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Mid Term Results of Transapical Aortic Valve Replacement via real-time MRI Guidance
*Keith A Horvath, Dumitru Mazilu, Michael Guttman, Ming Li
National Institutes of Health, Bethesda, MD
BACKGROUND:
Percutaneous valve replacements are presently being evaluated in clinical trials. As delivery of the valve is catheter based, the safety and efficacy of these procedures may be influenced by the imaging employed. To assist the operator and improve the success of the operation, we have performed transapical aortic valve replacements (AVR) using real-time MRI guidance (rtMRI).
METHODS:
Twenty eight domestic pigs (50-65 kgs.) were anesthetized and underwent rtMRI AVR on the beating heart without unloading by rapid ventricular pacing or cardiopulmonary bypass This was accomplished using commercially available stentless bioprostheses (21-25mm) mounted on platinum iridium stents. MR imaging (1.5T) was used to precisely identify the anatomic landmarks of the aortic annulus, coronary artery ostia, and the mitral valve leaflets. In addition to anatomic confirmation of adequate placement of the prosthetic valve in relation to the aortic annulus and the coronary arteries, functional assessment of the valve and left ventricle was also obtained with MR imaging. Intraoperative perfusion scanning documented adequacy of myocardial blood flow after valve placement. Phase velocity scans confirmed adequate opening of the prosthetic valve leaflets and lack of valvular or paravalvular regurgitation. Avoidance of injury to the mitral valve or subvalvular apparatus was also confirmed and readily assessed. A series of acute feasibility experiments were conducted (n=18) in which the animals were sacrificed after valve placement and MRI assessment. Ten additional animals were allowed to survive and had follow-up MRI scans and confirmatory echocardiography (2D and 3D) at 1, 3 and 6 months postoperatively. Post-mortem gross and histopathology was also performed.
RESULTS:
rtMRI provided superior visualization of the landmarks needed to implant the aortic valve prostheses compared to fluoroscopy or echocardiography. The time to implantation after the apical access was obtained to deployment of the valve was 74 + 18 seconds. The average procedure duration was less than forty minutes. Perfusion scanning demonstrated adequate coronary flow and functional imaging documented preservation of ventricular contractility in all animals following successful deployment. Phase contrast imaging revealed minimal intra or para-valvular leaks. Longer term results demonstrated stability of the implants with preservation of myocardial perfusion and function over time. Necropsy confirmed the MRI and echo results.
CONCLUSIONS:
rtMRI provides excellent visualization for intraoperative guidance of aortic valve replacement on the beating heart. Additionally it allows assessment of tissue perfusion and organ function that are not obtainable by conventional imaging alone. Expansion of rtMRI guidance to facilitate other types of cardiac surgical procedures should be considered to minimize trauma and enhance patient benefit.
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