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.