Abstract Body (Do not enter title and authors here): A 71-year-old female with a history of severe aortic stenosis status post aortic valve replacement with a 23mm bioprosthetic valve and aortic root enlargement with bovine pericardial patch 6 years prior, presented with progressive dyspnea and NYHA class III symptoms. Echocardiography revealed preserved left ventricular function and a normally functioning bioprosthetic aortic valve, but severe mitral annular calcification (MAC) with significant mitral stenosis (Mean gradient: 11 mmHg, Mitral valve area: 1.6 cm^2). Cardiac CTA showed severe circumferential MAC with >75% annular involvement and a MAC score of 8. Following multidisciplinary discussion, consensus was reached to proceed with a hybrid open transcatheter mitral valve-in-MAC replacement via right thoracotomy using an atrialized (80% atrial, 20% ventricular) approach to mitigate the risk of LVOT obstruction identified on preprocedural CT planning.

Right femoral cannulation and right anterior thoracotomy were performed. Following the initiation of cardiopulmonary bypass, the pericardium and left atrium were opened. The mitral valve was severely stenotic with extensive calcification involving both leaflets and the annulus, precluding full debridement. Consequently, only a portion of the anterior leaflet was resected to prevent displacement into the left ventricular outflow tract during valve inflation. A balloon-expandable valve was deployed using nominal volume plus 5 mL for optimal expansion and anchoring, aided by atrial sutures and an atrialized cuff. The prosthesis successfully circularized the annulus and was confirmed competent with no evidence of a paravalvular gap or leak.

A hybrid open transcatheter mitral valve-in-MAC procedure offers a viable option for patients unsuitable for conventional surgery or transcatheter approaches. While standard implantation places the valve 2 mm above the annulus (20% atrial, 80% ventricular), we employed an atrialized technique to minimize LVOT obstruction and avoid valve-septal contact. Additional balloon inflation facilitated annular circularization, ensuring full valve expansion and sealing. Meticulous preoperative planning with cardiac CT is essential to guide procedural strategy and optimize outcomes.