Author + information
- Received February 15, 2017
- Revision received August 7, 2017
- Accepted August 14, 2017
- Published online November 15, 2017.
- Karima Addetia, MDa,
- Denisa Muraru, MD, PhDb,∗ (, )
- Federico Veronesi, PhDc,
- Csaba Jenei, MDb,
- Giacomo Cavalli, MDb,
- Stephanie A. Besser, MSAS, MSA, MACJCa,
- Victor Mor-Avi, PhDa,
- Roberto M. Lang, MDa and
- Luigi P. Badano, MD, PhDb
- aSection of Cardiology, Department of Medicine, University of Chicago, Chicago, Illinois
- bDepartment of Cardiac, Thoracic and Vascular Sciences, University of Padua, Padua, Italy
- cDepartment of Electrical, Electronic and Information Engineering, University of Bologna, Bologna, Italy
- ↵∗Address for correspondence:
Dr. Denisa Muraru, Department of Cardiac, Thoracic and Vascular Sciences, University of Padua, Via Giustiniani 2, 35128 Padua, Italy.
Objectives The authors used transthoracic 3-dimensional transthoracic echocardiography (3DE) to characterize tricuspid annulus (TA) geometry and dynamics in healthy volunteers.
Background Accurate sizing of the TA is essential for planning tricuspid annuloplasty and for implantation of new percutaneous tricuspid devices.
Methods 3DE of the TA from 209 healthy volunteers was analyzed using custom software to measure TA area, perimeter, circularity, and dimensions at end diastole (equals tricuspid valve closure), mid-systole, end systole, and late diastole. TA intercommissural distances were measured at mid-systole. For comparison, TA diameters were measured at the same time points on multiplanar reconstruction of the 3DE datasets and on 2-dimensional transthoracic echocardiography (2DE) apical 4-chamber and right ventricular focused views. In 13 subjects with both 3DE and computed tomography, TA parameters were compared.
Results 3DE TA area, perimeter, and dimensions were largest in late diastole and smallest at mid-systole/end systole. Normal tricuspid valve parameters in end diastole were 8.6 ± 2.0 cm2 for area; 10.5 ± 1.2 cm for perimeter; 36 ± 4 mm and 30 ± 4 mm for longest and shortest dimensions, respectively; and 0.83 ± 0.10 for circularity. There were no age-related changes in TA parameters. Women had larger indexed TA perimeter and longer long-axis dimensions compared with men. The longest 3DE TA dimension was significantly longer than diameters measured from both 2DE and 3D multiplanar reconstruction. 3DE TA area, perimeter, and dimensions correlated with both right atrial and right ventricular volumes, suggesting that both chambers may be determinants of TA size. TA fractional area change was 35 ± 10%. Fractional changes in both perimeter and dimensions were ≥20%. When compared with computed tomography, 3DE systematically underestimated TA parameters.
Conclusions Gender and body size should be taken into account to identify the reference values of TA dimensions. 2DE underestimates TA dimensions.
Recent studies focusing on the optimal timing for surgical intervention on the tricuspid valve (TV) in patients with functional tricuspid regurgitation (TR) caused by left-sided heart disease have suggested a role for tricuspid annular (TA) size to evaluate the spectrum of TV dysfunction (1–3). In these studies, it was shown that when TV repair is performed based on TA dilatation instead of TR severity, surgical outcomes were improved. In the long term, these patients had less TR, less heart failure, and were more likely to undergo reverse right ventricular (RV) remodeling (1,4). In light of these results, the latest valvular heart disease guidelines assign a class IIa recommendation for TV repair in the presence of TA dilatation irrespective of TR severity in patients undergoing surgery for left-heart disease.
Although numerous studies have described the geometry and function of the mitral valve apparatus, few have focused on the morphoanatomy of the TV complex (5,6). TA geometry is currently assessed using 2-dimensional transthoracic echocardiography (2DE), despite significant controversy regarding the optimal view and timing in which TA measurements should be performed (7). Similar to the mitral annulus, the TA has a complex saddle-shaped morphology that is incompletely characterized by 2DE (8–10). Previous studies have shown that TA enlargement occurs predominantly in the septal-lateral (SL) direction, a dimension that is difficult to assess using 2DE (10). To our knowledge, no study has systematically characterized the dynamic behavior of the TA in normal subjects using 3-dimensional transthoracic echocardiography (3DE). Filling this knowledge gap might be an important step toward advancing the understanding of the pathophysiology of functional TR and perhaps help guide the selection of patients for percutaneous transcatheter procedures, such as tricuspid annuloplasty (11,12). Accordingly, we designed a 3DE study to: 1) describe the normal geometry of the TA and its dynamic changes throughout the cardiac cycle in a large number of healthy volunteers using novel analysis that accounts for the nonplanarity of the TA; 2) assess the impact of age and gender on these parameters; and 3) compare TA area and diameters measured using the novel 3DE analysis with those obtained with conventional 2DE and multiplanar slicing of 3D datasets.
Study subjects were enrolled from 2 institutions (University of Chicago, n = 99; University of Padua, n = 110). Subjects were included if they were ≥18 years of age, had no prior history of cardiovascular or lung disease, were on no medications with normal 2DE, defined as normal left ventricular and RV and atrial size and function with left ventricular ejection fraction ≥55%, and no evidence of valvular heart disease (i.e., no stenosis and <mild regurgitation) together with a pulmonary artery systolic pressure <35 mm Hg. Two hundred and fifteen subjects fit the inclusion criteria. Of these, 6 were excluded because of poor 3D image quality. This study was approved by the Institutional Review Boards of both institutions.
2DE was performed using either the iE33 imaging system (Philips, Andover, Massachusetts) equipped with an S5 transducer or the Vivid E9 scanner (GE Vingmed, Horten, Norway) equipped with an M5S probe. TA diameter was measured from the apical 4-chamber view and RV-focused apical view at 4 time points during the cardiac cycle (7): 1) end diastole (ED) (frame after TV closure); 2) mid-systole (MS) (frame between TV closure and end systole [ES]); 3) ES (the frame just before TV opening); and 4) late diastole (LD) (TV leaflet opening after atrial contraction, p wave) (Figure 1).
3DE was acquired using either the X5 transducer (Philips), equipped or the 4V probe (GE Vingmed Ultrasound). Full-volume acquisitions of the RV, TV, and right atrium (RA) were performed from the RV-focused apical view. 3DE datasets were obtained using electrocardiogram gating over 4 to 6 consecutive cardiac cycles during a single breath-hold. Gain settings were optimized and temporal resolution was maximized by optimizing sector width and minimizing depth. The average frame rate was 27 ± 8 vps. All 3D datasets were analyzed offline.
3DE datasets of the RV were analyzed to quantify RV, ED, and ES volumes and ejection fraction using commercial software (4D RV-Function 2.0, TomTec, Unterschleissheim, Germany), which has been previously validated against cardiac magnetic resonance. 3DE datasets of the RA were analyzed using software designed for the volumetric analysis of the left atrium (LA Analysis, TomTec) (13).
TA measurements were made using 3DE custom software developed for the TV (Figure 2). Initially, the operator sliced the 3DE data set to obtain 2 planes corresponding to the optimal 4-chamber and its respective orthogonal view (Figures 2A and 2B). Initialization of the TA was manually performed in MS by marking the TV leaflet hinge points (Figures 2A and 2B, purple dots). This initialization was performed in 8 rotated planes around the TA center point. Further editing was allowed on any additional user-identified rotational plane. Input from the user was interactively resampled to 80 points along the TA using smooth spline interpolation. These 80 points were then automatically tracked to obtain the following TA measurements throughout the cardiac cycle: area (computed as the sum of the triangles composing the 3D mesh connecting all annulus points), perimeter (computed as the sum of the distances between consecutive annulus points), and long- and short-axis dimensions and circularity (ratio of short- to long-axis dimensions). In MS, the transverse plane was adjusted to find the TV commissures (Figure 2E) and identify the commissural points: anteroseptal commissure, anteroposterior (AP) commissure, and posteroseptal commissure. Once these points were marked, the software provided measurements for the anterior (AP/anteroseptal), posterior (AP/posteroseptal), and septal (anteroseptal/posteroseptal) intercommissural distances (ICD). To estimate fractional change in area and long- and short-axis dimensions the following formula was used: [(maximum – minimum)/maximum] × 100%.
To assess the differences between the custom software and the commercially available multiplanar reconstruction (MPR) method, the TA was assessed with MPR using QLAB (version 9.0, Phillips) or EchoPAC BT 12, (GE, Milwaukee, Wisconsin) in a subgroup of 58 subjects. Measurements included TA area as well as AP and SL dimensions at ED, MS, ES, and LD. To perform these measurements, orthogonal long-axis views of the TA were identified in the multiplanar view mode. The transverse cut plane was then oriented to be at the hinge points of the TV leaflets in both longitudinal views (or above the annulus and toward the RA, if the hinge point view could not be obtained). Measurements were performed from the RA perspective (Figures 3 and 4⇓⇓). The short-axis plane of the RV was used to identify the SL dimension, the longest septal to lateral distance (Figures 4A and 4B). This plane was then moved to the level of the TA where the SL measurement was made. The AP dimension was made orthogonal to the SL dimension (Figure 4C). Because the lateral aspect of the TA displaced more than the septal aspect during the cardiac cycle, annular alignments to measure the SL and AP dimensions were adjusted at each point of the cardiac cyle.
Statistical analysis and interreader agreement
Continuous variables were expressed as mean ± SD. Two-tailed independent Student t test was used to compare TA parameters between genders, whereas a 1-way analysis of variance was used to compare the TA parameters among the following age groups (<30, 30 to 40, 40 to 50, 50 to 60, and >60 years of age). Pearson correlation coefficient was used to determine the correlation between annular parameters and RV and RA volumes. Software validation was performed on 13 additional subjects who underwent computed tomography (CT) angiography on the same day as 3DE. CT images were acquired according to standard clinical protocol at 78% of the cardiac cycle (256-channel iCT scanner, Philips) during suspended respiration using prospective gating. Imaging parameters were as follows: 270 ms gantry rotation time, slice thickness 0.625 mm, tube currents 600 to 1,000 mA, and voltage 100 to 120 kV (depending on body weight). An iodinated contrast agent (60 ml, 5 ml/s) was infused, followed by a 20-ml bolus of normal saline. The identical custom software was used to analyze the TA area and perimeter using CT and 3DE datasets. TA parameters obtained from CT and from 3DE were compared using Pearson correlation and Bland-Altman analyses. Interobserver variability (expressed in terms of percent variability) of TA measurements was performed in a subset of 20 patients by 2 independent blinded observers.
Demographics, clinical characteristics, and right heart parameters of the study population are summarized in Table 1. Post-processing of the TA using the custom software took 3 to 4 min on average. There was no correlation between age and TA geometry parameters (r < 0.3) and when comparisons were made between age groups (<30, 3 to 40, 40 to 50, 50 to 60, and >60 years of age) there were no significant differences in area, perimeter, and dimensions among age groups at any point during the cardiac cycle. All TA geometry parameters showed significant correlations with body size measures (body surface area [BSA], r = 0.5 to 0.6; height, r = 0.4 to 0.5).
TA changes during the cardiac cycle
Both TA area and perimeter decreased to a minimum during systole (Figures 5A and 5B, dotted black line), then increased during diastole reaching a maximum value in LD. In both the TA area and perimeter curves (Figures 5A and 5B) it is possible to appreciate an initial and a late diastolic slope (arrows) corresponding to early ventricular filling and atrial contraction, respectively. The mean reduction in TA area during the cardiac cycle was 35 ± 10%. Of this, 49 ± 29% occurred during atrial systole (between LD and ED). TA circularity augmented during systole, with the TA returning to a more oval shape in diastole (Figure 5C). Table 2 shows the area, perimeter, dimensions, and circularity values in ED, MS, ES, and LD. Fractional changes in perimeter and dimensions were all ≥20% (Table 3). The anterior ICD (28.1 ± 4.2 mm) (Figure 6) was significantly longer than both the posterior (24.3 ± 4.9 mm [p < 0.001] vs. anterior ICD) and the septal (26.6 ± 4.1 mm [p < 0.001] vs. anterior ICD) ICDs. Septal ICD was also longer than posterior ICD (p < 0.001).
The left panel of Figure 7 shows the relative anatomic positions of the 3DE TA long- and short-axes, the 2D apical 4-chamber, and the RV-focused view diameters on the reconstructed 3D TA. The right panel of Figure 7 depicts the changes in the 3DE TA long- and short-axis dimensions and of the TA diameters obtained from the 2DE apical 4-chamber and RV-focused views during the cardiac cycle. The 3DE long-axis dimension of the TA was significantly longer than the diameters measured from both the apical 4-chamber view and the RV-focused view throughout the cardiac cycle (p < 0.01). TA areas measured at ED, MS, ES, and LD obtained using 3DE correlated moderately with TA diameters from the 2DE apical 4-chamber view (r = 0.55, r = 0.55, r = 0.54, and r = 0.62, respectively) and from the RV-focused view (r = 0.54, r = 0.59, r = 0.59, and r = 0.58, respectively).
Gender differences in tricuspid annular parameters
As expected, RV ejection fraction was higher in women, and indexed and nonindexed RV end-diastolic and end-systolic volumes were significantly larger in men (Table 1). Nonindexed TA area, perimeter, and long- and short-axis dimensions were all larger in men (Table 4). When indexed to BSA, TA area also remained larger in men; however, indexed perimeter and long-axis dimensions were larger in women at ED, MS, ES, and LD. Indexed short-axis dimensions were not different between genders. Fractional changes in area, perimeter, and long-axis dimensions were all significantly larger in women than in men (Table 3).
Correlations with right heart chamber size
TA area, perimeter, long- and short-axis dimensions from 3DE correlated well with both RA maximal volumes (r = 0.53, r = 0.52, r = 0.48, and r = 0.43, respectively) and RV end-diastolic volume (r = 0.44, r = 0.41, r = 0.39, and r = 0.38, respectively). On 2DE, TA diameters from the RV-focused view and apical 4-chamber views correlated with RV end-diastolic volume (r = 0.49 and r = 0.42, respectively) and maximum RA volume (r = 0.45 and r = 0.32, respectively).
MPR analysis of the TA
In the subset of subjects in whom the 3DE datasets of the TA were analyzed using MPR, the TA area increased from ED to LD. (Table 2). The smallest TA area was measured at ED in 45 of 58 subjects (78%). Only 7% of subjects showed that the smallest annulus area occurred at ES. The SL and AP dimensions likewise increased progressively from ED to LD, also suggesting that the smallest dimensions were noted in ED. Perimeter and circularity could not be determined using MPR. When the MPR method was compared with the custom software, annular measurements obtained using MPR analysis were different from those obtained using custom software. With MPR, the smallest annular area was measured at ED, whereas with custom software the smallest TA area was measured at ES. The SL dimension was significantly smaller than the longest dimension measured using the custom software throughout the cardiac cycle (Figure 7).
CT validation of 3DE software and interobserver variability in annular measurements
CT- and 3DE-obtained perimeter and area measurements correlated highly, with r values of 0.91 and 0.94, respectively. CT-derived TA perimeter and area, as expected, were larger than those obtained from 3DE by 1.9 ± 0.7 cm and 3.8 ± 1.7 cm2, respectively, as shown on Bland-Altman analysis (Figure 8). Interobserver variability for annular measurements was higher for area (15%) and lower for perimeter and long and short axis (7%, 9%, and 9%, respectively).
This the first study to use 3DE in the assessment of TA function and size in a large cohort of healthy volunteers over a wide age range. We found that: 1) 3DE allows the assessment of static and dynamic TA dimensions and function, which were found to correlate well with CT reference values in a small validation cohort, although 3DE measurements were notably smaller than those made on CT; 2) 2DE diameters underestimate TA dimensions obtained with 3DE; 3) 3DE TA assessment using custom software, which takes into account the nonplanarity of the annulus, provides unique and different information from that obtained using MPR; 4) reference values of TA dimensions should be gender specific and indexed by BSA; and 5) TA dimensions correlate with both RA and RV volumes.
Recent data support the notion that TA diameter assessment is an important measure of TV dysfunction in functional TR. Current recommendations indicate that TV repair be considered in the setting of operable mitral valve disease, when the TA diameter is >40 mm (14). This is because substantial annular dilatation, and thereby TV dysfunction, may be present without significant associated TR because of the confounding influence of variable loading conditions (1). In fact, when TV repair is performed in patients undergoing mitral valve surgery with a dilated TA, irrespective of coexisting TR, post-operative outcomes are improved (1,4,15), and the occurrence of reverse RV remodeling more likely (4,15). These results underline the importance of an accurate and complete assessment of 3DE TA geometry.
Current guidelines state that the TA should be measured as a diameter obtained in diastole from the 2D apical 4-chamber view (14). Indeed, the TA is a highly dynamic, complex 3D structure (7,16). Moreover, the TA diameter measured with 2DE from any view underestimates the TA maximal dimension obtained with 3DE and cardiovascular magnetic resonance (2,8,17). Furthermore, current cutoff values used to guide TV annuloplasty indications or suture repair have been reported from different echocardiographic views, including the conventional 4-chamber 2DE view, the RV-focused 2DE view, and mid-esophageal transesophageal view (4,18). Finally, the timing within the cardiac cycle during which these measurements were obtained has varied significantly between studies (7). Accordingly, no uniform method for reliable TA assessment exists. A recent study (2) proposed 42 mm or 23 mm/m2 as cutoffs for TA enlargement but these cutoffs were not based on surgical outcomes and the TA diameters were loosely reported as obtained “in diastole at the time of maximal leaflet opening.” Furthermore, the proposed cutoffs were not obtained using 3DE but using 2DE, which is known to underestimate TA size. Finally, TA enlargement may not occur uniformly in 1 direction suggesting that a more comprehensive 3D evaluation of the TA seems more appropriate.
Despite the obvious need to develop 3D software to measure the TA accurately, neither ultrasound vendors nor software companies have addressed this need. We therefore developed a software tool that analyzes transthoracic 3D datasets of the TV acquired from the transthoracic approach, yielding a comprehensive dynamic assessment of the TA. We showed that TA area, perimeter, and dimensions progressively decrease during systole, reaching minimum values at around ES. These results parallel those obtained from animal studies with sonomicrometry and lead beads (19). According to our study, the largest annular measurements were noted in LD around the time of atrial contraction. Fractional change in TA area was ∼30% and in TA dimensions and perimeter ∼20%. Of note, on 2DE, TA dimensions obtained from the RV-focused view correlated more strongly with maximum RA and RV volumes than did those obtained from the apical 4-chamber view, possibly implying that the TA diameter measured from the RV-focused view may be more representative of the direction of TA enlargement than the diameter obtained from the apical 4-chamber view. Finally, long-axis measurements obtained using the hinge point initialization approach adopted by the new software were longer than measures obtained using either 2DE or 3D MPR. Of note, 3DE TA measurements significantly underestimated measurements made using CT data. Although this is not unexpected, it may have implications in the new era of catheter-based tricuspid procedures.
The first study to look at TA size throughout the cardiac cycle was reported by Tei et al. (20) using a transducer mounted on an inclinometer, which allowed multiple 2D planes to be acquired at 30° rotations. TA area measurements obtained in that study closely paralleled our measurements. TA size was found to be smallest at MS and not ES. With our approach, using 3DE, we found very little difference between the size of the TA in MS and the size in ES (Table 2). When individual cases were counted, 52% of subjects had smallest TA area at ES and the rest had smallest area at MS. The timing of ES derived from a 3DE dataset depends heavily on the frame rate of the dataset. The higher the frame rate the more precise and likely more accurate the measurement of ES. Future improvements in temporal resolution of 3D datasets will rectify this limitation.
Insights from dedicated 3D analysis
The novel software developed for this study allowed initialization of the TA in a series of rotated planes around the annulus. This feature allows the nonplanar nature of the TV to be factored into the TA measurements, a provision that is not available when using the commercially available MPR method (7). When performing annular measurements using the MPR method, the position of the annular plane is chosen by the operator. Although this plane should, ideally, be located at the hinge points of the TV, it is difficult to correctly position the plane because of the nonplanarity of the TA. Accordingly, if the chosen annular plane is located at the hinge points in 1 longitudinal view, it may not be possible to ensure that it is at the hinge points in the orthogonal plane. As a compromise, the plane is often placed above the annulus, toward the RA. As a result, the operator measures a projected area instead of the actual area. Measuring a projected area may explain why the smallest TA areas and dimensions are found in ED and not in ES using MPR (Table 2). Incorrect partial identification of the RA wall as TA annular boundary results in smaller ED measurements, because at this time the RA is the smallest. These findings explain why the use of custom software that accounts for the nonplanarity of the TV annulus is a potentially more reliable method for the assessment of TA size and dynamics.
Several studies have suggested that enlargement of the TA occurs in the SL or AP direction (1,10). The longest SL direction is difficult to reproduce on 2DE. Indeed, the apical 4-chamber view often does not depict the longest SL dimension (7). The RV-focused view may more closely represent the direction of TA enlargement but this 2D cut-plane cannot be uniformly obtained in every patient. Automated software identification of TA size from the 3D annular reconstruction, irrespective of its orientation in space, is not hampered by the view dependency of 2DE.
Gender and age differences
Gender is an important determinant of TA size and function. Without taking into account BSA, women had smaller TA areas, perimeters, and dimensions than men. After indexing to BSA, women were found to have larger TA perimeters and longer long-axis dimensions than men. Furthermore, fractional changes in area, perimeter, and long-axis dimensions were larger in women than in men, perhaps suggesting that the TA is more dynamic in healthy women than in men, in addition to reflecting the higher RV ejection fraction in women. Conversely, we found no difference in TA parameters among adults when ages were grouped by decade.
3D datasets must be optimized to minimize dropout of the anterior RV free wall and maximize frame rates to optimize the accuracy of dynamic measurements. Multibeat full-volume acquisitions do not always record the very last segment of diastole because during that time the probe is preparing to acquire the next sector of volume. This is a limitation of the 3D imaging technique. Consequently, atrial contraction is almost always partially missing in 3D datasets. Furthermore, to obtain accurate measurements in systole, 3D datasets must be acquired at high frame rates, ideally higher that what is possible with currently available technology. It is hoped that newer iterations of the custom software used in this study together with improved image acquisition techniques will correct this issue in the future.
Although TV disease is a significant contributor to morbidity and mortality, reliable data on the normal dynamic geometry of the TA are lacking. In this study we present a comprehensive analysis of the TA in healthy subjects. Gender and body size should be taken into account to identify the reference values of TA dimensions. Importantly, we conclude that conventional 2DE and 3D MPR measurements of tricuspid annular diameters, which do not account for the nonplanarity of the TV, underestimate true 3DE dimensions. This limitation is overcome by using quantification methods tailored for the TV. Knowledge garnered from this study could enhance the understanding of TR pathophysiology and have implications on preoperative planning of TV repair.
COMPETENCY IN MEDICAL KNOWLEDGE: Recent studies show an improvement in patient outcomes when tricuspid annuloplasty is performed using a threshold of TA size rather than degree of TR. The need to better characterize the TA is perhaps even more important today because of the new percutaneous tricuspid devices that have entered the market. Because of the complex morphology of the TV and its dynamic behavior a single 2D diameter cannot fully describe TA geometry. In this study we used 3DE datasets of the TV together with custom software to characterize the TA in healthy volunteers. We showed that it is possible to study the dynamics of TA size parameters (i.e., area, perimeter, and dimensions). In normal subjects these parameters did not differ significantly with aging, but women had larger indexed TA perimeter and longer long-axis dimensions when compared with men. Fractional changes in TA area and dimensions were ≥20%. Finally, TA parameters correlated with both RA and RV volumes, suggesting that both chambers may be important determinants of TA size.
TRANSLATIONAL OUTLOOK: 3DE analysis of the TA provides incremental information over 2D imaging on the dynamic behavior of multiple TA parameters including area, perimeter, dimensions, and circularity. With the knowledge provided in this study, a more comprehensive analysis of abnormal TV morphology is possible, with the goal of providing physicians and researchers with improved understanding of TV pathology and ultimately enabling new ideas for therapeutic interventions on these patients.
Dr. Muraru has served as a consultant, has received research support, and has served on the Speakers Bureau for GE Healthcare and TomTec Imaging. Dr. Veronesi has served as a consultant for GE Healthcare. Dr. Lang has served on the Speakers Bureau and Advisory Bureau and has received research grants from Philips Medical Imaging. Dr. Badano has received equipment grants from GE Vingmed and TomTec Imaging Systems; and has received speaker honoraria from GE Vingmed. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose. Drs. Lang and Badano contributed equally to this work.
- Abbreviations and Acronyms
- body surface area
- computed tomography
- end diastole
- end systole
- intercommissural distance
- late diastole
- multiplanar reconstruction
- right atrial
- right ventricular
- tricuspid annulus
- 3-dimensional transthoracic echocardiography
- tricuspid regurgitation
- tricuspid valve
- 2-dimensional transthoracic echocardiography
- Received February 15, 2017.
- Revision received August 7, 2017.
- Accepted August 14, 2017.
- 2017 American College of Cardiology Foundation
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