**4. Procedural planning**

Successful TMVR depends on accurate sizing of the mitral annulus and avoidance of LVOT obstruction. In the absence of a validated standard method for mitral annulus sizing at the present time, operators have extrapolated from transcatheter aortic valve replacement (TAVR) experience and used a variety of sizing approaches including echocardiography, 3-dimensional (3D) transesophageal echocardiography, cardiac CT, and balloon sizing techniques [10]. Cardiac CT is the most accepted imaging modality for annulus sizing. In general, pre-procedural imaging constitutes of contrast-enhanced CT to identify critical cardiac structures and anatomy, including sizing of the mitral annulus, which is the basal-most structure of the mitral leaflets [19]. In addition to annular sizing, CT also provides essential information for pre-procedural planning, including the amount and distribution of calcifications, as well as predictors of LVOT obstruction; the left ventricular cavity size, anterior leaflet length, aorto-mitral angulation, septal hypertrophy, among other features. CT is also helpful in identifying the trajectory and site of access, whether transapical or transseptal [10, 19].

Data utilizing 2-dimensional (2D) echo imaging correlated acute angulation of the mitral aorta-outflow-angle (mAOA) with higher risk of LVOT obstruction compared with that of more obtuse mAOA. However, risk of LVOT obstruction is not solely based on mAOA; this is because LVOT is a 3D anatomical structure and mAOA on 2D echo images may not provide the comprehensive assessment needed. CT overcomes this limitation as it provides a 3D assessment. Both the prosthetic valve and the anterior displacement of anterior mitral leaflet can result in severe LVOT obstruction. Additionally, utilization of computer-aided designs and 3-D printed models allows us to test devices in patient-specific anatomy and at different angulations and depths with estimation of risk for LVOT obstruction [19].

LVOT obstruction is a fatal complication; thus, pre-procedural planning in an attempt to predict neo-LVOT provides a key step in the success of TMVR procedure. In a multicenter study of 38 patients undergoing TMVR using balloon-expandable valves for severe mitral valve dysfunction because of degenerative surgical mitral ring, bio-prosthesis, or severe native mitral stenosis from severe mitral annular calcification, the investigators defined LVOT obstruction as increase of 10 mmHg or more in LVOT peak gradient following TMVR and found that 7 of the 38 patients had LVOT obstruction, with CT neo-LVOT surface area correlating well with measurements after TMVR [20]. Yoon and colleagues in their study of 194 patients undergoing TMVR found that LVOT obstruction was associated with higher procedural mortality compared with patients without LVOT obstruction (34.6% vs. 2.4%; p < 0.001) [21].
