Summary
Improvements in cardiac imaging have recently focused a great interest on the right ventricle (RV). In patients with congenital heart disease, the right ventricle (RV) may support the systemic circulation (systemic RV). There are 2 different anatomic conditions providing such physiology: the congenitally corrected transposition of the great arteries (ccTGA) and the TGA surgically corrected by atrial switch. During the last decades, evidence is accumulating that progressive systemic RV failure develops leading to considerable morbidity and mortality. Various imaging modalities have been used to evaluate the systemic RV, but echocardiography is still predominantly used in clinical practice, allowing an anatomic and functional approach of the systemic RV function and the potential associated anomalies. The goal of this review is to offer a clinical perspective of the non-invasive evaluation of the systemic RV by echocardiography.
Résumé
Les améliorations de l’imagerie cardiaque ont permis de se focaliser récemment sur l’étude du ventricule droit. Chez certains patients porteurs d’une cardiopathie congénitale, le ventricule droit peut se retrouver en position systémique. C’est le cas des patients porteurs d’une double discordance et patients porteurs d’une transposition des gros vaisseaux opérés par un switch à l’étage atrial. Durant les dernières décennies, les données scientifiques ont montré une dégradation progressive du ventricule droit systémique, qui est associée à une morbidité et une mortalité importante. Différentes modalités d’imagerie ont été utilisées pour évaluer le ventricule droit systémique mais l’échographie reste la méthode de choix dans l’activité quotidienne, permettant une analyse anatomique et fonctionnelle. L’objectif de cette revue est d’offrir une perspective clinique de l’évaluation non invasive du ventricule droit systémique par échocardiographie.
Introduction
Improvements in cardiac imaging have recently focused great attention on the right ventricle (RV), emphasising its importance in cardiac physiology. The RV has been studied in many physiopathological conditions, such as pulmonary hypertension and heart failure related to left ventricular (LV) dysfunction.
Specific congenital heart diseases (CHD) may provide an original condition where the RV is pumping through the systemic circulation. In these anatomical and haemodynamic conditions, the RV is called systemic RV (systemic meaning pumping through the systemic circulation as opposed to the pulmonary circulation). There are two different anatomical conditions that provide such physiology.
The first condition is congenitally corrected transposition of the great arteries (ccTGA), where the left atrium is connected to the RV, pumping in the aorta, and where the right atrium is connected to the left ventricle (LV), pumping in the pulmonary artery branches. The other condition is related to atrial switch, which is the surgical treatment developed in the late 1950s by Ake Senning and in the mid-1960s by William Mustard for surgical correction of transposition of the great arteries (TGA). In terms of live birth incidence, TGA is the more common condition (TGA 1:3100 live births vs ccTGA 1:33,000), but the absolute number of Mustard and Senning patients is decreasing because atrial redirection procedures for TGA were superseded by the arterial switch operation in the 1980s. As a consequence, almost all patients with TGA and atrial redirection reach adulthood.
For both populations, right ventricular (RV) failure is an important long-term concern, leading to severe late complications . Various imaging modalities have been used to evaluate the systemic RV, including angiography, radionuclide imaging and magnetic resonance imaging (MRI) . Nevertheless, in clinical practice, echocardiography is still used predominantly for the assessment of RV function, as it is non-invasive, widely available, relatively inexpensive and has no adverse side effects. However, because of complex geometry, the assessment of systemic RV function by echocardiography has remained mostly qualitative. Advances in digital echocardiography allow for a more refined assessment of the RV, as demonstrated in patients with pulmonary arterial hypertension and other clinical conditions. These novel echocardiography variables may also be valuable in the functional assessment of the systemic RV . The goal of this review is to offer a clinical perspective of the non-invasive evaluation of the systemic RV by echocardiography.
Echocardiographic assessment of systolic function in the systemic RV
Why the RV is different
By analogy with the LV, RV ejection fraction (RVEF) is considered to be the marker of RV function. However, attempting to extend this to the RV has been problematic. The shape of the RV does not allow the use of geometric formulae to calculate the RVEF. A range of echocardiographic variables has therefore been developed to evaluate RV function in the subpulmonary position, especially simple measurements of long-axis excursion, giving rapid and unambiguous results , which has proven utility as a measure of LV function in patients with coronary artery disease, valve disease and heart failure . The technique is equally straightforward for the RV, and is especially valid because most of the RV myocardial fibres are arranged longitudinally. These variables have been compared with other methods of calculating the RVEF, especially cardiac MRI, which is considered to be the gold standard for RV functional assessment . Logically, these echocardiographic variables have been used subsequently for systemic RV functional assessment ( Fig. 1 ).
Basic longitudinal function variables
The first studies focused on the analysis of longitudinal myocardial fibres using mainly tricuspid annular plane systolic excursion (TAPSE), based on the anatomical hypothesis that the majority of RV myocardial fibres originate at the apex of the heart and insert into the right atrioventricular junction, such that the main bulk of the RV myocardium is composed of longitudinally arranged fibres . Derrick et al. showed that systemic RV long-axis function was notably reduced compared with that of either the normal subpulmonary RV or the systemic LV. The authors hypothesized that impaired systemic RV longitudinal function reflected the response of the longitudinally arranged myocardial fibres to chronic increased afterload , but they underlined that the systemic RVEF could remain remarkably constant over long-term follow-up , and that there were no “normal” RVEF values under these circumstances. TAPSE was further compared with cardiac MRI when the technique became the gold standard for RVEF measurement in the early 2000s. Lissin et al. showed rather good correlation between TAPSE and MRI-based systemic RVEF in a small cohort of patients with TGA and previous atrial switch procedure.
Basic longitudinal function variables
Tissue Doppler imaging and tissue Doppler-derived strain and strain-rate imaging
Tissue Doppler imaging (TDI) is an echocardiographic technique that uses Doppler principles to measure the velocity of myocardial motion. TDI can be performed in pulsed-wave and colour modes. Pulsed-wave TDI is used to measure peak myocardial velocities, and is particularly well suited to the measurement of long-axis ventricular motion, because the longitudinally oriented endocardial fibres are most parallel to the ultrasound beam in the apical views, and it is a good surrogate measure of overall longitudinal RV contraction and relaxation. Strain and strain rate (SR) are TDI-derived modalities. SR measures the rate of deformation of a tissue segment, and is measured in s −1 . Peak systolic SR represents the maximal rate of deformation in systole. Strain is obtained by integrating the magnitude of deformation between end-diastole used as a reference point and end-systole. Isovolumic myocardial acceleration (IVA) is also a TDI-based index of contractile function, which has been applied to RV function assessment because it was shown experimentally not only to measure contractile function accurately, but also to be relatively independent of acute changes in ventricular preload and afterload . Vogel et al. confirmed reduced systolic contractile function in the systemic RV of Mustard and Senning patients. The ejection phase indexes, S wave velocity and acceleration, were both reduced compared with both systemic LV and subpulmonary RV in controls. Furthermore, in a subset of 12 patients with simultaneous conductance catheter measurements, IVA correlated with the change in elastance associated with dobutamine stress . These data were confirmed in a population of asymptomatic patients with ccTGA compared with subpulmonary RV in normal patients assessed by peak systolic SR and peak systolic strain .
Strain and SR imaging are useful for differentiating active and passive movement of myocardial segments, and to quantify and evaluate components of myocardial function, such as longitudinal myocardial shortening, which are not visually assessable; they allow comprehensive assessment of myocardial function, which is of particular importance for systemic RV assessment, considering the specific haemodynamic conditions. However, TDI-derived strain and SR have several disadvantages related to angle-dependency and the limited spatial resolution imposed by imaging at high temporal resolution. Other disadvantages of the TDI-derived strain and SR imaging techniques are the time-consuming steps for data acquisition and processing and the need for expert readers. Taking into account all of the factors mentioned above, it is not surprising that TDI-derived strain and SR measurements are not highly reproducible , as with IVA, which carries even higher variability . As a consequence, even if these variables are very interesting in terms of helping us to understand cardiac physiology, their applicability in clinical practice remains to be proved.
Speckle-tracking-derived two-dimensional strain imaging
Speckle-tracking echocardiography (STE) is a newer technique for obtaining strain and SR measurements, which analyses motion by tracking speckles (natural acoustic markers) in the two-dimensional (2D) ultrasonic image. These markers (“stable” speckles) within the ultrasonic image are tracked from frame to frame. Special software allows spatial and temporal image processing, with recognition and selection of such elements on ultrasound images. The geometric shift of each speckle represents local tissue movement. By tracking these speckles, strain and SR can be calculated. The advantage of this method is that it tracks in two dimensions, along the direction of the wall, not along the ultrasound beam, and thus is angle independent. STE requires only one cardiac cycle to be acquired, but strain and SR data can be obtained only with high-resolution image quality at a high frame rate. The need for high image quality is a potential limitation to routine clinical applicability in all patients. STE was recently introduced for assessment of RV function in patients with TGA , in the context of TDI-derived strain and SR imaging, suggesting that deformation-based variables may provide incremental prognostic information over standard variables in this high-risk population. In concordance with earlier studies that used TDI , it has been reported that STE-derived deformation variables of the RV are depressed in patients with TGA and atrial switch ( Fig. 2 ). A recent study confirmed these preliminary results, showing that systemic RV global longitudinal strain was able to discriminate TGA patients after atrial switch with and without a cardiac MRI systemic RVEF ≥ 45% .
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