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.