Concordia University



The implementation of percutaneous heart valveMickel Abd El MalekAbstract: Transcatheter aortic valve replacement is now a viable option in the treatment of high-risk severe symptoms for aortic stenosis. Although heart valve replacement is among the most common cardiovascular surgical procedures, the outcome is difficult to assess. These valves have been shown to be technically implantable, even though they have a high rate of morbidity and mortality. In the valvular market transcatheter procedure is presenting a short term potential for those who would have been rejected for the replacement. The ideal stent is inexpensive to manufacture, and easy to install. They are made of memory allow which is sufficiently rigid for support. Some stents also could dispense therapeutic agents, which reduces thrombosis formation, also the material used does not release harmful chemical in the body. Nowadays the technological advancement is very rapid which allows the manufacturing of a biocompatible stent which would make the perfect stent. 1. IntroductionScience has reached a crucial point which allowed them to create artificial organs. This may seem easy to understand; however, the knowledge required is vast and needed many years of trial and errors. For a long time doctors and researchers were baffled by the physiology of the heart and how it functions. The complexity and importance of the heart prevented doctors to think that this delicate organ can withstand the rigors of surgery.The diseased heart valve continues to be an important health care issue in our society. Although surgical valve replacement remains the standard treatment, a new less invasive procedure is gaining popularity among surgeons. Patients also are more attracted to percutaneous valve replacement because of the reduced recovery time, wider age spectrum of patient that can endure the procedure. Elderly patient cannot withstand the stress of open heart surgeries even younger patients at the early stage of valve disease, which cannot be treated until reaching a certain age to reduce the multiple surgeries.The field of interventional cardiology is one of the most rapidly evolving and vibrant areas in today's medicine. The percutaneous valve replacement is still at early stage of development which needs improved implantation procedure as well as design. Since it’s a new procedure a long term follow up regime is needed to assess the effectiveness of the treatment modality.2. Evolution of Treatment Options for Valvular Heart DiseaseThe occurrence of valvular heart disease is expected to increase over the next several decades as the "baby boomer" passes into advanced decades of life. Treatments of heart disease especially defective valves are expected to represent the core for maintaining health in this aging population. In the face of this challenge, the development in technological and scientific understanding of the anatomy and pathophysiology of diseased heart valves are offering an increasing array of interventional and surgical treatments of valvular heart disease. This development is to be able to meet the demand of these aging baby boomers. All current surgical approaches to valve replacement require cardiopulmonary bypass with or without induced heart stoppage. The start of many operations performed on the heart like cardioplegia delivery and aortic clamping allowed the development of the traditional thoracotomy incisions scenario to treat valvular heart surgery in the same manner with a small incision [1]. In the past, high-risk and inoperable patients were offered balloon aortic valvuloplasty. However this procedure does not alter the inevitable aortic stenosis nor provide improvement of survival rate of patients it’s merely a soothing option. Meanwhile we are in the era of transcatheter aortic implantation which established feasibility in 2002 [2].Cardiac surgery is heading towards a new drift in the future towards minimal invasive procedure to patients with trauma. In spite of the small intrusion of the procedure, patients are faced with great risks inflicted by the new trend of this operation. The emergence of the new breed of surgery resulted in percutaneous valve replacement. This technique was developed having in mind the cosmetic scarring for patients comfort, overall low cost, and without general anesthesia. Most of the time anesthesia induces drawbacks and many patients have allergies that could produce complication [3]. Evolution of Percutaneous Valve Replacement Investigators/YearExperimental SubjectsStent SystemDelivery TechniqueMajor ComplicationsAndersen et al 1992 [4]Short-term study in pigs.Porcine aortic valve mounted into an expandable stentRetrograde delivery through the suprarenal aorta into the ascending aorta for sub- and supracoronary implantationRestriction of coronary flow and trivial regurgitationPavcnik et al 1992 [5]Short-term study in dogs.Self-expanding caged-ball aortic valveRetrograde delivery through the carotid into the ascending aorta for subcoronary implantation.Significant aortic regurgitation. System dis-assembly with escaping of the ball out of cage.Bonhoeffer et al 2000 [6]Long-term study in lambs.Bovine jugular vein valves sutured into an expandable stentAntegrade delivery through the jugular vein into the native pulmonary valve position.Pannus formation of valve leaflets.Lutter et al 2001/2002 [7,8]Short-term study in pigs.Porcine aortic valves or pericardium sutured into a self-expanding stentRetrograde delivery through iliac artery or infrarenal aorta for descending or ascending implantation.Restriction of coronary flow. Twisting of stents leading to high grade regurgitation. Rhythm disturbances.Boudjemline et al 2002 [9]Short term study in lambs.Bovine jugular vein valves sutured into an expandable stentRetrograde delivery through the carotid into the aortic valve annulus, trapping native leaflets between the outer nitinol stent and the inner platinum stent.Obstruction of coronary orifices, mitral valve insufficiency, and premature stent migration.Bonhoeffer et al 2002 [10]Seven children and one adult with significant PR and/or RV outflow mercial biological valve sutured into a platinum stentAntegrade delivery through femoral vein into the pulmonary position.Possible valve degeneration.Cribier et al 2002 [11]57 year old man with calcific aortic stenosis and no other treatment mercially available bovine pericardial leaflets mounted within a balloon expandable stentAntegrade transseptal approach with implantation into the diseased native aortic valveMild paravalvular regurgitation. Non-cardiac complications leading to patient's death 17 weeks after valve implantation.Cribier et al. 2004 [12]Six patients with calcific aortic stenosisCommercially available bovine pericardial leaflets mounted within balloon expandable stentAntegrade transseptal approach with implantation within the diseased native aortic valve.Early migration with subsequent death in one patient. Mild paravalvular regurgitation.3. Valve history Technique:The Reduction in compliance of the valves often due to the deposit of calcium, this is called calcification which leads to aortic stenosis, which targets mostly elders. The application of the valve in a percutaneous manner is very challenging due to numerous reasons. The anatomy of the human body is very complex to gain access though vascular routing, also positioning the valve in the aortic annulus needs trained hands to ensure proper positioning. The following techniques were proposed some of them were discarded and others are more appealing for improvement. 3.1 The antegrade technique: This technique has been studied but was discarded which involved the access thought the femoral vein with a catheter pushing the valve through the interatrial septum and the mitral valve finally positioned in the diseased aortic valve [2,11,12]. This procedure was thought to have an advantage due to the large size of the femoral vein which could accommodate the large catheter sheath and the access site can be managed by manual compression. However, one or more disadvantages were in question. Firstly this technique has a great potential to damage the mitral valve resulting in mitral regurgitation. Secondly, the challenge of delivering the aortic valve to the right position [26]. Weighing the setbacks this practice was crossed.3.2 The retrograde techniqueIn order to access the femoral artery and advance the catheter with the prosthetic aortic valve to the stenotic aortic valve, the retrograde technique is used .implementing this technique would be easier and faster than the antegrade technique. The down side of the retrograde technique is causing an injury to the aortofemoral vessels and failing to cross the aortic arch or the diseased valve. [11, 17]3.3The transapical techniqueIn order to position the valve in the stenotic aortic valve, the transapical technique is used.To successfully implement this technique, a small incision is made between the ribs, then the apex of the left ventricle is punctured with a needle and at the same time the valve delivery system is inserted. This technique allows surgeons to have a more direct access to the aortic. It allows patients with peripheral vascular diseases to undergo surgeries with smaller access sites and minimal complications. [20, 21]Common features of the three approaches The three percutaneous approaches have certain final steps in common. The patient’s unique valvular structure and anatomy help surgeons determine the final position for deploying the prosthetic valve.For optimal results, surgeons use fluoroscopic imaging of the native aortic valve with the aid of supra-aortic angiography as well as the transesophageal echocardiography. Making sure not to invade the coronary ostium and not to obstruct the motion of the leaflet, the aortic valve prosthesis is then placed at mid-position in the patient’s aortic valve. In all three techniques, the prosthesis is then positioned by inflating the delivery balloon to the maximum, quickly deflating it and finally retracting it. This final step is taking place during the temporary high-rate of the right ventricular contraction. As a result, it creates a ventricular tachycardia at 180 to 220 beats per minutes and it could last up to 10 seconds. This leads to an immediate decrease in stroke volume, resulting in minimal forward flow through the aortic valve, which in turn facilitates precise positioning of the prosthetic valve. It has been determined that the only common valve used in all three techniques is Cribier-Edwards valve [2, 11, 12,]Figure 14. Device description After the implantation of the heart valve the stent promising results of anchoring to the aortic root. In pulsating flow of the systolic phase of the left ventricle the stents arms prevented the movement of the stent towards the flow in the aorta. In addition to that the stent flexibility mimics the expansion of the aorta (dicrotic node). Similarly, in the diastolic phase the stent was very efficient diminishing axial translation due to the conical basis. Under cyclic loading the cylindrical head managed to changed diameter to ensure contact even at higher pressure nevertheless, tilting was also prevented. On top of these meticulous considerations of the stent the anchorage remained intact at higher pressures with minimal static pressure leakage. fig.14.1 Stent DesignIn order to position properly the stent, the geometry of the native aortic root is to be taken into consideration. The diameter of a human aorta varies from 15mm to 25 mm depending on the age, sex and built of the patient. Many stent manufacturers have detailed tables relating allometric guidance for optimal stent size with respect to weight of patient. Two important parts make the stent the links and rings. Each has a different role links are mainly for flexibility, while the rings are to support radial forces. The unit consisted of 8 lips with two non-crossing struts making a circular diameter of 16mm. Two struts join to form a lip creating a diamond shaped port. The port angle is 45 degrees and the distance between the two struts at the center are 6.25mm, with 8 lips joining at the center to form the circular stent. The stent design has a constant strut thickness of 0.5 mm and a height of 18 mm. (Fig1)4.2 Stent MaterialMost stents are crafted from 316L stainless steel. However, many disadvantages arose from this specific material. Many patients suffered from thrombosis, and restenosis on and around the stent. Other complications were also present, for instance; bleeding, corrosion, and dilation of stented vessel. The ideal material for designing a stent should be chemically inert. Biocompatible material must not react with human cells in fact if the material reacts this will induce inflammations. Nevertheless, the material must withstand the retracting forces exerted by the targeted vessel.The prime candidate from the wide range of alloys was Nitinol in comparison to stainless steel. Nitinol is super elastic memory alloy which is biocompatible that can return to a specific shape when heated at post phase transition temperature. The composition of the Nitinol is a mix between 55% nickel and 45% titanium [13].The low interaction between the two components makes it an inert material; however, strongly interconnected by a metallic bond. This bond reduces corrosion and immunity response by reducing the platelets activation. [13,14]4.3 Stent configurationsStents components have two essential configurations. One of them is the self-expanding stents and the other one is the balloon expandable stents. There is a newly developed configuration, which has some characteristics of both self-expandable and balloon expandable stents. A self – expanding stent is within the catheter and has the same function as a spring upon insertion in a contracted body passageway. The self-expanding stent undergoes two successive thermal treatments. The first treatment helps fixing wire curvatures. The Final treatment fixes the desired 3D shape. The balloon expandable stents is mounted over a balloon and attached to the distal end of a catheter. After being guided by the catheter to the stenosis position, the balloon is inflated and expands the stent in a radial outward position. Once the stent is correctly expanded, the balloon is then deflated and withdrawn from the patient [15]5. Complications Many complication rises during the manipulating catheter. The greatest is arterial dissection or perforation that could occurs creating more damage than repair.[17, 22]Patients with aortic stenosis are in danger of myocardial ischemia and cardiogenic shock [23].Other concerns involve obstruction of the coronary arteries with the native or implemented valve.Valve in valve also reduces the flow since the area present for the flow is reduced but might not produce major effect [17, 20]. This reduction in the orifice requires further studies and improvement to be able to meet same requirement are surgical procedure [18].The reduction of the area also produces small vortices that could lead to haemolysis of the red blood cell, many test were performed to try to diminish these circulation zones [23].Some results were derived from many patients who showed 3% to 9% of patients reported strokes after the implementation [17, 25, 26]. After implantation patient are required for frequent follow-up up to 2 year however, in vivo valves failure might occur currently none has been reported [19,20].Hopefully these risks will diminish as the procedure improves and is applied to patients with less morbidity.6. ConclusionMost patients with aortic stenosis are candidates for Percutaneous Aortic Valve Replacement. Nevertheless, conventional valve surgery remains the proven therapy of choice for the majority of patients with symptomatic aortic stenosis, at this time. Percutaneous aortic valve replacement offers the potential for significant benefit but is not without risk. Optimal outcomes will require cautious application, technological and procedural improvements, formal training, and centers of expertise. Also some further trials, and ongoing surveillance during follow-up is required. Currently this procedure can be considered for symptomatic patients who are poor candidates for conventional surgery. A more mature procedure might soon offer a viable alternative to a much broader range of patients.References1. Sharony R, Grossi EA, Saunders PC, et al. Minimally invasive aortic valve surgery in the elderly: a case-control study Circulation 2003;108(Suppl):II43-7. 2. Cribier A, Eltchaninoff H, Bash A, Borenstein N, Tron C, Bauer F, Derumeaux G, Anselme F, Laborde F, Leon MB. Percutaneous transcatheter implantation of an aortic valve prosthesis for calcific aortic stenosis: first human case description. Circulation. 2002;106:3006–3008. 3. 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