SURGICAL TREATMENT FOR VALVULAR HEART DISEASE 1 Susan Raaymakers, MPAS, PA-C, RDCS (AE)(PE) Grand Valley State University, Grand Rapids, Michigan raaymasu@gvsu.edu BACKGROUND Review Rheumatic heart dz originates as throat infection from streptococcal infection Article in January 2009 JASE Rheumatic heart disease was the leading cause of death 100 years ago in people aged 5-20 years in the United States Incidence just above 0% in developed countries Chronic rheumatic heart disease is estimated to exist in 5-30 million children and young adults; 90,000 patients die from this disease each year. The mortality rate from this disease remains 110% Occurs generally in children 5-15 years old but may present in adult http://www.emedicine.com/ped/topic2007.htm 2 INITIAL SURGICAL TREATMENTS First successful attempt at surgical treatment Incising the left atrial appendage, placing finger through the incision into the left atrium, feeling the stenotic mitral and relieving the obstruction by simple finger pressure. 3 INITIAL SURGICAL TREATMENTS In the early days of cardiovascular surgery, procedures were done on the beating heart 1950’s cardiac and pulmonary bypass machines were developed This development made it possible to keep the patient alive while stopping the heart for surgical repair Ability to stop the heart allowed examination of valve pathology and repair Stimulated surgeons’ collaboration with mechanical engineers in developing prosthetic valves Erector set heart pump, 1950 Using a toy Erector set, William Sewell Jr. and William W. L. Glenn, Yale University medical students, built this section of a heart pump, which Sewell successfully used in experimental bypass surgery on dogs. Acquired in 1959 from Sewell's mother, this heart pump is one of many invention prototypes in the Smithsonian collections 4 INDICATIONS FOR SURGICAL REPAIR/REPLACEMENT OF VALVES 5 FIRST GENERATION OF SYNTHETIC VALVES Era of valve surgery proceeded the development of echocardiography by only a few years. 1960s One of the earliest applications of echocardiography was the evaluation of prosthetic valves. The first generation of synthetic valves retained in a cage Free-floating balls (mechanical ball and cage) or Disc occluders (caged disk) 6 INDICATIONS Valvular stenosis Valvular regurgitation Native valve endocarditis Aortic dissection with severe aortic regurgitation 7 VALVULAR REPLACEMENT 8 9 THREE TYPES OF PROSTHETIC HEART VALVES 10 THREE TYPES OF PROSTHETIC HEART VALVES Mechanical Bioprosthetic Homograft 11 MECHANICAL VALVES All mechanical valves have A Sewing ring Moving component Cage, strut or frame. Made from a compressed carbon material • • Hard enough and yet free of significant friction to provide long term durability Providing relative freedom from wear, breakage or excessive clotting. MECHANICAL VALVES TYPES Ball and Cage Caged Disc Tilting Disc Bileaflet Valved Conduit 13 MECHANICAL VALVE BALL AND CAGE 14 MECHANICAL VALVES BALL AND CAGE Starr-Edwards (used in the first clinically successful valve replacement) Was most Common Smeloff-Cutter Braunwald-Cutter Magovern-Surgitool Magovern-Cromie Harken DeBakey-Surgitool Hufnagel 15 MECHANICAL BALL AND CAGE To open, the ball moves forward into the cage, allowing blood flow around the entire circumference. To occlude, the ball is driven back into the sewing ring to prevent backflow. Hufnagel 1952 Smeloff-Cutter Starr-Edwards in Mitral 16 Position – introduced 1961 STARR-EDWARDS VALVE IN MITRAL POSITION •poppit •moving forward and backward in the cage. •Diastole, •poppet moves forward allowing blood to flow around the occluder. •These valves are highly echogenic, and small thrombi or vegetations can be easily hidden or overlooked. 17 14.5 Feigenbaum FLOW PROFILE BALL AND CAGE Open position Blood flows across sewing ring and around the ball occluder on all sides In Colorflow, observed as bilateral horns. Closed position Small amount of regurgitation: circumferentially around the ball as it seats in the sewing ring 18 M-MODE STARR-EDWARDS FROM APEX 19 SHORT-COMINGS OF BALL AND CAGE 1. Bulky in design and did not fit well into a small ventricle or aorta 2. Small internal orifice, making them relatively stenotic 3. Stimulated thrombus formation, which precipitated thromboembolic events, necessitating long-term anti-coagulation therapy 20 MECHANICAL VALVE CAGED DISC 21 MECHANICAL VALVE - CAGED DISC NO LONGER IN USE Beall-Surgitool Was the most common Kay-Shiley Kay-Suzuki Cooley-Cutter Cross-Jones 22 MECHANICAL VALVE - CAGED DISC Disc elevated by very slight pressure to Movable disc (discoid) demonstrate closure Beall-Surgitool Cooley-Cutter 23 MECHANICAL VALVE - CAGED DISC Advantage over ball and cage Caged disc occupied less area Disadvantage Similar to ball and cage Tissue overgrowth Chipping of the disc due to constant impact Mechanical problems 24 MECHANICAL VALVE SINGLE TILTING DISC 25 MECHANICAL VALVE SINGLE TILTING DISC Most common Medtronic-Hall Björk-Shiley No longer available in U.S. due to the problem of strut fracture Other tilting discs Lillehei-Kaster Hall-Kaster Wada-Cutter Omniscience Omnicarbon Van der Spuy "toilet seat" valve Blood tended to clot at the spring pivot of this 26 valve. http://www.hhmi.org/biointeractive/museum/exhibit98/content/h12info.html MECHANICAL VALVE SINGLE TILTING DISC •Single disk prosthesis •Round sewing ring and a circular disk fixed eccentrically to the ring via a hinge. • Disk moves through an arc of less than 90º allows: •Antegrade flow in the open position •Seating within the sewing ring to prevent backflow in the closed position. Björk-Shiley (1971 First Successful Tilting Disk) Medtronic-Hall Pivoting 27 Disc Valve FLOW PROFILE – SINGLE TILTING DISK Open position Two orifices of unequal size (major vs. minor) Asymmetric flow profile as blood accelerates along the tiled surface of the open disk Subtle variations dependent on shape of disk (concave vs. convex) and sewing ring design Closed position Small central jet of regurgitation occurs around the central hole 28 COMPLICATIONS OF SINGLE TILTING DISCS Björk-Shiley had issues with strut fractures 619 of the 80,000 convexo-concave valves implanted fractured with patient death in 2/3 of cases FDA removed from market in 1986 Perhaps the most infamous recall case on record Hinge is eccentrically positioned within the sewing ring and the disk opens less than 90 degrees. Major and minor orifices are created and some stagnation of flow occurs behind the disk. 29 BJORK-SHILEY 30 SINGLE TILTING DISCS Advantage Low profile, can be inserted into aortic and mitral positions Disadvantage High degree of leakage around central strut Region of stagnation behind disc Thrombus formation Tissue overgrowth 31 MECHANICAL VALVE BILEAFLET 32 MECHANICAL VALVE - BILEAFLET St. Jude Most frequently used mechanic valve Three orifices, which promote central flow Least stenotic mechanical prosthetic valve Carbomedics Duromedics (Hemex) Gott-Daggett 33 Non-dynamic MECHANICAL VALVE - BILEAFLET • Opening angle is generally more vertical (approx 80º) than single disk prosthesis • Results in three distinct orifices: • Two larger ones on either side and a smaller central rectangular-shaped orifice. St. Jude hyperlink Carbomedics 34 ST. JUDE MITRAL PROSTHESIS 35 BI-LEAFLET MECHANIC VALVE Colorflow profile Single large flow pattern or Two major jets on sides and one minor in middle. 36 FLOW PROFILE - BILEAFLET Complex Open fluid dynamics position Two large lateral valve orifices with a small narrow central “slitlike” orifice Three peak velocities corresponding to three orifices Highest velocity in middle orifice Local gradients are often substantially higher than overall valvular pressure Closed position Two crisscross jets of regurgitation are seen in plane parallel to the leaflet opening plane 37 OVERALL COMPLICATIONS OF MECHANICAL VALVES 38 COMPLICATIONS OF MECHANICAL VALVES Thrombus Indefinite anti-coagulation Stenosis Thrombosis Pannus ingrowth Fibrotic tissue which grows around a newly implanted prosthetic heart valve. Vigorous growth of this healing tissue can freeze or obstruct a replacement valve. May be related, in part, to the design or materials of the prosthesis, or to the degree of anticoagulation Dehiscensce Infective endocarditis Hemolysis 39 COMPLICATIONS OF MECHANICAL VALVES CONTINUED Mechanical failure Ball/disc/cage variance/strut fracture Heart-valve mismatch Left ventricular outflow tract obstruction Valve bed abnormality Pseudoaneurysm, valve ring abscess, fistula, hematoma Regurgitation Central, perivalvular 40 BIOPROSTHETIC VALVES 41 Constructed from either human or animal tissue. BIOPROSTHETICVALVES Heterograft (xenograft) Transfer from animal to human Longest replaced approximately 10 years Typically replaced at 5 years Auto-graft Transfer from self to self Homograft (allograft) Transfer from one human to another Last approximately 5 years 42 HETEROGRAFT 43 Transfer from Animal to Human HETEROGRAFT (XENOGRAFT) PORCINE VALVES Limited availability of heterograft prompted the use of porcine valves procured from slaughterhouses Pig’s aortic valve is placed on stents, attached to a sewing ring and glutaraldehyde stabilized Most common: Hancock I and II Carpentier-Edwards Intact (aortic) Hancock Porcine – Valve Closed 44 NORMAL FUNCTIONING PORCINE AORTIC PROSTHESIS •Leaflet opening during systole resembles that of a normal native valve. •Overall appearance is similar that bioprosthesis •Occasionally mistaken for native when historical information is not available. • Careful observation yields an echogenic sewing ring and struts. 45 NORMAL FUNCTIONING PORCINE MITRAL PROSTHESIS 46 HETEROGRAFT (XENOGRAFT) STENTLESS PORCINE VALVE A low-pressure glutaraldehyde fixed intact porcine valve supported by Dacron cloth Advantage: No stents allows larger valve to be implanted Two approved valves: Toronto SPV Freestyle Valve Toronto SPV 47 STENTLESS AORTIC VALVE Stentless Aortic Valve 48 SAME PATIENT - PSAX 49 HETEROGRAFT (XENOGRAFT) BOVINE PERICARDIUM Bovine (cow) pericardium fashioned into a trileaflet valve Mounted on stents and a sewing ring Most common brands: Carpentier-Edwards IonescuShiley (Withdrawn from United States Market) Mitroflow Carpentier Edwards – Valve Closed 50 AUTOGRAFT 51 Transfer from Self to Self AUTOGRAFT ROSS PROCEDURE Excision of the aortic valve Placement of the pulmonary valve annulus and trunk into the aortic position Reimplantation of the coronary arteries. Pulmonary side, a homograft conduit is placed between the right ventricle and pulmonary artery 52 HOMOGRAFT 53 Transfer from One Human to Another HOMOGRAFT (ALLOGRAFT) Rarely used to replace a MV or TV Aortic Homograft Harvested from human cadavers shortly after death, (cryopreseved) May be sown into the aortic annulus without stents. Customized by the surgeon in the operating room at the time of implantation. May be difficult to identify by echocardiography Aortic root may appear thicker than normal Valve failure is usually due to valvular regurgitation 54 ADVANTAGE OF BIOPROSTHETIC VALVES May avoid anticoagulation Lower pressure gradients Central flow dynamics Failure usually occurs slowly Valve of choice in the tricuspid/pulmonic position Stentless valve may be hemodynamically superior to stented heterograft Increased effect orifice area Lower gradients Greater regression of ventricular hypertrophy 55 COMPLICATIONS OF BIOPROSTHETIC VALVES Calcification/degeneration Infective endocarditis Vegetation, valve ring abscess, fistula Dehiscence (all valve replacements) Sewing ring around prosthesis becomes unsecured to surrounding structures Inherently stenotic Tissue preserved and fixed with within a prolypropylene mount attached to a Dacron sewing ring Less pliable than native valve tissue. 56 COMPLICATIONS OF BIOPROSTHETIC VALVES – CONTINUED Stenosis Degeneration, thrombotic Sewing ring may be too small relative to the flow In young patients, what was normal as a child is now too small as an adult Effective orifice area is significantly smaller than the area of the sewing ring Valve assembly (i.e. occluder mechanism) occupies some of the central space 57 COMPLICATIONS OF BIOPROSTHETIC VALVES - CONTINUED Deterioration of tissue valve Occurs at an accelerated rate Older patients, especially in those with a risk of falling, Younger patients Patients with end-stage renal disease on hemodialysis. Tissue valve may be the most appropriate choice. Tissue valves are less durable than mechanical valves with a reported failure rate of 25% at 10 years 42% at 12 years 60% at 15 years 58 The failure rate is higher in young patients (less than 35 years of age) and in chronic renal failure patients VALVED CONDUITS 59 VALVED CONDUITS Used in congenital heart surgery and ascending aortic repairs When a new passageway for blood flow and a valve are needed May be biologic (i.e. homograph) or artificial (i.e. Gore-Tex or Dacron) material May incorporate either tissue or a mechanical valves Fluid dynamics similar to those for a valve implanted in the native annulus 60 CARPENTIER-EDWARDS BIOPROSTHETIC VALVED CONDUIT 61 OTHER CONDUITS 62 EVALUATION OF PROSTHETIC VALVES BY TRANSTHORACIC ECHOCARDIOGRAPHY 63 EVALUATION OF PROSTHETIC VALVES BY TRANSTHORACIC ECHOCARDIOGRAPHY Confirm stability of the sewing ring Determine the specific type of prosthesis Confirm the opening and closing motion of the occluding mechanism Can be difficult but with careful interrogation the rapid motion of the leading edge of the disk or ball generally can be recorded. Evaluate for gross structural abnormalities such as vegetations and thrombi 64 TEE EVALUATION OF PROSTHETIC VALVES 65 EVALUATION BY TEE GENERAL QUESTIONS THAT SHOULD BE ANSWERED Is there valve dehiscence? Is there evidence of torn/flail leaflets, ball/disc variance? Are there mass lesions? Vegetations, thrombi, pannus Is there valve ring abscess / pseudoaneurysm/ fistula? How much volume/leakage volume/ pathological valvular regurgitation / paravalvular leak is present? Is there valvular stenosis? 66 TEE EVALUATION OF PROSTHETIC VALVES Helpful in patients who are too unstable to undergo cardiac catheterizations Surface study is inadequate for diagnosis Regurgitation jets appear larger as compared to transthoracic 67 Non dynamic PRESSURE RECOVERY 68 PRESSURE RECOVERY Downstream pressure after an obstruction After flow passes through orifice Will be lower than the upstream pressure before Pressure recovers toward its original value Rate and magnitude: variable depending on valvular geometry Difference between cardiac catheterization and echocardiography pressure gradients 69 ROUTINE EVALUATION OF PROSTHETIC VALVES 70 ROUTINE EVALUATION OF PROSTHETIC VALVES Chamber dimension and function Valve type and movement Peak flow velocity Maximum and mean pressure gradients Pressure half time Generally overestimates valve area in presence of mitral prosthesis 71 ROUTINE EVALUATION OF PROSTHETIC VALVES Effective orifice area by continuity equation Pulmonary artery pressures Diastolic filling profile Color flow jet length, duration and area, pulmonary vein (mitral regurgitation) Color flow jet or descending thoracic aorta flow (aortic regurgitation) 72 GENERAL M-MODE/ 2-D/ CARDIAC DOPPLER FINDINGS POST-PROSTHETIC VALVE SURGERY 73 POST-PROSTHETIC VALVE SURGERY 14.23 Feigenbaum 74 GENERAL M-MODE/ 2-D/ CARDIAC DOPPLER FINDINGS POST-PROSTHETIC VALVE SURGERY Aortic Stenosis Left ventricular systolic/diastolic function Left ventricular hypertrophy Will be reduced compared to pre-op but a residual peak and mean gradient will be present due to aortic valve replacement If mitral regurgitation was present before surgery, Should regress Peak/mean gradient Should improve is decreased preoperatively May be decreased in severity post-op Left ventricular intracavitary systolic gradients May predict a poor prognosis 75 GENERAL M-MODE/ 2-D/ CARDIAC DOPPLER FINDINGS POST-PROSTHETIC VALVE SURGERY Aortic Regurgitation Left ventricular dimensions Should decrease with an improvement of ventricular systolic function Peak/mean gradients Will be increased for prosthetic heart valve compared to native aortic valve 76 GENERAL M-MODE/ 2-D/ CARDIAC DOPPLER FINDINGS POSTPROSTHETIC VALVE SURGERY Mitral Stenosis May be a slight increase in Left atrial dimension Will be reduced compared to pre-op Mitral valve area May be left intact Peak/mean gradients May be obliterated at surgery Valve leaflets, chordae tendineae, papillary muscles May decrease slightly but usually will not normalize Left atrial appendage Left ventricular dimensions Larger than pre-op Pulmonary artery pressures May decrease 77 GENERAL M-MODE/ 2-D/ CARDIAC DOPPLER FINDINGS POST-PROSTHETIC VALVE SURGERY Mitral Regurgitation LV dimension LA dimension May be left intact Decreased compared to pre-op with mitral valve replacement Should decrease but will not normalize Valve leaflets, chordae tendineae, papillary muscles Should decrease with an improvement in systolic function Transmitral peak velocity, peak pressure gradient, mean pressure gradient Pulmonary artery pressures may decrease 78 NORMAL OR PHYSIOLOGICAL REGURGITATION 79 NORMAL OR PHYSIOLOGIC REGURGITATION Regurgitation occurs in Virtually all types of mechanical prostheses Seating regurgitation or "closure backflow" appears only briefly Due to retrograde volume displacement as the valve leaflets close. Divided into two types Closure backflow Leakage 80 COMPLICATIONS 81 AORTIC ROOT ABSCESS Echo-free space is seen posterior to the aortic root and associated perivalvular regurgitation. 14.27b Feigenbaum 82 14.27 Feigenbaum PERIVALVULAR LEAK Example of stentless aortic prosthetic valve Mild degree of perivalvular regurgitation is seen. 14.28a Feigenbaum 83 OBSTRUCTION The most common cause of prosthesis obstruction is the presence of a thrombus. 84 THROMBUS ECHOCARDIOGRAM Large thrombus Left atrial aspect of the mitral prosthesis 14.37 Feigenbaum 85 VEGETATION Prosthetic valve Most common site for attachment of a vegetation is the sewing ring. Large vegetation can be seen in the left atrium Attached to the sewing ring of a St. Jude mitral prosthesis. 14.46 Feigenbaum 86 RING ABSCESS 14.51b Feigenbaum 87 14.51c Feigenbaum DEHISCENCE Dehiscence of porcine mitral prosthesis Excessive motion of the prosthetic valve was evident. Abnormally high peak flow velocity (2.8 cm/sec) Increased gradient (14 mm Hg) 88 14.52 Feigenbaum VALVED CONDUITS 89 VALVED CONDUITS Part of repair of some forms of complex congenital heart disease Not all conduits contain valves and those that do may use either bioprosthetics or mechanical prostheses. Conduit itself often has a characteristic echocardiographic appearance due to the conduit material and the ribbed design 90 REPAIR Adult patients with aortic valve pathology are seldom candidates for valve repair. Valve replacement is usually necessary for significant aortic stenosis or regurgitation. 91 MITRAL VALVE REPAIR 92 MITRAL VALVE REPAIR Repairing rather than replacing Several advantages and is being performed with increasing frequency. Mitral and tricuspid valve pathologies should be considered for valve repair Operative mortality associated with repair of these valves is lower than that associated with their replacement. 93 MITRAL VALVE REPAIR Selection of repair vs. replace is dependent upon Etiology, morphology and severity as well as the status of the left ventricle. Replacement for :severe scarring and deformation by a disease process such as advanced rheumatic heart disease advanced lupus another inflammatory process 94 MITRAL VALVE REPAIR SUCCESS RATE IN PATIENTS WITH MYXOMATOUS DEGENERATION AND MITRAL VALVE PROLAPSE Posterior Carries repair leaflet prolapse a greater likelihood of successful Than anterior or bi-leaflet prolapse 95 http://www.escardio.org/communities/EAE/CasePortal/Pages/Case159.aspx 96 SOME DEGREE OF REGURGITATION MAY REMAIN AFTER REPAIR Stable mitral ring in mitral position Well preserved leaflet excursion 14.59 Feigenbaum 97 14.60a Feigenbaum FUTURE OF VALVULAR REPLACEMENT? 98 99 SAFETY AND EFFICACY STUDY OF THE MEDTRONIC COREVALVE® SYSTEM IN THE TREATMENT OF SYMPTOMATIC SEVERE AORTIC STENOSIS IN HIGH RISK AND VERY HIGH RISK SUBJECTS WHO NEED AORTIC VALVE REPLACEMENT Clinical Trial for transcatheter aortic valve implantation (TAVI) >1,300 patients Subjects have one of two options: 1. 2. Open heart surgical aortic valve replacement Transcatheter aortic valve implantation (only available through the clinical trial) 100 CLINICAL TRIAL FOR TRANSCATHETER AORTIC VALVE IMPLANTATION (TAVI) 45 Sites Across the US. Trial Locations in Michigan Detroit Medical Center Spectrum Health Hospitals University of Michigan Health Systems 101 COREVALVES Inclusion criteria Predicted risk of operative mortality ≥15% Senile degenerative aortic valve stenosis Mean > 40 mmHg/left velocity >4.0 m/sec AND Initial AVA ≤0.8 cm2 (or AVA index ≤0.5 cm2/m2) Symptomatic; NYHC Functional Class II or greater Subject informed of the nature of the trial, agrees and has provided written informed consent as approved by IRB of the respective clinical site Subject and treating physician agree that the subject will return for all post-procedure follow-up visits 102 COREVALVES Exclusion Criteria Evidence of an acute myocardial infarction ≤ 30 days before the index procedure. Any percutaneous coronary or peripheral interventional procedure performed within 30 days prior to the index procedure. Blood dyscrasias as defined: leukopenia (WBC < 1000mm3), thrombocytopenia (platelet count <50,000 cells/mm3), history of bleeding diathesis or coagulopathy, or hypercoagulable states. Untreated clinically significant coronary artery disease requiring revascularization. Cardiogenic shock manifested by low cardiac output, vasopressor dependence, or mechanical hemodynamic support. Need for emergency surgery for any reason. Severe ventricular dysfunction with left ventricular ejection fraction (LVEF) < 20% as measured by resting echocardiogram. Recent (within 6 months) cerebrovascular accident (CVA) or transient ischemic attack (TIA). 103 COREVALVES Exclusion Criteria End stage renal disease requiring chronic dialysis or creatinine clearance < 20 cc/min. Active Gastrointestinal (GI) bleeding within the past 3 months. A known hypersensitivity or contraindication to any of the following which cannot be adequately pre-medicated: aspirin Heparin (HIT/HITTS) and bivalirudin (only for Extreme Risk patients) nitinol (titanium or nickel alloy) ticlopidine and clopidogrel contrast media 104 COREVALVES Exclusion Criteria Ongoing sepsis, including active endocarditis. Subject refuses a blood transfusion. Life expectancy < 12 months due to associated non-cardiac co-morbid conditions. Other medical, social, or psychological conditions that in the opinion of an Investigator precludes the subject from appropriate consent. Severe dementia (resulting in either inability to provide informed consent for the trial/procedure, prevents independent lifestyle outside of a chronic care facility, or will fundamentally complicate rehabilitation from the procedure or compliance with follow-up visits). Currently participating in an investigational drug or another device trial. Symptomatic carotid or vertebral artery disease. Additional Exclusion for High Risk Surgical only: Subject has been offered surgical aortic valve replacement but declined. Anatomical 105 COREVALVES (TAVI) Exclusion Criteria Native aortic annulus size < 20 mm or > 29 mm per the baseline diagnostic imaging. Pre-existing prosthetic heart valve any position. Mixed aortic valve disease (aortic stenosis and aortic regurgitation with predominant aortic regurgitation (3-4+). Moderate to severe (3-4+) or severe (4+) mitral or severe (4+) tricuspid regurgitation. Moderate to severe mitral stenosis. Hypertrophic obstructive cardiomyopathy. Echocardiographic evidence of intracardiac mass, thrombus or vegetation. Severe basal septal hypertrophy with an outflow gradient. Aortic root angulation (angle between plane of aortic valve annulus and horizontal plane/vertebrae) > 70° (for femoral and left subclavian/axillary access) and > 30° (for 106 right subclavian/axillary access). COREVALVES Ascending aorta diameter > 43 mm unless the aortic annulus is 20-23 mm in which case the ascending aorta diameter > 40 mm. Congenital bicuspid or unicuspid valve verified by echocardiography. Sinus of valsalva anatomy that would prevent adequate coronary perfusion. Vascular Transarterial access not able to accommodate an 18Fr sheath. 107 SOURCES CoreValve U.S. Pivotal Trial. Medtronic. [Online] 2010. [Cited: February 20, 2012.] http://www.medtronic.com/corevalve/ous/system.htm. Feigenbaum H, Armstrong W. (2004). Echocardiography. (6th Edition). Indianapolis. Lippincott Williams & Wilkins. Goldstein S., Harry M., Carney D., Dempsey A., Ehler D., Geiser E., Gillam L., Kraft C., Rigling R., McCallister B., Sisk E., Waggoner A., Witt S., Gresser C.. (2005). Outline of Sonographer Core Curriculum in Echocardiography. Kardon, Eric. Prosthetic Heart Valves. Medscape Reference. [Online] February 08, 2010. 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