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| FUTURE TRENDS |
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| Year : 2002 | Volume
: 3
| Issue : 2 | Page : 3 |
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Percutaneous valve implantation: Past, present and future.
Younes Boudjemline1, Philipp Bonhoeffer2
1 Service de Cardiologie Pédiatrique, Hôpital Necker Enfants Malades, Paris, France
2 Cardiothoracic Unit, Great Ormond Street Hospital, London, UK, and The Institute of Child Health, London, United Kingdom
| Date of Web Publication | 22-Jun-2010 |
Correspondence Address:
Philipp Bonhoeffer
Cardiothoracic Unit, Great Ormond Street Hospital for Children NHS Trust, Great Ormond Street, London WC1N 3JH
United Kingdom
 Source of Support: None, Conflict of Interest: None  | Check |
 Abstract | | |
The percutaneous approach for valve replacement has recently been introduced. This procedure is presently available for patients with artificial right ventricular outflow tract conduits only. The technique is safe, but is limited to rare stereotyped clinical situations. Further technical improvements will broaden the spectrum of indications to pulmonary valve replacement regardless of the anatomy of the outflow tract to the aortic valve and possibly to atrio-ventricular valves. In this report, we review our experience in valve replacement through a percutaneous technique in humans, describing the ongoing experimental work. Keywords: Catheterization, valve diseases, stent
How to cite this article:
Boudjemline Y, Bonhoeffer P. Percutaneous valve implantation: Past, present and future. Heart Views 2002;3:3 |
| Introduction | |  |
The first surgical attempt of valve replacement was by Hufnagel in the early 1950's. He surgically implanted a mechanical ball valve in the descending aorta to palliate chronic aortic insufficiency [1],[2] . With the development of extracorporeal circulation, native valve replacement became the conventional treatment for valve diseases making implantation in heterotopic position a forgotten entity. In parallel, over the last years, cardiac catheterization has developed progressively, shifting from diagnostic to interventional procedures. Various diseases namely valvular or vascular stenosis, and septal defect or shunt closures, are now treated primarily by a transcatheter technique [3],[4],[5],[6] . Until recently, valve replacement has, however, remained entirely in the surgical domain. In this manuscript, we give an insight into our experience in transcatheter implantation of cardiac valves, describing the device and discussing present and future indications.
| Methods | | |
The valve
The valve of choice for percutaneous implantation is a valve which 1) is easily available at variable sizes; 2) is biocompatible; 3) has excellent intrinsic properties; 4) has a low profile; 5) can be sutured into an expandable stent; and 6) does not lose its property after crimping and re-expansion. After testing different types of valves [Figure 1], we finally opted for the bovine jugular venous valve. Bovines have native valves in their jugular veins which allow the filling of the right heart, and avoid stasis of the blood while these animals hold their head at a low level such as during feeding. The bovine jugular venous valve was introduced in the late 1990s in surgical practice as a right ventricular to pulmonary artery substitute (Contegra, Medtronic) and preliminary results were excellent [7] . Various sizes are available from 8mm to 22mm. The leaflets of the valve are highly mobile, thin and redundant. Despite its excellent properties, the size of the venous wall is too thick to allow it to be used as a substitute for percutaneous insertion. Fortunately, the wall can be reduced in profile significantly without interfering with the valve function.
Preparation of the device
After removal of unnecessary tissue from the external venous wall, a section of the prepared valved vein is sutured into a platinum-iridium stent, which we developed in cooperation with Numed Inc [Figure 2]. This is a highly malleable stent that can be crimped and re-expanded several times without damage. After preparation, the device is sterilized, cross-linked with a buffered glutaraldehyde solution and stored in an alcoholic solution according to industrial protocols.
Delivery system
A custom-made delivery system was developed in parallel with the valved stent. It is based on a frontloading technique using the BiB (balloon in balloon) technology (Numed Inc) for stent delivery. At the time of implantation, the device is delicately hand-crimped on the outer balloon of the delivery system and then covered [Figure 3]. The outer diameter of the device when fitted on the balloons is approximately eighteen French.
First steps towards percutaneous pulmonary valve replacement
Eleven lambs underwent catheterization for percutaneous pulmonary valve implantation under general anesthesia [8] . The valve device was inserted percutaneously in pulmonary position on to a previously positioned guide wire according to standard stent placement technique. Seven of the lambs had a pulmonary valve insertion through a right jugular venous access. Technical failure occurred in the remaining four lambs because of the narrow angle between the tricuspid valve and the right ventricular outflow in our model. In the other seven lambs, five valved stents impinged on the function of the native valve and two were unsatisfactorily positioned just adjacent to the native valve. At 2 months evaluation, one stent was slightly stenotic, with a systolic pressure gradient between the right ventricle and the pulmonary artery of 15-mm Hg. Six of the seven successfully implanted valves were angiographically competent. One mild regurgitation was noted on the last animal, probably aggravated by the position of the catheter through the pulmonary valve during the contrast injections. Two months after the insertion, valves were electively explanted. Macroscopic examinations showed that four of the five precisely inserted valves had no sign of valvar calcification. The remaining valve showed early signs of degeneration including macroscopic calcification, cuspal retraction and partial fusion of the commissures. This was attributed to the sub optimal sterilization process used in this initial setting. The two stents incorrectly positioned were malfunctioning. Fibrous tissue covered the leaflets of these valves. Their functions were restored in vitro after the removal of fibrous tissue. This animal experimentation confirmed the feasibility of the technique. Technical implantation difficulties were mainly related to the animal model.
The main concern was about the durability of the valves. However, as far as the durability is concerned, the surgery and the percutaneous approach shared the same limitations. Therefore, the excellent results of the initial experience of the bovine jugular vein in the surgical setting encouraged us to introduce percutaneous approach in humans.
First human implants in pulmonary position
We initially limited the indication to pulmonary valve insertion to patients with surgically created communications between the right ventricle and the pulmonary artery namely prosthetic conduits, valves, and patch reconstructions [9] . This stereotyped situation allowed us the precise knowledge in advance of the anatomy of the right ventricular outflow tract. Nine patients with significant pulmonary regurgitation and/or right ventricular outflow tract obstruction were selected for a preliminary study. Approval for percutaneous pulmonary valve replacement was given by a certified ethical committee (CCPPRB, Paris Cochin, Paris, France). Fully informed consent was obtained from the parents, where the patient was a child, and from the patient themselves, if an adult. The seven children ranged in age between 10-17 years, with a mean age of 12.1 2.3 years. Three had a tetralogy of Fallot without pulmonary atresia, and 3 with pulmonary atresia. One patient had absent pulmonary valve syndrome. Six of the seven patients had previous palliative surgery with one or more modified Blalock-Taussig shunts followed by a total repair with conduit placement. Most of the children underwent reoperation for replacement of the initial conduit with a larger one during infancy. One adult patient (aged 38 years) had a tetralogy of Fallot repaired initially at 3 years of age, followed by two reoperations to replace the pulmonary valve 15 and 25 years after the original repair. The last patient (aged 18 years) had a congenital aortic stenosis with a sub-valvular membrane. His membrane was resected at 5 years of age. In July 2001, a Ross procedure was performed to replace his aortic valve, and a 26mm homograft was inserted in the pulmonary position. All patients were symptomatic with effort intolerance and breathlessness, and needed surgery for a conduit replacement or pulmonary valvulation due to significant stenosis and insufficiency of the conduit. Before the procedure, six patients were in New York Heart Association (NYHA) class II and three were in NYHA class III with cardiomegaly, moderate to severe right ventricular dilatation and dysfunction on echocardiography. The valved/stent was successfully implanted in all nine patients. Immediately after implantation, the hemodynamic and angiographic evaluation confirmed competence of the newly implanted valve in seven patients [Figure 4]. In two patients, the valve was deployed slightly lower than intended, lying slightly in the infundibulum leading to insignificant paraprosthetic regurgitation. The relief of the conduit obstruction was partial in three patients. The fluoroscopy time ranged from 17-129 minutes with a mean of 48 minutes. The time for the procedure of valved/stent implantation improved significantly after the first cases. All the patients were discharged between 1 and 5 days after the procedure with aspirin at 2mg/kilo/day. Echocardiography immediately before discharge confirmed the perfect competence of the implanted valve in all patients [Figure 5]. The slight paraprosthetic leak present in two patients after the procedure disappeared on color Doppler echocardiography. At the latest follow-up ranging from 1-17 months (mean 11 months), all the patients had improvement of their symptoms especially in adults patients who were the most symptomatic. Color Doppler echocardiography showed a fully competent pulmonary valve in six, and trivial regurgitation in the remaining three.
A reduction of the right ventricular size and an improvement of systolic function of the right ventricle was suspected on transthoracic echocardiography evaluation, and confirmed by MRI scan. Echocardiography also showed the persistence of a moderate elevation of the right ventricular systolic pressure in four patients.
| Future indications in the right heart | | |
One of the major challenges of the future will be to implant such a valve in more variable anatomy of the right heart. Indeed, conduit valvulation is a limited indication. Most pulmonary insufficiency occurs after surgical repair of tetralogy of Fallot. These patients have extremely dilated pulmonary trunks that make percutaneous implantation of valved stents as presently designed impossible. One alternative would be to implant a valve in the two proximal pulmonary branches. However, this stenting would leave a regurgitant fraction originating from the pulmonary trunk. The clinical benefit of this technique would need to be demonstrated. The valvulation of a Fontan circulation could also be an interesting indication to evaluate in the near future. Indeed, right atrial dilatation is a frequent complication occurring in long-term follow-up [10] . It generates arrhythmia that has major repercussions on cardiac function [11] . The valvulation of such patient might prevent the occurrence of arrhythmia.
| First steps towards percutaneous aortic valve replacement | | |
Percutaneous aortic valve replacement is a major challenge. This has been considered not possible because of 1) the proximity of coronary arteries, and 2) the anatomic continuity between the aortic and the mitral valve. The design of the valved stent in the present study does not allow its implantation in sub-coronary position. But, as a first step, we evaluated the function of the venous valve at high pressure in vitro and more recently in an animal setting. To achieve this goal, we implanted valved stent in the descending aorta of lambs after the creation of a moderate or severe aortic regurgitation. All animals with severe aortic insufficiency died suddenly despite perfect competence of the implanted valve. The deaths were attributed to coronary flow impairment secondary to aortic regurgitation. All animals with moderate aortic regurgitation survived. All implanted valves were perfectly functioning during the first 2 months of implantation. At the 3-month evaluation, the aortic regurgitation has disappeared and none of the implanted valves were competent. At macroscopic evaluation, all implanted valves were covered with a fibrinous tissue and no tear was found on the aortic valve. According to our previous study, which showed that unused valves are rapidly covered with a fibrous tissue, we speculated that the healing of the aortic valve led to the dysfunction of the implanted valves. In this initial experiment [12,13] , we demonstrated the feasibility of implantation in the descending aorta, reproducing through a percutaneous approach the pioneering work of Hufnagel et al [1] . As a second step to approach the native aortic valve, we redesigned the device [14],[15] . Initially, we wrongly thought that the venous wall was necessary for the valve to function. In experimental studies, we first verified that the removal along the commissures of the venous wall did not alter the function of the valve in vitro and secondarily that these dissected venous valves could function at high pressures (in the descending aorta) in an animal setting very similar to the previous one. The next step was implantation in the native position. We first tested second-generation valved stents. This type of stent liberates space for coronary orifices but its orientation is impossible. Therefore, the stent was deployed 1 cm below the aortic annulus. All lambs died suddenly during the procedure despite "successful" (but inappropriate) delivery of the device: one from a severe mitral valve insufficiency; the second from an acute obstruction of the coronary artery orifices; the third migrated prematurely in the ascending aorta. The last device was inappropriately placed in the left ventricle leading to a paraprosthetic leak. This study highlighted the need for perfect orientation and anchoring of the device. To achieve these goals, we redesigned the valved stent ("third generation") with a deployment strategy in two steps. The first step assured the orientation and the locking of the device in the aortic orifice. The second acted as a supporting structure for the graft. To guaranty the orientation of the device, we fixed an autoexpandable nitinol stent on a second generation valved stent [Figure 6]. The branches congruous with the commissures of the valve were interdependent with the platinum stent wires and could not be deployed separately. Contrarily, the branches congruous with the leaflets were not sutured to the platinum stent wires so that when the platinum stent was reduced, the non-sutured branches of the nitinol stent were deployed at a diameter of 23mm defining a free space between the inner valved stent and the nitinol stent. In an acute setting, we successfully implanted this third generation-valved stent in five animals. No coronary orifices were obstructed and no mitral valve impairment was noted. There was no stent migration. Two valves were dilated to the proper diameter and were perfectly competent [Figure 7]. Three implanted valves were overdilated and were incompetent from mild to severe because of a non-coaptation of the valve leaflets. Further experiments are obviously needed to confirm these early results. In particular, questions regarding the durability of the valve in systemic pressure have to be answered before considering human application.
| Conclusions | | |
We report in this study the development of a non-surgical technique to implant valves. This technique has already been applied in humans in patients with artificial pulmonary artery trunk. The technique is safe but is presently limited to a rare and stereotyped clinical situation. Further technical improvements will allow expanding the spectrum of indications to patients with large pulmonary trunk or with Fontan circulation. More recently, we approached the aortic valve and opened the field of percutaneous valve replacement in heterotopic position as well as in native position using newly designed valved stents. Studies with longer follow-up are presently missing but will permit evaluating the function and durability of bovine venous valve in systemic pressures. However, even if the results of these studies are good, large diameters of valves are difficult to find making its use as a common substitute in adults improbable. Therefore, other valvular substitutes (artificial or biological) have to be found to overlap all the sizes used in clinical practice and to be suitable for right and left sided valves. The technology we developed for bovine valve placement can be used for any valvular substitutes. Non semi-lunar valves were not approached yet through a percutaneous technique but would probably be in the next decade.
| Acknowledgements / Grants | | |
The "Fondation de l'Avenir", Paris, France and the "Fιdιration Franηaise de Cardiologie", Paris, France. Research at the Institute of Child Health and Great Ormond Street Hospital for Children NHS Trust benefits from R&D funding received from the NHS Executive.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
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