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Page 8 of 14 Morgan et al. Vessel Plus 2020;4:6 I http://dx.doi.org/10.20517/2574-1209.2019.32

IMAGING OF THE TV
Imaging of the right heart, specifically the TV, is imperative to not only understanding TV anatomy and
function in each individual to optimize future management, but also to identify the underlying etiology of
TR. Thus, evaluation of the structural integrity of the TV leaflets in multiple views with cardiac imaging is
critical, as this is the distinguishing feature between primary and secondary TR. With respect to assessing
severity of functional TR, special attention should be paid to the tricuspid annulus (TA) size. This can be a
challenge, as the tricuspid annulus is a dynamic structure with changing shape and size during respiratory
[49]
and cardiac cycles, due to the contraction of the surrounding myocardium . Echocardiography is the
primary modality used for evaluating the TV [50,51] . The application of 2D TTE to image the TV has several
limitations. First, 2D TTE does not provide a complete visualization of the TV; only two TV leaflets can be
seen at the same time, while 3D TTE or TEE provides a view of all leaflets simultaneously. Second, 2D TTE
[52]
underestimates the maximal dimension of the tricuspid annulus compared to 3D TTE or cardiac MRI .
This is important to emphasize because there is a growing body of evidence demonstrating the importance
[53]
of TA diameter as a marker for TV dysfunction, even in the absence of clinically significant TR : normal
TA diameter in an adult is 28 ± 5 mm; concomitant TV surgical intervention at the time of left-sided valve
surgery is recommended when the TA diameter is ≥ 40 mm [9,54] . Third, 3D TTE provides a more reliable
means to identify the TV leaflets and commissures compared to 2D TTE, which is important in evaluating
the effect of damage secondary to pacemaker leads or other implanted devices in patients with primary,
[26]
catheter-related TR . Lastly, in addition to TA diameter, decreased TV leaflet coaptation and degree of
leaflet tethering are important prognostic factors predicting outcomes in patients undergoing tricuspid
[55]
repair . Both of these anatomic pathologies are better visualized using 3D (rather than 2D) TTE.

MODELING TR
Large animal models
Foundational in-vivo experiments with sonomicrometry crystals placed in ovine RVs demonstrate that
acute RV failure increases RV size, RV free wall strain, and size of the TA with corresponding increase
in grade of TR [56,57] . In these large-animal models, tricuspid repair with annuloplasty led to decreased RV
size, normalized RV strain, and resolution of TR, supporting tricuspid annuloplasty as a therapy of choice
[56]
for TR, with potential to improve RV function . Such animal models provide valuable insights into the
origins of RV failure and choice of intervention; however, they are invasive, cumbersome, and by nature
cannot be replicated in humans. Development of non-invasive techniques to reproduce these results is
consequently an important clinical target.

Computational models of the RV, LV, and TV
With the above advances in cardiac imaging, it is possible to accurately define RV structure, including fiber
angles, shape, and areas of ischemia and fibrosis; RV function, including strain in varying regions; TV
structure, function, and pathology; and the geometric and functional relationships among the RV, LV, and
TV. Using these imaging modalities, an important clinical target is development of accurate computational
models of the LV, RV, and TV. Such models will help predict the mechanical effects of tricuspid repair and
identify patients at risk of RV dysfunction after such repair. The following efforts are underway in model
development.

Shape analysis
Cardiac shape is intimately related to function and provides visual evidence of the pathological changes
of cardiac disease. This has been demonstrated most extensively in the LV, where remodeling towards
[58]
a spherical shape correlates with decreased exercise tolerance and increased mortality . However, as
discussed above, the shape of the RV is not simple to describe, and more nuanced methods are required.
One such method is statistical shape analysis, a mathematical tool allowing non-invasive identification of