft valves resulting in a significantly higher mean cuspal weight, height, and width. In the valveless autograft group, wall circumference, thickness, and height increased significantly (p < or = 0.001). The circumference increased significantly more than that of the native pulmonary wall. Compared with the native aortic wall, the pulmonary autograft media showed retained pulmonary architecture on microscopic study. CONCLUSION: These data suggest that the dimensional increase of the pulmonary autograft in the aortic position in the growing pig is determined by growth and dilatation, that the valve mass increases more than that of the native pulmonary valve, and that the characteristic pulmonary microscopic architecture is retained.

Beneficial effects of leukocyte depletion of transfused blood on postoperative complications in patients undergoing cardiac surgery: a randomized clinical trial.

BACKGROUND: Leukocytes in transfused blood are associated with several posttransfusion immunomodulatory effects. Although leukocytes play an important role in reperfusion injury, the contribution of leukocytes in transfused blood products has not been investigated. To estimate the role and the timing of leukocyte filtration of red cells in cardiac surgery, we performed a randomized study. METHODS AND RESULTS: Patients scheduled for cardiac surgery were randomly allocated to receive either packed cells without buffy coat (PC, n = 306), fresh-filtered units (FF, n = 305), or stored-filtered units (SF, n = 303) when transfusion was indicated. We evaluated the periods of hospitalization and stay at the intensive care unit, and the occurrences of postoperative complications up to 60 days after surgery. The average hospital stay was 10.7 days, of which 3.2 days were in the intensive care unit, without significant differences between the groups. In the PC trial arm, 23.0% of the patients had infections versus 16.9% and 17.9% of the patients in the leukocyte-depleted trial arms (P=.13). Within 60 days, 45 patients had died, 24 patients in the PC trial arm (7.8%), versus 11 (3.6%) and 10 (3.3%) patients in the FF and SF trial arms, respectively (P=.015). CONCLUSIONS: In cardiac surgery patients, especially when more than three blood transfusions are required, leukocyte depletion by filtration results in a significant reduction of the postoperative mortality that can only partially be explained by the higher incidence of postoperative infections in the PC group.

Stentless aortic bioprostheses: compelling data from the Second International Symposium.

BACKGROUND: Stentless aortic xenografts are an important addition to the range of prosthetic valves. So far their use has been restricted to a limited number of study centers. This report summarizes the principal findings from the Second International Symposium on Stentless Bioprostheses. Attention is focused on the Toronto SPV and Freestyle valves recently approved by the United States Food and Drug Administration. METHODS: Stentless xenografts are used predominantly in elderly patients with aortic stenosis. Implant techniques are more complex than for stented valves, as reflected by longer ischemic and cardiopulmonary bypass times. The valves have been subjected to detailed serial echocardiographic assessment and clinical follow-up. RESULTS: The hemodynamic characteristics resemble those of the aortic homograft. There is a progressive increase in effective orifice area and decrease in transvalvular pressure gradients with time. Left ventricular mass index and wall thickness normalize between 6 and 12 months postoperatively. Left ventricular remodeling is accompanied by improved symptomatic status and a low incidence of valve-related complications. Limited comparative studies suggest important benefits over stented xenografts. Improved hemodynamics may translate into better bioprosthetic durability. CONCLUSIONS: Reproducible and reliable implant methods should be taught carefully, but the hemodynamic advantages are substantial. Stentless xenografts are ideal for the elderly patient with aortic stenosis.

Tissue chimerism in human cryopreserved homograft valve explants demonstrated by in situ hybridization.

BACKGROUND: The presence of viable cells may contribute to increased homograft valve durability. These cells may be of infiltrating recipient or persisting donor origin. In this study, in situ hybridization was used to assess the origin of cells in cryopreserved homograft valve explants. METHODS: A total of 10 homografts with a donor-recipient gender mismatch were acquired from patients whose graft had been explanted at reoperation or at autopsy. The period of implantation varied from 14 days to 70 months. Frozen sections were made and alternately examined with hematoxylin and eosin staining and in situ hybridization. Male cells were distinguished from female using a biotinylated Y-chromosome-specific deoxyribonucleic acid probe. RESULTS: No endothelial cells were found. Thirty percent of the leaflets showed large acellular zones and 30% were completely acellular. The homograft arterial wall was occupied by a vast majority of penetrating host fibroblasts in 80% of the studied specimens. Donor and recipient cells were coexistent in the wall in 60% of the studied specimens and in 50% of the leaflets. In 30% only host cells could be identified. CONCLUSIONS: This finding of tissue chimerism may lead to new insights in homograft pathology. The technique of in situ hybridization may provide an indispensable contribution in further homograft research.