Asian Cardiovasc Thorac Ann 2003;11:280-284
© 2003 Asia Publishing EXchange Ltd
Fate of Polytetrafluoroethylene Monocusp Pulmonary Valves in an Animal Model
Luca A Vricella, MD1,
Steven R Gundry, MD,
Hironori Izutani, MD,
Michael A Kuhn, MD,
Neda Mulla, MD,
Leonard L Bailey, MD
Divisions of Cardiothoracic Surgery and Pediatric Cardiology, Loma Linda University Medical Center, Loma Linda, California, USA
1 Division of Cardiac Surgery, The Johns Hopkins Hospital, Baltimore, Maryland, USA
For reprint information contact: Luca AVricella, MD Tel: 1 909 559 2979 Fax: 1 909 558 0348 email: lvricella{at}jhmi.eduDivision of Cardiac Surgery, Johns Hopkins University, 600 N Wolfe St. – Blalock 618, Baltimore, MD 21287-1824, USA.
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ABSTRACT
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Creation of a competent pulmonary monocusp valve facilitates transition from pressure to volume overload following right ventricular outflow tract reconstruction. To determine intermediate-term results and performance of the different types of polytetrafluoroethylene membrane used to construct monocusp valves and transannular patches, 12 infant lambs underwent excision of the native pulmonary valve and insertion of a monocusp valve and transannular patch made from one of 4 types of membrane. Echocardiography was performed after 3, 6, 9, and 12 months, and cardiac catheterization was carried out prior to animal sacrifice at 6 (n = 4) or 12 (n = 8) months. There was no postoperative morbidity or mortality. On echocardiography, 6 valves were mobile (50%), 4 had diminished mobility (33%), and 2 were fixed (17%) prior to sacrifice. At catheterization, mild, moderate, and severe pulmonary regurgitation was observed in 4 valves each (33%), with no stenosis. Right ventricular outflow tract reconstruction with polytetrafluoroethylene monocusp valves can be safely accomplished with good early competence, variable degrees of late insufficiency, and no stenosis. Compared to an open microstructure, the closed polytetrafluoroethylene microstructure showed a milder fibroinflammatory reaction and fewer foci of microcalcification, with sparing of the free edge of the monocusp; this correlated with better intermediate-term hemodynamic performance.
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INTRODUCTION
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The ideal method and material for right ventricular outflow tract (RVOT) reconstruction have been sought for over 40 years, as acute pulmonary insufficiency and progressive right ventricular (RV) failure are well-recognized complications of un-valved reconstruction for tetralogy of Fallot, valvular and supravalvular pulmonary stenosis, and absent pulmonary valve complex. A variety of synthetic and biological valved conduits have been used for this purpose, each with its own specific complications and limitations.1,2 Pulmonary artery (PA) patch arterioplasty with insertion of a pericardial monocusp valve has been used with success by our group to preserve RV function in the early and intermediate postoperative period, particularly for infants and children with diminished pulmonary runoff, pulmonary hypertension, and tricuspid regurgitation.3 A competent pulmonary valve allows a more gradual transition from pressure to volume overload of the right ventricle, especially in the setting of a right ventriculotomy. Monocusp valves have been constructed from several materials including autologous and bovine pericardium, autologous left atrial appendage or PA, and cryopreserved homografts.4–9 Although the short-term fate of autologous or xenograft tissue is excellent, intermediate and long-term results have been disappointing, prompting us to examine polytetrafluoroethylene (PTFE) as a potential solution.10,11 The purpose of this study was to assess the fate of RVOT reconstruction with PTFE in an infant animal model, using four different types of PTFE membrane.
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MATERIALS AND METHODS
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Twelve infant lambs (< 1 year old; median weight, 16.1 ± 1.5 kg) underwent endotracheal intubation after induction of general anesthesia with intravenous ketamine (1.5 mg•kg-1) and pancuronium bromide (0.1 mg•kg-1), followed by maintenance with inhaled halothane (0.5%–2%). Systemic antibiotics (cefazolin 25 mg•kg-1) were administered prior to incision. A left thoracotomy was performed at the 3rd intercostal space, and cardiopulmonary bypass with a median lowest temperature of 25.5°C ± 1.7°C was instituted by means of aortic and right atrial cannulation (median duration, 59 ± 11 min). With the heart perfused and beating, a longitudinal transannular incision was made in the RVOT, and the pulmonary valve leaflets were excised. The RVOT was reconstructed using PTFE for both the transannular patch and a monocusp valve.6 To optimize monocusp membrane motion, 5 to 7 titanium clips were applied to the free margin of the monocusp membrane. Four different types of PTFE Gore-Tex membrane (WL Gore & Assoc., Inc., Flagstaff, AZ, USA) were used in 3 animals each: material A was Preclude, a commercially available expanded PTFE pericardial membrane with a closed microstructure; material B was an experimental expanded PTFE membrane with a markedly open microstructure; material C was an experimental expanded PTFE membrane with a moderately open microstructure; and material D was a PTFE membrane combining material B on one side and material C on the other.
Two-dimensional transthoracic echocardiography was carried out at 3, 6, 9, and 12 months postoperatively, with optimal visualization of both monocusp and tricuspid valves during 35 of the 40 examinations performed (87.5%). No anticoagulation was used in the postoperative period. Cardiac catheterization was performed in all animals prior to sacrifice. In each group, 1 and 2 animals were sacrificed, respectively, at 6 and 12 months postoperatively. All animals received humane care in compliance with the "Guide for the Care and Use of Laboratory Animals" (NIH publication 85-23, revised 1985).
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RESULTS
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The 12 animals experienced no postoperative morbidity or mortality, and achieved a median weight of 47.0 ± 6.3 kg at the time of sacrifice. Echocardiographic and cardiac catheterization results are summarized in Tables 1
and 2. These results and the subsequent histology (Figures 1 and 2) suggest that the closed PTFE structure (material A) was the best of the four types.
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Figure 1. Longitudinal section of right ventricular outflow tract 12 months after reconstruction. Material A (closed microstructure) was used to fashion the monocusp valve. Bland loosely adherent fibroinflammatory tissue is seen at the interface between the monocusp valve (MV) and the transannular patch (TAP). The free edge of the valve is devoid of fibrotic tissue. Verhoeff-van Gieson stain, original magnification x1.5.
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Figure 2. Longitudinal section across the free edge of material C (open microstructure) 12 months after implantation. There is a dense fibroinflammatory reaction (F) around the free edge of the monocusp valve (MV) and foci of calcification within the open microstructure of the membrane. Hematoxylin and eosin stain, original magnification x10.
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Follow-up echocardiography focused on RV function and size, as well as on tricuspid and monocusp valvular function. The echocardiograms performed prior to sacrifice showed 6 valves to be mobile (50%), 4 valves had diminished mobility (33%), and 2 were fixed (17%). All animals had some echocardiographic evidence of pulmonary insufficiency; mild, moderate, or severe in 4 animals each. In the short-term (3 months postoperatively), 8 (67%), 3 (25%), and 1 (8%) animals had, respectively, mild, moderate, or severe pulmonary insufficiency. Only one animal had echocardiographic evidence of pulmonary stenosis (RVOT velocity of 2.8 m•sec-1), a finding not confirmed at catheterization. Two animals (17%) had mild tricuspid insufficiency.
Catheterization prior to sacrifice disclosed no significant RVOT stenosis (median peak RV-left ventricular pressure ratio of 0.20 ± 0.00) and variable degrees of pulmonary regurgitation. Most animals had a low median diastolic gradient (PA end-diastolic pressure – RV end-diastolic pressure) of 3 ± 4 mm Hg, with a gradient greater than 8 mm Hg in 3 animals. Nevertheless, there was no long-term increase in median RV end-diastolic pressure (1 ± 1 mm Hg).
Gross evaluation of specimens at 6 and 12 months confirmed the results of echocardiography and cardiac catheterization. Median right ventricular wall thickness was 7.0 ± 1.0 mm, with 8 of 12 monocusp valves (67%) still showing good pliability. Two valves had diminished mobility, and two were fixed to the transannular patch. Histologically, material A (closed microstructure) outperformed materials B, C, and D. The bland fibrotic tissue loosely attached to material A almost completely spared the distal two thirds of the monocusp membrane at 12 months postoperatively. Overall, a paucity of inflammatory cells and microcalcification deposits was seen with this material (Figure 1
). In contrast, the open structures of materials B, C, and D appeared to be foci for cellular migration, wrinkling of the monocusp with dense inflammatory fibrotic changes and calcification. Although the majority of the membranes were still functional on hemodynamic and echocardiographic assessment, the pronounced fibroinflammatory changes seen in materials B, C, and D extended to the free edge of the monocusp valve at 12 months postoperatively (Figure 2).
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DISCUSSION
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The long-term sequelae of pulmonary insufficiency have been well described. If tolerated in the acute postoperative phase, right ventricular adaptation to chronic pulmonary insufficiency occurs, with a progressive increase of RV end-diastolic pressure and narrowing of the PA-RV diastolic gradient; the left ventricular/RV wall thickness ratio decreases, and hypertrophy and dilatation allow long-term functional tolerance.12–14 This process, if associated with persistent left-to-right shunting or pulmonary hypertension, can be acutely accelerated with rapid progression to RV failure and tricuspid insufficiency.
Pulmonary valve competency in the perioperative phase is of particular importance for those infants and children with associated labile pulmonary vascular resistance and/or poor pulmonary runoff.15 Following repair of congenital anomalies with RVOT obstruction, progressive deterioration of RV function is often seen. Pulmonary regurgitation and residual stenosis remain the major determinants of long-term cardiac performance, freedom from reoperation, and mortality. During the first 2 to 3 decades after repair in which pulmonary insufficiency is usually subjectively well tolerated, objective exercise-tolerance testing invariably detects subclinical deterioration in RV performance.16
Several techniques and materials have been utilized for RVOT reconstruction: valved conduits (homografts or xenograft valved Dacron conduits), or where anatomically feasible, a transannular patch with insertion of a monocusp valve. The absolute (secondary to pseudo-intimal proliferation or calcified stenosis) or relative (caused by somatic growth of the patient) reduction in luminal diameter of the RV-PA connection is usually the cause of late structural failure.1 Of additional concern, aortic or pulmonary homografts are expensive or unavailable. Although PTFE has been used experimentally and clinically for monocusp construction, investigation of the optimal type of PTFE has not been undertaken previously.17–19 The more frequently described transannular patch or monocusp valve made from bovine or autologous pericardium demonstrates early competence but late fibrosis and insufficiency.4 In general, PTFE shows less calcification, thrombus formation, and neointimal hyperplasia than glutaraldehyde-fixed pericardium.20
The use of PTFE for construction of both the transannular patch and monocusp valve in this study resulted in no intermediate-term hemodynamically significant calcified stenosis, with variable well-tolerated degrees of pulmonary insufficiency. However, this occurred in an experimental model without pulmonary hypertension or branch pulmonary stenosis. Hemodynamic and histologic changes in cases of clinical branch pulmonary stenosis or pulmonary hypertension may differ and should be the object of further investigation. Although still functional in the intermediate-term, the processes of calcification and cellular migration appear to be accelerated in valves made from bovine pericardium or an open PTFE microstructure.11 The preliminary data from this study indicate that PTFE can be effectively used to construct both a transannular patch and monocusp valve in growing animals, with no residual RVOT stenosis at one year postoperatively. The closed microstructure of the Gore-Tex Preclude membrane was superior to the various open microstructure PTFE membranes studied, showing much less cellular migration, inflammatory reaction, and calcification.
This study was supported in part by WL Gore & Assoc., Inc., Flagstaff, AZ, USA.
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ABSTRACT
INTRODUCTION
