Study Design and Population

This was a cross-sectional, observational study of 234 healthy volunteers, with a wide age range ( Figure 1 , Table 1 ), prospectively recruited from October 2011 to June 2013 as part of a large PhD project (the Padova three-dimensional [3D] echocardiography project) whose aim was to provide normative data for quantitative parameters obtained with 3D echocardiographic (3DE) imaging. Volunteers were recruited among hospital employees, fellows in training, their parents, and individuals who underwent medical visits for driving or working licenses and met the following inclusion criteria: age > 17 years, no history of cardiovascular or lung disease, no symptoms, absence of cardiovascular risk factors (i.e., systemic hypertension, smoking, diabetes, dyslipidemia), no cardioactive or vasoactive treatment, normal electrocardiographic results, and normal results on physical examination. Exclusion criteria were athletic training, pregnancy, body mass index > 30 kg/m ² , more than mild tricuspid or pulmonary valve regurgitation, pulmonary artery systolic pressure > 36 mm Hg, and poor 2DE image quality from either the parasternal or the apical approach.

Enrollment flow chart.

The study was approved by the University of Padua Ethics Committee (protocol no. 2380 P, approved on October 6, 2011), and all volunteers provided written informed consent before undergoing physical examination, blood pressure assessment, anthropometric measurement, and echocardiography. Body surface area was calculated according to the formula of Du Bois and Du Bois.

Image Acquisition

Study subjects underwent standard 2DE imaging using a commercially available Vivid E9 ultrasound machine (GE Vingmed Ultrasound AS, Horten, Norway) equipped with an M5S probe. Two-dimensional echocardiographic studies included an apical four-chamber (4CH) view optimized for the right ventricle, as well as parasternal long-axis RV inflow (PLAX) and parasternal short-axis at aortic plane (SAX) views. Three consecutive cardiac cycles were recorded during a breath hold with stable electrocardiographic tracing, to minimize respiratory movements and obtain images suitable for TA diameter measurement. All patients were examined in the left lateral decubitus position using grayscale second-harmonic 2DE imaging, with adjustments of image contrast, frequency depth, and sector size for adequate images.

At the end of the 2DE examination, four consecutive electrocardiographically gated subvolumes were acquired from the apical approach during a breath hold to generate two dedicated 3DE full-volume data sets of the right ventricle and right atrium, using a 4V matrix-array transducer (GE Vingmed Ultrasound AS) and taking care to encompass the entire cavities in the data sets. Data sets were stored digitally and exported to a separate workstation for offline analysis.

Image Analysis

Two-dimensional echocardiographic images were analyzed using EchoPAC version 112.1.3 (GE Vingmed Ultrasound AS). TA diameter was measured as the distance between the insertion points of the TV leaflets (inner edge to inner edge) in the three 2DE views (4CH, PLAX, and SAX) at five time points during the cardiac cycle ( Figure 2 ). The reference frames during the cardiac cycle were determined using both electrocardiographic and valve dynamic visualization: TV closure (end-diastole, the first frame after TV closure), midsystole (the beginning of the T wave), end-systole (the end of the T wave), TV opening early filling (the frame with the TV wide open during passive flow), and TV opening late filling (the frame with the TV wide open during active flow, after the P wave) ( Figure 2 ). For each 2DE view, fractional shortening of TA diameter was calculated using the largest and the smallest dimensions from the serial measurements respectively and expressed as percentage.

TA diameter measurement. Composition of sequential still images throughout the cardiac cycle obtained from three different 2DE views: SAX view (A) , PLAX view (B) , and apical 4CH view optimized for the right ventricle (C) . The time points during the cardiac cycle at which measurements were performed were determined according to electrocardiographic and RV filling Doppler tracings (D) . The anatomic orientation of the echocardiographic views is shown on the cartoons on the right side.

Three-dimensional echocardiographic right atrial (RA) maximum volume was measured using a software designed for the volumetric analysis of the left atrium (LA Analysis; TomTec Imaging Systems GmbH, Unterschleissheim, Germany), according to a previously described methodology. Two-dimensional echocardiographic RV geometry and functional parameters were obtained according to current guidelines. Three-dimensional echocardiographic RV volumes and ejection fraction were measured using dedicated software (4D RV Analysis; TomTec Imaging Systems GmbH) according to a previously described methodology.

To assess the actual size and shape of the tricuspid annulus, 3DE data sets with the TV were analyzed in the same reference frames during the cardiac cycle used to analyze the 2DE images in a subset of 50 randomly chosen subjects.

Using the “flexi slice” option, the TV data set was displayed in a multiplanar review mode ( Figure 3 ). The TV full volume was automatically sectioned in two longitudinal and one transverse cut planes, displaying one 3D volume-rendered image and three 2D slices of the data set ( Figure 3 ). The 2D slice with the solid borders displayed the same view as the 3D volume-rendered image. The position of the cut planes and their spatial orientation were manually adjusted to obtain the view of interest in both the 3D volume-rendered image and the corresponding 2D slices. The transverse cut plane was set at the level of the TA and rotated by 180°, to visualize the TA from the RA side. Then, the corresponding 2D slice was rotated to display the mitral valve on the right and the aorta at the top ( Figure 3 , top left ). Finally, the longitudinal cut planes were adjusted to section the TA in its largest anteroposterior and septal-lateral diameters.

TA geometry assessment by 3D transthoracic echocardiography. ( Top left ) Volume rendering of the tricuspid annulus shown from the RA perspective. The “laser lines” superimposed on the 3DE rendering indicate the orientation of septal-lateral ( yellow ) and its orthogonal diameter ( white ), which are correspondingly depicted in the longitudinal 2D views of the TV in the right upper panel and the left lower panel , respectively. After careful orientation of the annular plane position ( green dotted line ) using these two orthogonal 2D views, the 2D projection of the tricuspid annulus was obtained in the right lower panel and used for measurements of TA area and diameters.

The 2DE and 3DE gain settings were manually adjusted to obtain the best visual delineation of the TA borders ( Figure 3 ) and remained unchanged throughout the entire cardiac cycle, to avoid consequent variability in TA measurements. On the transversal cut plane representing the 2D projection of the tricuspid annulus, the following parameters were measured at each reference frame: projected TA area, anteroposterior diameter, septal-lateral diameter, and sphericity index as the ratio between the two diameters.

Statistical Analysis

Normal distribution of study variables was checked using the Kolmogorov-Smirnov test. Continuous variables are summarized as mean ± SD. Variables were compared between men and women using unpaired t tests. TA diameter variations throughout the cardiac cycle for each 2DE view were analyzed using analysis of variance. Pearson correlation was used to analyze the relationships between TA diameters and their fractional shortening with demographic and echocardiographic variables. A multivariate linear regression analysis was performed to identify the determinants of TA diameters among demographic and echocardiographic variables. Variable selection was performed in the multivariate linear regression using an interactive stepwise backward elimination method according to Durbin-Watson statistics.

Intra- and interobserver variability for 2DE diameters was analyzed in 15 random subjects using the Bland-Altman method. To obtain intraobserver variability, one observer repeated the measurements on the same images 3 weeks after the first measurements. For the interobserver variability assessment, two independent observers performed the analysis on the same 15 subjects.

All analyses were performed using SPSS version 21.0.0.0 (IBM Inc, Chicago, Illinois) and MedCalc version 10.0.1 (MedCalc Software, Mariakerke, Belgium). Differences among variables were considered significant at P < .05. Upper and lower limits of normality were computed as the mean ± 1.96 SDs.

Three-Dimensional Size and Shape of TA Changes during the Cardiac Cycle

The cohort of 50 subjects in whom TA size and geometry were assessed using 3DE imaging showed demographic characteristics similar to those of the overall study population (mean age, 45 ± 15 years; range, 19–73 years; mean body surface area, 1.81 ± 0.22 m ² ; 56% women). Anteroposterior diameter was significantly larger than septal-lateral diameter ( P < .001), and the tricuspid annulus showed an elliptical shape, with the sphericity index < 1 ( Table 2 ). TA diameters and area changed significantly throughout the cardiac cycle ( P < .0001), whereas the sphericity index remained rather constant ( Table 2 ).

TA Diameters Measured Using 2DE Imaging

TA diameters obtained from the three 2DE views at five time points during the cardiac cycle are summarized in Tables 3 to 5 . Irrespective of the 2DE view and timing during the cardiac cycle, TA diameters were significantly larger in men than in women ( Tables 3 to 5 ). Indexing to body surface area eliminated gender differences in TA diameter obtained from the 4CH view only.

Table 5

TA diameters obtained from the SAX view at five time points during the cardiac cycle

Timing	Overall ( n = 121)		Men ( n = 50)				Women ( n = 71)
			Absolute (mm)		Indexed to BSA (mm/m 2 )		Absolute (mm)		Indexed to BSA (mm/m 2 )
	Absolute (mm)	Indexed to BSA (mm/m 2 )	Average	Limit ∗	Average	Limit ∗	Average	Limit ∗	Average	Limit ∗	P †	P ‡
TV closure	29.2 ± 4.6	16.5 ± 2.6	30.7 ± 4.7	40.1	15.8 ± 2.2	20.2	28.2 ± 4.2	36.6	17 ± 2.7	22.4	.004	.01
Midsystole	27 ± 4.7	15.2 ± 2.6	28 ± 4.8	37.6	14.4 ± 2.3	19	26.3 ± 4.5	35.3	15.7 ± 2.7	21.1	NS	.007
End-systole	32.5 ± 4.5	18.4 ± 2.6	34 ± 4.2	42.4	17.6 ± 2.1	21.8	31.5 ± 4.5	40.5	19 ± 2.7	24.4	.01	.02
TV opening early filling	37.2 ± 5	20.9 ± 2.9	38.7 ± 4.6	47.9	20 ± 2.1	24.2	36 ± 5.1	46.2	21.6 ± 3.2	28	.005	.002
TV opening late filling	34.4 ± 5	19.4 ± 2.9	36 ± 4.7	45.4	18.6 ± 2.3	23.2	33.2 ± 5	43.2	20 ± 3.1	26.2	.003	.008

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