DISCUSSION
Despite its important function, the RAV is often considered as the
“forgotten valve”. However, our study further shows the complexity and
variability of the RAV’s morphology. This valve is much more irregular
than its sister valve, the bicuspid valve, which has a remarkably more
simple and predictable structure (15). While the mitral valve is almost
always composed of 2-leaflets, the RAV often presents with a 4-leaflet
configuration. In scientific literature, the RAV is often referred to as
the “tricuspid valve”, although we strongly suggest avoiding this
nomenclature, since it may mistakenly suggest the valve’s construction
(10). Moreover, before attempting any interventions withing the RAV
region, it is important to determine the amount of leaflets using either
echocardiography or cardiac computed tomography (17–19). Finally, it is
important to know there are considerable differences in leaflet and
commissure sizes between 3- and 4-leaflet RAVs (Table 1). Our study has
shown that the supero/septal region of the RAV is the least variable
area, whereas the infero/mural region differs significantly between
3-and 4-leaflet RAVs. It seems that the formation of the additional
fourth leaflet takes place at the expense of the size of the muro-septal
commissure and of the septal leaflet, which also explains the shifted
position of the muro-septal and infero-septal commissures (Table 1).
Besides the variable morphology of the RAV’s leaflets and commissures,
there are other causes which affect the valve’s hemodynamic
characteristics. Taken together, they make RAV imaging and interventions
(both surgical and percutaneous) challenging (13). The RAV annulus is a
relatively large, flexible, fragile and highly irregular structure and
it is easily affected by changes in shape and size of the right
ventricle (20). The nonplanarity of the RAV annulus and its
heterogeneous regional behavior may have a strong association with
leaflet configuration. This study has shown that there is a significant
difference in the size of the annulus between 3- and 4-leaflet valves.
Initially larger right atrioventricular orifice size may force the
creation of the additional leaflet and commissure to seal the orifice
that are located with the inferior/mural region of the annulus (10,19).
When planning therapeutic procedures on the RAV, it is important to
determine its morphology, position and neighboring cardiac structures
(17). The RAV annulus has important anatomical relationships with the
right coronary artery, the acute marginal branch, the small cardiac vein
(located within the vestibule of the right atrial appendage), the
terminal crest, the cavo-tricuspid isthmus, the coronary sinus (and its
ostium), the atrio-ventricular node and the aortic valve (21–24). The
above-mentioned structures are subject to injury and clinicians must
factor in their respective positions before attempting any invasive
procedures. Although the inferior/mural region of the RAV has the most
unpredictable morphology, it should be considered the safest area for
interventions due to its remoteness from crucial heart structures (the
only exception being the right coronary artery) (25).
Transcatheter RAV annulus reduction techniques involve placing special
pledgeted sutures on the superior and mural leaflet. These procedures
reduce the entire circumference of the valve annulus and seal the
orifice. This in turn improves the RAV’s hemodynamic properties (26,27).
Our observations have shown that the location of the supero-mural
commissure is quite consistent. It is usually located near the middle
isthmus of the vestibule of the right atrial appendage and is relatively
easy to discern. However, the locations of the commissures and leaflets
of the mural/inferior region of the valve are not always clear. Even
RAVs with the same amount of leaflets present with a lot of
heterogeneity (see Table 1). In 3-leaflet valves, the larger septal
leaflet ends much closer to the terminal crest and further away from the
ostium of the coronary sinus when compared to 4-leaflet RAVs. The
muro-septal commissure in a 3-leaflet valve may be located anywhere
between the left side of Koch’s triangle and the right side of the
middle isthmus of the right atrial appendage vestibule. In 4-leaflet
valves, the smaller septal leaflet pushes the inferior leaflet to the
left into the territory of the cavo-tricuspid isthmus, but never to the
region of the vestibule of the right atrial appendage. The presence of
an additional inferior leaflet and its adjacent commissures shifts the
mural leaflet to the right side of the valve. Therefore, we recommend
determining the detailed morphology of leaflet valves before qualifying
patients for invasive procedures. It is equally important to study the
course of coronary vessels within the RAV region in order to avoid any
damage to these vessels (17,25,28).
There are some limitations to this study. The main one is that this
study was performed on autopsied material preserved in paraformaldehyde
solution. Consequently, there may have been slight changes to the size
and shape of the studied hearts due to fixation. Nevertheless, our
earlier studies have demonstrated that using paraformaldehyde solution
did not cause important changes in cardiac dimensions (29,30).
Furthermore, since we analyzed post-mortem material, we were not able to
assess the behavior and dimensional changes of RAV components during the
cardiac cycle. Additionally, we only investigated hearts from healthy
donors and therefore our results may not be applicable to patients with
structural valvular diseases. Despite these limitations, we consider
that this study has provided insight on the morphological and
topographical analyses of the RAV and on the relationships between
individual RAV components and right atrial structures.