Aortic valve replacement with pulmonary homografts

Biological and mechanical evaluation of a Bio-Hybrid scaffold for autologous valve tissue engineering

Major challenge in heart valve tissue engineering for paediatric patients is the development of an autologous valve with regenerative capacity. Hybrid tissue engineering approach is recently gaining popularity to design scaffolds with desired biological and mechanical properties that can remodel post implantation. In this study, we fabricated aligned nanofibrous Bio-Hybrid scaffold made of decellularized bovine pericardium: polycaprolactone-chitosan with optimized polymer thickness to yield the desired biological and mechanical properties. CD44⁺, αSMA⁺, Vimentin⁺ and CD105^- human valve interstitial cells were isolated and seeded on these Bio-Hybrid scaffolds. Subsequent biological evaluation revealed interstitial cell proliferation with dense extra cellular matrix deposition that indicated the viability for growth and proliferation of seeded cells on the scaffolds. Uniaxial mechanical tests along axial direction showed that the Bio-Hybrid scaffolds has at least 20 times the strength of the native valves and its stiffness is nearly 3 times more than that of native valves. Biaxial and uniaxial mechanical studies on valve interstitial cells cultured Bio-Hybrid scaffolds revealed that the response along the axial and circumferential direction was different, similar to native valves. Overall, our findings suggest that Bio-Hybrid scaffold is a promising material for future development of regenerative heart valve constructs in children.

Biomechanical properties of hybrid heart valve prosthesis utilizing the pigs that do not express the galactose-α-1,3-galactose (α-Gal) antigen derived tissue and tissue engineering technique

The aim of the study was to estimate the biomechanical properties of heart valves conduit derived from transgenic pigs to determine the usefulness for the preparation of tissue-engineered heart valves. The acellular aortic and pulmonary valve conduits from transgenic pigs were used to estimate the biomechanical properties of the valve. Non-transgenic porcine heart valve conduits were used as a reference. The biomechanics stability of acellular valve conduits decreased both for the transgenic and non-transgenic porcine valves. The energy required to break the native pulmonary valve derived from transgenic pigs was higher (20,475 ± 7,600 J m(-2)) compared with native non-transgenic pigs (12,140 ± 5,370 J m(-2)). After acellularization, the energy to break the valves decreased to 14,600 and 8,800 J m(-2) for the transgenic pulmonary valve and non-transgenic valve, respectively. The native transgenic pulmonary valve showed a higher extensibility (42.70 %) than the non-transgenic pulmonary valve (35.50 %); the extensibility decreased after acellularization to 41.1 and 31.5 % for the transgenic and non-transgenic valves, respectively. The pulmonary valves derived from transgenic pigs demonstrate better biomechanical properties compared with non-transgenic. Heart valves derived from transgenic pigs can be valuable for the preparation of tissue-engineered bioprostheses, because of their biomechanical properties, stability, reduced immune response, making them safer for clinical applications.

A new construction technique for tissue-engineered heart valves using the self-assembly method

Tissue engineering appears as a promising option to create new heart valve substitutes able to overcome the serious drawbacks encountered with mechanical substitutes or tissue valves. The objective of this article is to present the construction method of a new entirely biological stentless aortic valve using the self-assembly method and also a first assessment of its behavior in a bioreactor when exposed to a pulsatile flow. A thick tissue was created by stacking several fibroblast sheets produced with the self-assembly technique. Different sets of custom-made templates were designed to confer to the thick tissue a three-dimensional (3D) shape similar to that of a native aortic valve. The construction of the valve was divided in two sequential steps. The first step was the installation of the thick tissue in a flat preshaping template followed by a 4-week maturation period. The second step was the actual cylindrical 3D forming of the valve. The microscopic tissue structure was assessed using histological cross sections stained with Masson's Trichrome and Picrosirius Red. The thick tissue remained uniformly populated with cells throughout the construction steps and the dense extracellular matrix presented corrugated fibers of collagen. This first prototype of tissue-engineered heart valve was installed in a bioreactor to assess its capacity to sustain a light pulsatile flow at a frequency of 0.5 Hz. Under the light pulsed flow, it was observed that the leaflets opened and closed according to the flow variations. This study demonstrates that the self-assembly method is a viable option for the construction of complex 3D shapes, such as heart valves, with an entirely biological material.

Homograft implantation for aortic valve replacement since 15 years: results and follow-up

BACKGROUND

The use of homografts in aortic valve replacement is an alternative to other prostheses and has been established in our department for 15 years.

METHODS

Since 1992, 360 homografts (HG) have been implanted in adult patients (mean age 51.6 years, 72.8% male). Prospective follow-up was done on an annual basis.

RESULTS

Thirty-day mortality was 5.0% (n = 17); after 5, 10, and 15 years, survival was 88.3%, 84.6%, and 76.0%, respectively. Out of 39 late deaths, 11 were valve-related (10 HG infections, 1 aortic aneurysm). Freedom from reoperation was 99.4% 1 year after operation; after 5, 10, and 15 years it was 94.1%, 78.2%, and 67.3%, respectively. Indications for HG explantation were graft infections (n = 20), calcification (n = 16), regurgitation > grade II (n = 17), perforation (n = 8), and paravalvular leakage (n = 1). Eleven transitoric ischemic attacks, 2 strokes, and 1 cerebral bleeding event were recorded. In echocardiography, the transvalvular pressure gradient changed from 10.55 to 15.02 (P = .004), 19.9 mmHg (P = .056), and 37 mmHg (not applicable) after 5, 10, and 15 years, respectively. Mean HG regurgitation was grade 0.49 before discharge and increased to 1.0 (P < .001), 0.91, and 2.5 after 5, 10, and 15 years, respectively. Ejection fraction increased from 61.9% to 64% after 5 years and to 66% after 10 years (P = .021) and then decreased to 63.5% after 15 years.

CONCLUSIONS

Comparing HG with other valve prostheses, survival and graft durability seem to be confirmed. They are vulnerable to infections. The hemodynamic performance is good, and hemorrhagic or thrombo-embolic events are rare.

Homograft aortic valve replacement--the experience of one unit

BACKGROUND

Homograft valves offer advantages including avoidance of anticoagulation and less susceptibility to infection especially in the setting of endocarditis. However, there is concern about their durability and possible accelerated degeneration particularly in cases of second time replacement with homografts.

AIM

This study aimed to evaluate the pattern of homograft failure and the quality of life in patients after homograft implantation.

METHODS

Between 1990 and 1998, 58 patients underwent aortic valve replacement with a homograft (aortic homograft = 47, pulmonary homograft = 11). Evaluation was based on clinical and echocardiographic examination, patient questionnaires and explanted valve pathology. Survival and freedom from cardiac related death were expressed by actuarial methods.

RESULTS

Follow up ranged from 1 to 10 years (mean 5.5 years). Analysis of questionnaires revealed 60% of respondents to be in good performance status and 20% in moderate and 20% in poor performance status groups. Eleven patients (18.9%) required subsequent redo valve replacement after initial homograft insertion (pulmonary = 6, aortic = 5) due to either valve dehiscence (n=4) or valve degeneration (n=7). The mean interval of re-replacement was 5.4 years.

CONCLUSIONS

Pulmonary homografts have a high failure rate in the aortic position. Overall subjective and clinical improvement after surgery is less than expected for a "physiological" device. In the setting of low availability of homografts the use of off-the-shelf devices such as stentless xenografts may be preferable in most cardiac surgical units in the current era.

Cell composition of the human pulmonary valve: a comparative study with the aortic valve--the VESALIO Project. Vitalitate Exornatum Succedaneum Aorticum labore Ingegnoso Obtinebitur

BACKGROUND

Cell populations present in human semilunar valves have not been investigated thoroughly. The aim of this study was to characterize the cell phenotypes in pulmonary valve leaflets (PVL) in comparison with aortic (AVL) valve leaflets.

METHODS

AVL and PVL were dissected from hearts (n = 4) harvested from transplanted patients. Leaflets were processed for immunocytochemistry analysis and Western blotting procedures using a panel of monoclonal antibodies specific for cytoskeletal/contractile antigens.

RESULTS

The fibrosa and the ventricularis layers of AVL had a higher cellularity than PVL. In PVL and AVL most cells were reactive for vimentin and nonmuscle (NM) myosin, though vimentin-positive cells were more abundant in AVL than in PVL. Sparse cells positive to anti-smooth muscle (SM) alpha-actin, calponin, and anti-SM myosin antibodies were found only at the outer edge of fibrosa. In Western blotting, AVL and PVL extracts were shown to be equally reactive for vimentin, SM alpha-actin, and NM myosin, whereas both valves were negative for SM myosin and SM22.

CONCLUSIONS

Three distinct cell phenotypes have been identified in both valves: fibroblasts, myofibroblasts, and fetal-type SM cells whose distribution is specifically related to the valve layers. Although PVL and AVL cell populations differ quantitatively, some minor qualitative differences exist for vimentin and NM myosin distribution. These data are essential for studies aimed at repopulating valve scaffolds by using tissue engineering technology.

Procollagen synthesis by fresh and cryopreserved rat pulmonary valve grafts

OBJECTIVE

Allograft valves are frequently used in the repair of congenital cardiac anomalies. The failure rate may differ depending on the type of allograft used. Previous studies have shown that rat aortic valve grafts exhibit synthesis of procollagen, suggesting a capacity for repair and regeneration after implantation. No studies of pulmonary valve grafts in the heterotopic rat implant model have thus far been reported. This study was designed to investigate whether pulmonary valve grafts maintain in vivo viability, as demonstrated by procollagen synthesis, and whether cryopreservation, histocompatibility, or both affect this property.

METHODS

Cryopreserved and fresh rat pulmonary valves were implanted into the abdominal aorta of syngeneic and allogeneic recipients. The grafts and native valves were excised 3 to 21 days after implantation. Valves were sectioned and immunohistochemically stained for procollagen. Computerized morphometry was used to calculate changes in intima, media, and adventitia as a percentage of cross-sectional area of the graft. Procollagen content was graded by semiquantitative methods.

RESULTS

Pulmonary valve grafts had significantly greater collagen density in the intima and adventitia compared with native aortic and pulmonary tissues, but collagen density in the media was similar in all groups. The grafts demonstrated appreciably greater procollagen than the corresponding native valves. These findings were consistent in all grafts (ie, both fresh and cryopreserved, both syngeneic and allogeneic), irrespective of duration of implantation.

CONCLUSIONS

Procollagen synthesis occurs in pulmonary valve grafts early after implantation, indicating viability of these tissues. This model of pulmonary valve implantation may have wide applicability to questions of allograft biology.

The in vitro hydrodynamic characteristics of the porcine pulmonary valve and root with regard to the ross procedure

OBJECTIVE

The hydrodynamic parameters and leaflet motion of the porcine pulmonary root and valve and the performance of the pulmonary autograft implanted in subcoronary position or as a free-standing root were investigated at systemic and pulmonary pressures in vitro.

METHODS

Ten fresh pulmonary and aortic roots (anulus diameter, 20-25 mm) were tested in a pulsatile flow simulator. Five free-sewn pulmonary valves were implanted in aortic roots in the subcoronary position, and 5 pulmonary roots were implanted as free-standing roots. The external diameter of the roots was measured at the sinotubular junction in a pressure range of 0 to 120 mm Hg. The transvalvular gradient and regurgitation were measured, and the effective orifice area was calculated. The leaflet motion was recorded on video tape.

RESULTS

The fresh pulmonary roots were more compliant than their aortic counterparts (33% +/- 3. 0% vs 7% +/- 1.5% with dilatation at 0-30 mm Hg and 46% +/- 8.4% vs 35% +/- 7.8% with dilatation at 0-120 mm Hg). The pulmonary roots had a lower pressure drop at systemic than at pulmonary pressures. The pressure drops of the pulmonary roots were also lower than those of the aortic roots in the systemic pressure range. The leaflet opening of the pulmonary valve was triangular, with low bending deformation at all pressures. Implanting the free-sewn pulmonary valve in the subcoronary position or the pulmonary root as a free-standing root did not affect the hydrodynamic parameters and leaflet motion adversely.

CONCLUSION

The pulmonary valve and root could easily withstand aortic pressures in vitro. A biphasic dilatation curve ensures that higher pressures did not overdilate the pulmonary root. Moreover, valve performance was better at systemic pressures.

Pulmonary homograft: should it be used in the aortic position?

OBJECTIVE

Retrospective analysis was performed to determine the suitability of pulmonary homograft as an aortic valve substitute.

METHODS

From January 1994 through June 1999, 147 patients (mean age, 32.2 +/- 17.3 years) underwent aortic valve replacement with either an aortic homograft (group 1: n = 103, 25 fresh antibiotic preserved and 78 cryopreserved) or a pulmonary homograft (group 2: n = 44, 11 antibiotic preserved and 33 cryopreserved). In group 1 a scalloped subcoronary technique was used in 64 patients, and a root replacement technique was used in 39 patients. In group 2 the scalloped subcoronary technique was used in 34 patients, and the root replacement technique was used in 10 patients.

RESULTS

There were 131 operative survivors (group 1 = 91; group 2 = 40). Follow-up ranged from 2 to 62 months. In group 1 none of the patients had significant aortic regurgitation during the hospital stay. Three patients (all having undergone the scalloped subcoronary technique) had moderate aortic regurgitation after 6 to 32 months. In group 2, 10 patients (9 having undergone the scalloped subcoronary technique and 1 having undergone the root replacement technique) developed significant regurgitation: 2 intraoperatively, 5 in the early postoperative period before discharge from the hospital, and 3 during late follow-up 6 to 12 months postoperatively. Among the various risk factors analyzed for overall homograft failure, use of a pulmonary homograft was the single independent predictor of valve failure (odds ratio, 8.6; 95% confidence interval, 1.9-39; P =.006).

CONCLUSION

Pulmonary homograft, when inserted by means of a scalloped subcoronary technique, is not a suitable aortic valve substitute.

Are pulmonary homografts which were subjected to pulmonary hypertension more appropriate for aortic valve replacement than normal pulmonary homografts? A long-term multicentric echography study

OBJECTIVE

To compare long-term results of the European Homograft Bank (LHB) cryopreserved pulmonary homograft in left ventricular outflow tract (LVOT) subjected to pulmonary hypertension with those subjected to normal pulmonary pressure. The mid-term study of this material published in 1997 showed different results.

METHODS

Statistical analysis is calculated by the Kaplan-Meier survival curves, while differences in prevalence by the Log-Rank test.

RESULTS

Follow-up (FU) was available in 69 cases (76.7%): 46 in group 1 and 23 in group 2. Five patients have been excluded from the study because of early homograft explantation (technical problems or early valve incompetence). Fourteen out of 43 cases of group 1 (32. 6%) and seven out of 21 cases of group 2 (33.3%) have been explanted after 2.5-88 months and 7-88 months, respectively. Significant echography changes have been found in 19 of 43 (44.18%) of group 1 and 11 of 21 cases (52.38%) of group 2 during the follow-up. Histology showed essentially wear and tear induced lesions. Mean FU was 36.9 (range, 6-88) and 41.3 months (range, 4-88) for group 1 and 2, respectively. No significant difference in the long-term outcome have been found between the two groups (P=0.38).

CONCLUSION

Contrary to our previous echocardiography study of mid-term implants the long-term follow up of the PHGs implanted in the LVOT did not show better function of the pulmonary homografts subjected to pulmonary hypertension than those with normal pulmonary pressure. The high failure rate of the PHGs should discourage their use for LVOT reconstruction. Further echocardiography studies of remaining PHGs implanted in the LVOT, and gross and microscopic explant studies are required to judge on the definitive outcome of these grafts.

Inflammatory reactions following homograft insertion in children with congenital heart disease

A study was carried out to assess significant short-term postoperative complications in children undergoing valved homograft surgery for congenital heart defects. Clinical and laboratory parameters of 50 patients undergoing this type of surgery were compared with those of 50 randomly selected but age-matched controls in whom open-heart surgery was performed without a homograft. Extracorporeal circulation, aortic clamping and intensive care times and the duration of febrile response were significantly longer in the study patients than in the controls (each p < 0.0001). In laboratory data the only significant difference between the two groups was in the C-reactive protein response, which was high in the study group (p < 0.0004) and did not correlate with extracorporeal circulation time or with positive blood cultures. The prolonged febrile and acute-phase reactant response following valved homograft surgery is an inflammatory reaction to the foreign antigen.

Prevention of calcification in glutaraldehyde-treated porcine aortic and pulmonary valves

BACKGROUND

The problem of calcification in porcine aortic (AVs) and pulmonary (PVs) valves and its relationship to glutaraldehyde (GA) is of current interest. We proposed an anticalcification treatment to develop noncalcifying porcine AVs and PVs.

METHODS

Porcine AVs and PVs were cross-linked in GA. Partially degraded heparin was coupled to the GA-treated AVs and PVs through intermediate surface-bound substrate containing amino groups. Control AVs and PVs were cross-linked in 0.625% GA but had no heparin coupling. All specimens were implanted subdermally in 3-week-old rats for 5 months for calcification studies.

RESULTS

Control AVs (Ca, 233.69 +/- 42.61 mg/g) and PVs (Ca, 181.48 +/- 4.06 mg/g) were severely calcified. Coupling of partially degraded heparin revealed complete prevention of calcification in GA-treated AVs (Ca, 0.73 +/- 0.27) and PVs (Ca, 1.125 +/- 0.22 mg/g) implanted subcutaneously in weanling rats for 5 months.

CONCLUSIONS

The proposed anticalcification treatment is effective in preventing calcification of GA-treated AVs and PVs implanted subcutaneously in weanling rats for 5 months.

Are pulmonary homografts subjected to pulmonary hypertension more appropriate for aortic valve replacement than normal pulmonary homografts? Results of echocardiography in a multicentric study

OBJECTIVE

To compare the function in aortic position of cryopreserved pulmonary homografts subjected to pulmonary hypertension with that of normal cryopreserved pulmonary homografts.

METHODS

Pulmonary valves (52) were implanted in aortic position in different cardiothoracic centres. The valves were classified as follows: Group I-pulmonary hypertension (procured from recipients of heart/heart-lung transplantation, 31 valves), Group II-normal pulmonary pressure (procured from cadavers and multiorgan donors, 21 valves). Regular echocardiographic follow-up was obtained by the implanting centers. Significant echo changes were defined as insufficiency > 2+ and/or stenosis producing a delta P > 30 mm Hg.

RESULTS

Pulmonary homografts showed the following significant echo changes: in the Pulmonary Hypertension Group, 7, 27 and 33% at 12, 24 and 36 months, respectively; in the normal PA Group 10, 37.5 and 80% at 12, 24 and 36 months, respectively. In both groups the most common echocardiographic alteration was homograft insufficiency rather than stenosis. Thus, pulmonary homografts subjected to long-term pulmonary hypertension have significantly less echo changes than normal pulmonary homografts, especially after 12 months (chi 2: P < 0.036).

CONCLUSIONS

These findings suggest that pulmonary valves subjected to pulmonary hypertension might be more appropriate than normal pulmonary homograft for aortic valve replacement, constituting a possible alternative in case of lack of aortic valve homografts. However, the failure of two out of five valves in the longer term must dictate caution while waiting further long-term results.

A new experimental model for simultaneous evaluation of aortic and pulmonary allograft performance in a composite graft

A model was developed in pigs for simultaneous evaluation of aortic and pulmonary allograft performance in a composite graft. The composite graft consisted of vascular prosthesis and aortic and pulmonary allografts. Following antibiotic preservation, it was anastomosed to the recipient's thoracic descending aorta by an extrapleural approach without using cardiopulmonary bypass. Aortic blood flow was completely diverted into the composite graft. All 12 recipient pigs recovered well, 4 of which were assigned for the initial study to design the suitable experimental schedule. Calcification readily occurred in the aortic allografts and aneurysmal dilatation without calcification developed in the pulmonary allografts. These morphological findings were consistent with those of previous reports. This model has several benefits. First, aortic and pulmonary allograft conduits can be implanted and evaluated simultaneously under the same conditions by making a composite graft. Second, the magnitude of the operation is minimum, and postoperative circulatory and respiratory management is uncomplicated. Third, wound infection rarely occurs, because the skin incision is made on the back. These preliminary studies suggest that this model will allow future study concerning aortic and pulmonary allograft conduits under different conditions.

A comparison of macroscopic lipid content within porcine pulmonary and aortic valves. Implications for bioprosthetic valves

Lipid droplets have been demonstrated within both explanted porcine bioprostheses and normal porcine aortic valves. Because of the increasing interest in pulmonary valves as an allograft or xenograft aortic valve substitute, we examined the incidence and distribution of such lipid deposits in 50 porcine aortic valves and 50 matched porcine pulmonary valves. All 300 cusps were removed with surgical scissors and, under a dissecting microscope, the ventricularis layer was removed to expose the spongiosal layer. Macroscopic extracellular lipid droplets analyzed by means of a dissecting microscope with an eyepiece grid and stereology point-counting techniques to provide an area-density average spatial probability map for each cusp. Only 8% of porcine aortic valves were free of lipid, with the distribution of the lipids being 52% +/- 14% right coronary cusp, 90% +/- 8% left coronary cusp, and 68% +/- 13% noncoronary cusp. Of the pulmonary valves, 60% were free of lipid, with the incidence of lipids being 26% +/- 12% left cusp, 6% +/- 7% right cusp, and 12% +/- 9% anterior cusp. Subsequently, lipid cluster samples underwent thin-layer chromatography, which showed them to be phospholipids, oleic acid (fatty acid), triglycerides, and unesterified cholesterol. One primary mode of bioprosthetic valve failure is leaflet calcification. The similarity of distribution within the spongiosal layer between leaflet calcification and intrinsic cusp lipids suggests that the observed lipids might act as a nucleation site for calcification. The substantially lower incidence of lipid in pulmonary valves therefore may represent a potential benefit when these valves are considered for use as aortic valve replacements.

Aortic valve replacement with cryopreserved pulmonary allografts: five years' follow-up

Excellent clinical results with pulmonary autografts and experimental evidence that pulmonary valves can withstand the higher stress in the systemic circulation led us to use the cryopreserved pulmonary allograft for aortic valve replacement. From September 1988 until March 1993, 126 consecutive patients (61 +/- 10 years; 74 men and 52 women) underwent aortic valve replacement with a cryopreserved pulmonary allograft. All allografts were inserted freehand in the subcoronary position. There were four in-hospital deaths (3.2%), and 1 patient had severe valvular incompetence immediately postoperatively, requiring reoperation after 4 weeks. One hundred twenty-one patients were followed up in 3- to 6-month intervals for 25.3 +/- 16.3 months (range, 6 to 66 months), and valve performance was assessed routinely by means of color-flow Doppler echocardiography. Nine patients (7.1%) died during follow-up. Two patients died of multiple septic emboli during bacterial endocarditis, and 1 patient died of a massive stroke. The other 6 patients died of myocardial infarction (4), respiratory insufficiency due to chronic obstructive lung disease (1), and carcinoma (1). Ninety-four patients (78%) had absent or trivial aortic valve regurgitation. Valvular incompetence class II was present in 3 patients (2.5%), whereas 5 others (4%) demonstrated class II to III. Severe aortic regurgitation (class III or IV) could be detected in 10 patients (8.3%). All underwent reoperation and replacement of the valve with a prosthetic device. Bacterial endocarditis caused graft incompetence in 3 patients, valve degeneration was detected in another 3, and technical mistakes at valve implantation caused valve failure in the other 4.(ABSTRACT TRUNCATED AT 250 WORDS)