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Percutaneous valve surgeryReviewer: Andrew D. Maslow, MD
Percutaneous valve surgery has received tremendous interest as a direct result of advancements in endovascular techniques.1,2 A minimally invasive percutaneous system that allows consistently accurate positioning and rapid deployment (< 5 min) of a collapsible/expandable stent-valve system has become a realistic expectation. The success of any such system depends, in part, on the nature and extent of valve dysfunction and the severity of comorbidities. Although feasible, long term outcomes are not yet available. Percutaneous approaches to repair and or replace the semilunar and atrioventricular valves have been reported.1,2 Repairs include balloon valvuloplasty for stenotic lesions, and, for mitral regurgitation, annular reduction (via the coronary sinus) or performance of an Alfieri procedure using a 'clip' placed on the tips of the anterior and posterior leaflets. Although feasible, percutaneous repair of the mitral valve has not yet enjoyed the success of open repair techniques, and long-term outcome data are not available. For patients with rheumatic mitral stenosis, for which leaflet mobility is preserved and minimal calcification is noted on the valve apparatus, balloon valvuloplasty has been associated with good long-term results, however, progression of disease is the norm and these patients will eventually present for valve replacement. Similar data has been reported in the pediatric literature for valvuloplasty or angioplasty of the right ventricular outflow tract and/or pulmonic valve, and aortic valve. Balloon valvuloplasty for aortic valve stenosis in the elderly, however, has not enjoyed long-term success and has been complicated by embolization during the procedure and early restenosis. Although valvuloplasty, for some, has delayed the need for open-heart surgery and the complications associated with valve replacement, most, if not all, demonstrate progression of the underlying pathology. Valve replacement still remains the definitive treatment for valvular dysfunction. While open-heart procedures report short and long term successes, major morbidity is associated with sternotomy, thoracotomy, and excision of cardiac tissues. In addition, reoperation for prosthetic valve dysfunction, complications of thromboembolism and anticoagulation, and risks of endocarditis have prompted clinicians and researchers to explore a variety of less invasive techniques including valve repairs, minimally invasive surgical approaches, and, more recently, percutaneous approaches toward valve replacement. A number of basic features are required for percutaneous valve replacement. These include accurate and continuous imaging of the targeted valve, development of a safe ablation technique of the native valve, a non-invasive flexible delivery system, a collapsible/expandable stent-valve apparatus, and the ability to retract the expanded valve if positioning is not ideal. Percutaneous valve replacement has been successfully performed under experimental conditions using a variety of designs including bovine and porcine tissues (jugular venous valves, or cardiac valves). These are sutured into a self-expanding metal (nitinol) scaffold, which is advanced toward the desired location guided by a host of imaging technologies. Limitations include the suturing technique of the valve to the scaffold, the expandability of narrowed and/or calcified tissue, and proper sizing in both the diameter of the valve and the length; the latter, perhaps more important for aortic valve procedures to avoid obstruction to coronary ostial flow. The preoperative assessment is crucial to guide selection and preparation of the stent-valve prosthesis. While the feasibility of placing a prosthesis using percutaneous approach has been demonstrated in the descending aorta, replacing the native aortic valve is more complicated.3-5 A number of different approaches including retrograde approach via the axillary and subclavian arteries, the ileo-femoral arterial tree, and antegrade via the left ventricular apex have been studied.3-5 Decisions regarding these approaches depend on the extent and severity of aortic atheroma and the size of the arterial tree. Although successful, percutaneous aortic valve replacement has been complicated by myocardial tears and perforations, ventricular arrhythmias, coronary ostial occlusion, stent valve migration, and stent-valve twisting.3-5 Autopsy reports of the explanted prosthesis have demonstrated significant degrees of valve thrombosis, emphasizing the importance of imaging and evaluation as well as imaging and guidance prior to and during the procedure. In addition, proper sizing of the stent-valve is important to avoid both patient-prosthesis size mismatch, and residual regurgitation. Procedural imaging may include fluoroscopy, intravascular ultrasound, intracardiac ultrasound, and surface and transesophageal echocardiography. Echocardiographic monitoring and evaluation has an advantage in that it is not only able to provide guidance and evaluation of the percutaneous technique, but also allows a comprehensive evaluation of both heart function and of the new valve. To date, once in place, these valves have demonstrated excellent hemodynamic profiles.3-5 Experimental animal data and early human experience shows that significant work remains regarding all components of the procedure.3-5 While the stent-valve apparatus can be successfully placed, perforations of cardiac tissues, improper positioning, arrhythmia and residual insufficiency occur.6-7 While one group reported successful placement and function of percutaneously placed aortic valve in five patients,6 a second group reported a high complication rate for 12 patients, which included cardiac perforation, arrhythmias, perivalvular regurgitation, and need for urgent conversion to sternotomy and open cardiac valve replacement.7 Furthermore, the need to ablate native tissue would cause transient aortic insufficieny until a new valve is placed, and also increases the risk of distal embolization of native valve fragments requiring a 'catching' device.3 Assessment during this time reports significant inceases in heart rate and decreases in diastolic blood pressure. Long-term data regarding valve function and the difficulties associated with future explantation should open heart replacement be required are not known. However, percutaneous valve replacement, especially for high-risk patients, has great potential to significantly reduce procedural morbidity of an open-heart procedure. Percutaneous approach for pulmonary valve replacement has also received great interest for both pediatric patients and for the adult survivors of congenital heart disease and previous surgery.8-11 The latter may address the need for pulmonary valve surgery after previous repair of Tetralogy of Fallot to prevent worsening of right heart dilation and failure.8,9 These patients present with either right ventricular outflow tract narrowing and obstruction, pulmonic valve insufficiency, or a combination of both. Surgical therapy depends on the pre-procedure evaluation of the right ventricular outflow and pulmonary artery diameter to determine whether or not enlargement of the RVOT is necessary to place an adequately sized prosthetic valve to prevent increases in afterload to the already dysfunctional RV. While open-heart procedures for the RVOT are reported, percutaneous approaches have been studied, and in small numbers of humans, have been successfully utilized with excellent hemodynamic profiles after implantation.10,11 Although feasible, similar complications are reported for percutaneous placement of the pulmonary valve as during replacement of the aortic valve,8 reemphasizing the importance of pre-procedural evaluation and planning, and continuous monitoring and imaging during the procedure. Finally, percutaneous replacement of the tricuspid valve has been studied using an animal model.12 Although feasible, these procedures have been complicated, in some, by difficult positioning, trapping within the chordae, and residual regurgitation. Nevertheless, successful percutaneous valve replacement for atrio-ventricular (AV) valves will continue to advance and, with greater experience, become a therapeutic option for patients with AV valve dysfunction. Although widespread acceptance is premature, the future of percutaneous valve procedures is of great interest and will likely be embraced. Obvious benefits are avoidance of a major surgical procedure, improved and more rapid recovery, and a faster return to daily activities. However, issues regarding proper valve sizing, and native tissue ablation require further attention. Operator experience will likely contribute to procedural morbidity. Finally, the longevity of such techniques and ability to repair or replace failed percutaneously placed valves is not known. References Table of Contents:
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