Right Ventricular Dimensions and Function: Why do we Need a More Accurate and Quantitative Imaging?

The right ventricle plays an important role in the morbidity and mortality of patients presenting with symptoms and signs of cardiopulmonary disease. This cardiac chamber has a unique crescent shape, which adds complexity to the quantification of its size and function. Until recently, little uniformity in echocardiographic imaging of the right heart existed because of a lack of familiarity with various techniques, and the enormous attention directed towards left heart quantification. Three-dimensional (3D) echocardiography, a major technological breakthrough in the field of cardiovascular imaging, provides several advantages over two-dimensional (2D) imaging in the quantitative evaluations of right ventricle because of its independence from any geometrical assumption. In this review, we focus on the contribution of this new modality to the evaluation of right ventricle.

INTRODUCTION

Despite the right ventricle plays an important role in determining the morbidity and mortality of patients with cardiopulmonary disease,[1234] the systematic assessment of right heart function is not uniformly carried out.

Recent guidelines[56] recommend that in all clinical studies, a comprehensive examination of the right ventricle should be performed, taking into account the study indication and available clinical information.

This review covers the contribution of three-dimensional (3D) echocardiography in the evaluation of right ventricular (RV) volumes and function, its advantages in comparison to the standard echo-Doppler evaluation, and its accuracy versus cardiac magnetic resonance imaging (CMR).

STANDARD ECHOCARDIOGRAPHIC EXAMINATION OF THE RV

The parameters to be performed and reported should include a measure of RV size, right atrial (RA) size, RV systolic function (at least one of the following: Fractional area change (FAC), tricuspid annular plane systolic excursion (TAPSE), peak systolic velocity (PSV); with or without RV index of myocardial performance (RIMP)), and systolic pulmonary artery (PA) pressure (SPAP) with estimate of RA pressure on the basis of inferior vena cava (IVC) size and collapse. In many conditions, additional measures such as PA diastolic pressure (PADP) and an assessment of RV diastolic function are indicated.[56]

Starting with the standard methods to evaluate RV dimensions; apical four-chamber, RV-focused apical four-chamber and modified short-axis, left parasternal RV inflow, and subcostal views provide the images required for a comprehensive assessment of RV size.

Measurements by two-dimensional (2D) echocardiography are challenging because of the complex geometry of the RV and the lack of specific right-sided anatomic landmarks to be used as reference points. The conventional apical four-chamber view results in considerable variability in how the right heart is sectioned, and consequently, RV linear dimensions and areas may vary widely with relatively minor rotations in transducer position. Specifically recent guidelines(5,6) suggest that diameter >42 mm at the base and >35 mm at the midlevel indicates RV dilatation. Similarly, longitudinal dimension >86 mm indicates RV enlargement. Concerning RA dimensions, these are routinely estimated in the apical four-chamber view. RA area >18 cm2, RA length (defined as the major dimension) >53 mm, and RA diameter (also known as the minor dimension) >44 mm indicate at end-diastole RA enlargement.

As far as RV systolic function is concerned, TAPSE is easily obtainable and represents a measure of RV longitudinal function, as well as PSV of the tricuspid annulus at tissue Doppler imaging (TDI), while FAC (four-chamber view) is a simplified method considering both longitudinal and radial shortening. We previously evaluated the feasibility of a routine use of FAC, TAPSE, and PSV as measures of RV systolic function in 900 patients, including 750 pathological and 150 normal subjects matched for age, referred for transthoracic echocardiography in our laboratory.[7] The methods were easy, not time consuming, and inter- and intra observer variabilities were very low for TAPSE (0.24 ± 1.3 and 0.17 ± 1.4 mm, respectively) and PSV (0.15 ± 1.8 and 0.21 ± 1.5 cm/s) and higher for FAC (4.22 ± 7 and 1.1 ± 3.9%), due to a limited definition of the endocardial border in RV lateral free wall and in the apex. A cutoff of 17 mm for TAPSE, 12 cm/s for PSV, and 37% for FAC identified abnormal RV function with high specificity (even though with low sensitivity).

These previous data are in accordance with recent guidelines[56] recommending these values as indexes of RV dysfunction: TAPSE <16 mm; FAC <35% as index of RV systolic dysfunction, and PVS <10 cm/s.

RIMP provides an index of global RV function. RIMP >0.40 by pulsed Doppler and >0.55 by TDI indicates RV dysfunction. By measuring the isovolumic contraction time (IVCT), isovolumic relaxation time (IVRT), and ejection time (ET) indices from the pulsed TDI velocity of the lateral tricuspid annulus, one avoids errors related to heart rate variability. RIMP can be falsely low in conditions associated with elevated RA pressures, which will decrease the IVRT.

Strain and strain rate are useful parameters for estimating RV global and regional systolic function. Longitudinal strain is calculated as the percentage of systolic shortening of the RV free wall from base to apex, while longitudinal strain rate is the rate of this shortening. RV longitudinal strain is less confounded by overall heart motion, but depends on RV loading conditions as well as RV size and shape. RV longitudinal strain should be measured in the RV-focused four-chamber view. Compared with speckle tracking-derived strain, the angle dependency of TDI strain is a disadvantage. RV speckle-tracking echocardiographic strain is influenced by image quality, reverberation, and other artifacts, as well as attenuation. However, peak global longitudinal RV strain excluding the interventricular septum has been recently reported to have prognostic value in various diseases; such as heart failure, acute myocardial infarction, pulmonary hypertension, and amyloidosis; and to predict RV failure after left ventricular assist device implantation. Maffessanti et al., compared indices of RV function obtained using different echocardiographic modalities, before and after cardiac surgery in 42 patients. They demonstrated that TAPSE, PSV, and FAC significantly decreased after surgery, while 3D RV ejection fraction (EF) was preserved; speckle-tracking echocardiographic results were dependent on the considered direction with preserved radial, but decreased longitudinal strain values.[8] Figure 1 shows examples of 2D methods for the RV evaluation.

Figure 1

Examples of different methods for RV function evaluation. Top left: Fractional shortening area; top middle: TAPSE; top right: DTI of the tricuspid annulus. Bottom left, mid, and right: Images of RV strain evaluation. LV = Left ventricle, PSV = Peak systolic velocity, TAPSE = Tricuspid annulus peak systolic excursion, RV = Right ventricular, TAPSE = Tricuspid annular plane systolic excursion, DTI = Doppler tissue imaging

All dimensional and functional parameters of the right chambers should be associated to the estimation of the SPAP differentiating volume and pressure overload. Tricuspid regurgitation velocity reliably permits estimation of RV systolic pressure with the addition of RA pressure, assuming no significant RV outflow tract obstruction. It is recommended to use the RA pressure estimated from IVC and its collapsibility, rather than arbitrarily assigning a fixed RA pressure.

In 1994, we described a new formula for echo-Doppler estimation of RV systolic pressure based on tricuspid regurgitation and IVC collapsibility.[9] In 110 patients, the new method based on IVC collapsibility correlated better than the old ones with invasive RA, RV, and pulmonary pressures. In brief, we suggested that RA pressure may be better defined assigning 6, 9, and 16 mmHg, when IVC collapsibility was >45%, between 35-45% and <35%, respectively. Consequently, based on these IVC indexes, different RA pressure values may be added to the tricuspid gradient. Several papers confirmed the importance of IVC collapsibility and redefined the method considering IVC dimensions also. Guidelines[56] suggested that IVC diameter <2.1 cm that collapses >50% with a sniff suggests a normal RA pressure of 3 mmHg (range, 0-5 mmHg); whereas, an IVC diameter >2.1 cm that collapses <50% with a sniff suggests a high RA pressure of 15 mmHg (range, 10-20 mmHg). In indeterminate cases in which the IVC diameter and collapse do not fit this paradigm, an intermediate value of 8 mmHg (range, 5-10 mmHg) may be used, or, preferably, secondary indices of elevated RA pressure should be integrated. These include restrictive right-sided diastolic filling pattern, tricuspid E/E’ ratio >6, and diastolic flow predominance in the hepatic veins (which can be quantified as a systolic filling fraction <55%). In indeterminate cases, if none of these secondary indices of elevated RA pressure are present, RA pressure may be downgraded to 3 mmHg. If there is minimal IVC collapse with a sniff (<35%) and secondary indices of elevated RA pressure are present, RA pressure may be upgraded to 15mmHg. If uncertainty remains, RA pressure may be left at the intermediate value of 8 mmHg. In patients who are unable to adequately perform a sniff, an IVC that collapses <20% with quiet inspiration suggests elevated RA pressure. This method of assigning an RA pressure is preferable to assuming a fixed RA pressure value for all patients.

3D EXAMINATION OF THE RV

Because of the peculiar RV morphology and function, 2D echocardiography has several limitations in the evaluation of the right ventricle, which can be readily overcome by a 3D echo gated wide-angled acquisition, which enables complete assessment of its geometry, volumes, and EF. The right ventricle is composed of three anatomic and functional subunits, which extend from the tricuspid valve annulus to the proximal in fundibulum, the RV body to the apex, and the RV outflow tract to the pulmonary valve. This divides the RV cavity into three sections: Inlet, apical trabecular, and outlet, respectively.

Several methods and software packages have been used to evaluate the right ventricle.[10] 3D data are generally acquired in a full-volume dataset from the four-chamber apical view adapted to include the entire right ventricle. 3D echocardio graphic data sets are typically digitally stored and then post-processed offline. After full-volume acquisition, current RV analysis software displays 2D cut planes of the RV sagittal, four-chamber, and coronal views; allowing calculation of RV volumes from end-diastolic and end-systolic endocardial border tracings of these three views. The RV volumes are calculated by summation of the volumes for each slice through the complete data set. Curves of global and regional RV function may be generated and analyzed [Figure 2].

Figure 2

Examples of 3D right ventricular imaging. Top left: 3D imaging of a normal RV; top right: 3D imaging of a severe dilatation and hypertrophy of the RV. Bottom: Images of RV assessment by 3D reconstruction. The inflow, outflow, and apical segments of the RV are clearly delineated. Curves of global function and RV ejection fraction are also shown (bottom right panel). 3D = Three dimensional

Several studies utilized this method in normal and pathologic heart to evaluate feasibility and accuracy. In 2008, Tamborini et al.,[11] investigated the feasibility of transthoracic 3D RV analysis in 200 subjects (48 normal, 104 patients with valvular heart disease, 20 patients with idiopathic dilated cardiomyopathy, and 28 patients with pulmonary hypertension). This study compared and correlated 3D RV data with classic 2D and Doppler parameters, including TAPSE and PSV on TDI, RV FAC, RV stroke volume by the Doppler method, and SPAP. 3D-derived RV diastolic and systolic volumes were 103 ± 38 and 46 ± 28 mL, respectively. RV EF was correlated negatively with SPAP and positively with TAPSE, PSV, and FAC. 3D echocardiography showed that patients with pulmonary hypertension had the largest RV volumes and the lowest RV EF and those with idiopathic dilated cardiomyopathy were characterized by lower RV EF than those patients with valvular disease. At least one good 3D acquisition of the RV was achieved in all individuals in a mean time of 3 ± 1 min and the RV image quality was good in most individuals (85%). Thus, the investigators concluded that the new quantitative 3D method to assess RV volumes and function was feasible, relatively simple, and not time consuming.

More recently, another study provided normal reference values for RV volumes and function that may be useful for the identification of clinical abnormalities.[12] The mean RV end-diastolic and end-systolic volumes were 49 ± 10 and 16 ± 6 mL/m2, respectively, and the mean RV EF was 67 ± 7%. Significant correlations were observed between RV parameters and body surface area. Normalized RV volumes were significantly correlated with age and gender. RV EF was lower in men, but differences across age deciles were not evident. In a larger population and a multicenter study[13] RV volumes (end-diastolic volume and end-systolic volume), stroke volume, and EF were measured by 3D echocardiography. In 540 healthy adult volunteers, prospectively enrolled, and evenly distributed across age and sex the relation of age, sex, and body size parameters was investigated in detail using bivariate and multiple linear regression. Analysis was feasible in 507 (94%) subjects (260 women; age, 45 ± 16 years; and range, 18-90). Age, sex, height, and weight significantly influenced RV volumes and EF. Sex effect was significant (P < 0.01), with RV volumes larger and EF smaller in men than in women. Older age was associated with lower volumes (end-diastolic volume, −5 mL/decade; endsystolic volume, −3 mL/decade; and EF, −2 mL/decade) and higher EF (+1% per decade). Inclusion of body size parameters in the statistical models resulted in improved overall explained variance for volumes (end-diastolic volume, R2 = 0.43; end-systolic volume, R2 = 0.35; and stroke volume, R2 = 0.30), while EF was unaffected. Ratiometric and allometric indexing for age, sex, and body size resulted in no significant residual correlation between RV measures and height or weight.

These 3D references values confirmed 2D standard measurements showing that gender and body surface area are important determinants of RV dimensions and systolic function as measured on 2D echocardiography.[14]

ADVANTAGES OF 3D VS 2D EXAMINATION IN SPECIFIC DISORDERS

In 2009, Tamborini et al.,[15] evaluated RV function in 40 patients with mitral valve prolapse undergoing surgical valvular repair and to compare and correlate 3D RV EF with 2D derived TAPSE and PSV of tricuspid annulus before and after surgery. Transthoracic 2D and 3D echocardiography were performed presurgery and 3, 6, and 12 months postsurgery. TAPSE (15.5 ± 3, 16.5 ± 3, and 18.5 ± 4 mm at 3, 6, and 12 months, respectively) and PSV of the tricuspid annulus (11.9 ± 2, 12 ± 2, and 12.8 ± 3 cm/s at 3, 6, and 12 months, respectively) were significantly lower after surgery in comparison with presurgical values. On the contrary, preoperative RV EF (58.4 ± 4%) did not change after surgery (56.9 ± 5, 59.5 ± 5, and 58.5 ± 5% at each step). This explains why in the literature after cardiac surgery (independently on the type of surgery) TAPSE is always reduced, even though a damage and dysfunction in all cases is not reasonable. Therefore in a patient with previous cardiac surgery, TAPSE is not a surrogate of RV systolic function and 3D echocardiography may overcome this limit.

Moreover in several clinical conditions such as congenital heart disease, artificial heart, and LV device assistance; the value of 3D echocardiography has been proved.[1016]

CMR FOR RV EVALUATION AND CORRELATIONS BETWEEN CMR AND 3D ECHOCARDIOGRAPHY

CMR is the second-line imaging modality after echocardiography for comprehensive RV evaluation. This technique is well suited for the visualization of RV anatomy and the quantitative measurement of RV parameters (dimensions, mass and function) because of the variable configuration of this chamber that requires a 3D volume acquisition.

CMR-derived RV volumes show good correlation with in vivo standards, and this technique has shown optimal accuracy and reproducibility for RV measurements;[171819] on the basis of these characteristics, CMR has become the clinical reference technique for accurate assessment of global RV function. Short-axis images and the summation disc method are used for calculation of RV volumes and EF without any geometrical assumption and RV normal clinical ranges for these parameters have been established from both a spoiled gradient echo sequence[2021] and more recently, from the steady-state free precession technique[2223] that yields significantly improved blood-myocardium contrast, acquisition speed, and the ability to greatly enhance the temporal resolution of the cines with improved image quality.

CMR also provides advanced imaging of the RV myocardium that includes tissue characterization. Different T1- and T2-weighted sequences combined with late-enhancement imaging after gadolinium administration can be used for the assessment and differentiation of cardiomyopathies including arrhythmogenic RV cardiomyopathy, metabolic storage diseases, and cardiac tumors.[242526] Late enhancement imaging shows intramyocardial fibrosis, inflammation, and scars; and it has been demonstrated to be important in various clinical settings as pulmonary hypertension or ischemic RV disease.[272829]

Given that CMR represents the reference technique for the RV comprehensive evaluation and has been used as the gold standard for quantitative RV assessment; however, its widespread use in clinical settings is limited because it is expensive, time-consuming, and sometimes contraindicated. 3D echocardiography is considered theoretically ideal for estimation of RV volumes and function, otherwise not feasible by standard 2D echocardiography. This new echocardiographic method has been reported to be in good agreement with validation studies in vitro and in vivo.[3031323334] In clinical studies, however, it has often been reported that, mostly because of technical factors[35] and despite the excellent correlation, 3D echocardiography underestimates RV volumes compared with CMR. The accuracy of the 3D echo method has been analyzed in healthy individuals and in patients with different pathologies.

Gopal et al.,[36] investigated the accuracy of 2D and 3D echocardiography with both disc summation and apical rotation methods compared to CMR for the assessment of RV volume and function in 71 normal subjects. 3D echocardiography with the disc summation technique showed the least underestimation of, and thus the best agreement with CMR derived RV volumes. Also, in this study, normal reference ranges of RV indexed volumes, stroke volume, and EF were reported.

In 2006, Prakasa et al.,[37] evaluated 58 patients with RV dysplasia concluding that 3D echocardiographic measurements of RV volumes and EF closely correlate with CMR values and may be useful in the follow-up of patients with arrhythmogenic RV dysplasia. However, real-time 3D echocardiography underestimated CMR- derived RV volumes, especially end-systolic volumes, significantly.

Jenkins et al.,[38] investigated in 50 patients with left ventricular wall motion abnormalities whether 3D echocardiography was superior to 2D echocardiography for the follow-up of RV function. In this study, RV volumes and EF were studied with 2D echocardiography with three different methods (area-length, the modified 2D subtraction method, and the Simpson method of discs). CMR was again used as a reference standard. CMR-derived RV end-diastolic and end-systolic volumes were slightly underestimated by 3D echo with a greater mean difference for 2D echocardiography with the area length method and the Simpson method of discs, and were greatly overestimated by the 2D subtraction method.

In 2010, Grewal et al.,[16] demonstrated that 3D echocardiography compared with CMR was both accurate and reproducible for assessing RV volumes and EF in adult patients with Tetralogy of Fallot and severe pulmonary regurgitation.

In 2010, Sugeng et al.,[39] undertook volumetric analysis of the RV by real-time 3D echocardiography, CMR, and cardiac computed tomography (CCT) on images obtained in RV-shaped phantoms and in patients with a wide range of pathologies. The in vitro measurements showed that volumetric analysis of CMR images yielded the most accurate measurements; CCT measurements showed slight (4%) but consistent overestimation; and 3D measurements showed small underestimation, but considerably wider margins of error. In humans, both 3D echo and CCT measurements correlated highly with the CMR reference and showed the same trends of underestimation and overestimation noted in vitro.

These observations confirm the fact that evaluating the RV morphology is challenging, in part because of its particular crescent shape, leading us to wonder whether it is valuable to search for strict correlations between dimensions obtained with different imaging techniques.