4 DISCUSSION
This is the first study to assess the newly created LVOT cross sectional area after TMVI using 3D TEE. 3D assessment of cross-sectional image directly shows the anatomical relationship among the stent valve, mitral leaflet, and basal LV wall after TMVI. Moreover, the current study shows that narrowed LVOT area at the valve stent distal (ventricular side) edge depends on small LV size, preserved LVEF, and small 3D derived aorto-mitral angle. Only real-time 3D TEE can identify the relationship between the transcatheter heart valve and basal LV with precise measurements of LVOT area at any level during the procedure.
4.1 Cause of LVOT narrowing in patients undergoing transcatheter ViV implantation
In patients with degenerated bioprosthetic valves, who do not have their native mitral valve, the stent valve itself encroaches into LVOT space and alters the LVOT geometry and flow. The causes responsible for LVOT obstruction appear to be multifactorial. Previously, LVOT obstruction after surgical mitral valve replacement has been identified when the valve was implanted in a small ventricular cavity or hypertrophic left ventricle.15 The results of the current study also showed that small LV size with preserved LV contraction and small AM angle before procedure might decrease LVOT area at the valve stent distal edge and increase LVOT gradient after transcatheter mitral ViV implantation, because the stent valve implanted in the mitral position with small AM angle could protrude into the outflow tract near the basal septum especially in patients with small LV cavity.
4.2 Cause of LVOT narrowing in patients undergoing transcatheter ViR or ViN implantation
LVOT obstruction may more commonly occur in ViR or ViN than in ViV implantation because, in addition to the valve encroachment, the native redundant mitral leaflet can cause SAM. Previously, LVOT obstruction with SAM has been described in patients undergoing MV repair for myxomatous MV prolapse.16 The large posterior leaflet and the reduction of the anteroposterior diameter using an annuloplasty ring cause the leaflets to coapt anteriorly, increasing the likelihood of SAM and LVOT obstruction.17 Ro et al. reported that the angle of systolic flow directed onto the posterior surface of the mitral leaflet was important to produce the pushing force (drag force), resulting initially in SAM and later the Venturi effect.18 Similarly, in patients undergoing ViV or ViN procedure, valve implantation in the mitral position with small AM angle can cause displacement of the mitral anterior leaflet anteriorly toward LVOT, making the leaflet more susceptible to the effect of systolic outflow and receiving the drag force and Venturi effect.