Right ventricular function: the comeback of echocardiography?

Please see page 104 for the article by van den Bosch et al. ( doi: 10.1016/S1525-2167(03)00048-9 ) and page 123 for the article by D'Andrea et al. ( doi: 10.1016/S1525-2167(03)00053-2 ) to which this editorial pertains. Right ventricular (RV) function has been a puzzle to clinicians for a long time. Originally considered of not much more than a conduit and reservoir, the importance of the RV in both acquired as well as congenital heart disease has become increasingly clear. RV function is an important factor in outcome of patients with various types of congenital heart disease, including those operated for tetralogy of Fallot, and those with the right ventricle supporting the systemic circulation. 1,2 In acquired heart disease RV function is of particular importance in patients with pulmonary hypertension from various causes, and in RV infarction. 3 Considering the importance of RV function, there has been a need for a simple, reproducible, accurate and easy-to-use method to assess systolic and diastolic RV function, and for assessment of tricuspid and pulmonary valve function. Conventional echocardiography fulfilled the criteria of a simple, bedside tool. However, 2D echocardiography has not been an accurate technique for assessment of systolic RV function. Well known problems have been limitations to image the partly retrosternal RV and the poor demarcation of the heavily trabeculated RV endocardial border. Most important limitation has been the complex shape of the RV. Volume calculations of the RV are the basis for quantification assessment of RV systolic function. Of various models used to approach the complex RV shape, none has found widespread acceptance, because of intrinsic inaccuracies and inability to cover the wide range of RV shapes with different RV loading conditions. 4 Non-volumetric measures, such as the excursion of the tricuspid annular plane during systole (TAPSE) have provided an alternative way of estimating global systolic function. Again, this method has not found widespread clinical use, in part because of its load dependence. The same is true for the ‘myocardial performance’ index. 5 Many of the limitations of 2D echo for assessment of RV function have been overcome with 3D techniques, such as 3D echo or MRI. MRI has been considered the ‘gold standard’ for non-invasive assessment of RV function. 6 With the recent commercial launch of on-line 3D echocardiography, renewed interest in volumetric assessment of the RV can be expected. With many echocardiographic techniques, endocardial border detection is important. For the LV, contrast echocardiography has been an important tool to improve border detection. 7 In the current issue of EJE, Van den Bosch and co-workers report their experience with contrast echocardiography for the right ventricle. 8 Not surprisingly, this initial report of RV contrast echocardiography is from a congenital cardiology group, underlining the importance of RV function in this growing population. Van den Bosch et al. clearly demonstrate the improvement that can be obtained in visualisation of the RV with a combination of second harmonic imaging, reduced mechanical index setting and a second generation contrast agent (SonoVue). 7,8 This was true for direct and for transpulmonic injection of contrast agent, in the group of patients with tetralogy of Fallot and with systemic RV group, respectively. In 11 of 13 RV segments, imaged from the apical 4 chamber view and from 3 different levels of parasternal short axis views, visualisation of the RV endocardial border improved with contrast, resulting in a different visual estimate of RV contraction pattern (RV function) in 55% of the patients. In all patients this change was towards a better RV function. The largest change was established in the patients with poorest echo windows. In 7 out of 20 patients there were problems in obtaining adequate images in all required views, particularly in the parasternal short axis views. This finding demonstrates that contrast enhances images, it does not allow visualisation of structures that cannot be included in the acoustic window. The primary importance of the observations by Van den Bosch is that enhancement of RV visualisation can be obtained with echo contrast. However, the study of Van den Bosch does not solve the main problem of 2D echo of the RV, that is that it can at best be used to ‘eyeball’ the function of the RV. This clearly calls for studies of the use of contrast media in 3D echo. Diastolic function of the RV is generally assessed from transtricuspid and caval vein Doppler signals. Contrast media have also been demonstrated to improve echo Doppler signals. 7 Van den Bosch et al. did not test this for the RV. One of the reasons may be that transtricuspid Doppler signals are of limited value in the setting of pulmonary regurgitation, as is commonly the case in tetralogy of Fallot. In the other patient group in their study, caval vein flow patterns may be altered by atrial baffles as occur in atrially corrected transposition of the great arteries. The limitations of 2D and conventional Doppler echocardiography for assessment of RV function have resulted in a search for alternative methods. MRI has become the technique of choice. 9 MRI allows accurate volumetric assessment of RV volumes, ejection fraction and mass. Blood flow can be volumetrically assessed, allowing quantification of diastolic RV function, even if pulmonary regurgitation is present. However, MRI has clear limitations, such as relatively limited temporal resolution, time consuming data acquisition and analysis, high costs and relatively limited availability. Recently, a new means of assessing ventricular function has become available: Doppler tissue imaging (DTI). DTI allows quantification of regional and local myocardial velocities, either by color Doppler or pulsed Doppler technique. Color DTI superimposes wall motion velocity on 2D echo image. Color DTI provides rapid qualitative assessment of wall dynamics, differentiation of velocity between subendo- and epicardial layers, analysis of various myocardial regions simultaneously and good spatial resolution, but has limited temporal resolution. Pulsed DTI allows imaging of high quality Doppler signals that provide measurements of velocity, mean and instantaneous local acceleration. Limitations are the poor spatial resolution and the impossibility to simultaneously record different wall segments. 10 In the current issue of EJE the use of DTI to assess RV function is addressed by D'Andrea and co-workers. 11 In their report of a highly selected group of adult patients operated for tetralogy of Fallot, RV function was assessed with conventional 2D and Doppler echocardiography, and with pulsed DTI. Systolic as well as early and late diastolic regional peak velocities were assessed at the tricuspid valve annulus. Pulsed DTI was also used to measure precontraction time and interventricular activation delay (the difference of precontraction time between RV and LV). It was demonstrated that tetralogy of Fallot patients had lower peak systolic and peak early diastolic velocities, increased RV precontraction time and increased interventricular delay. 11 Increased interventricular delay was associated with an increased risk of ventricular arrhythmias. 11 Decreased RV early diastolic velocity and increased interventricular delay were associated with decreased maximal workload. 11 The results of the study by D'Andrea et al. confirm the usefulness of DTI to study regional patterns of ventricular contraction. The finding of an increased interventricular activation delay could be interpreted, as the authors do, as evidence for impaired synchronicity between LV and RV. This may be related to inhomogeneous electrical activity. In an earlier study Weidemann et al. also used DTI in Fallot patients, and found that peak systolic strain rate (a measure of the local rate of myocardial deformation) of the RV free wall correlated with QRS duration. 12 Prolonged QRS duration and other ECG parameters of inhomogeneous distribution of electrical activity have been associated with increased risk for ventricular tachycardia in tetralogy of Fallot. 13 However, the assessment of electrical inhomogeneity from the surface ECG is cumbersome and time consuming. The DTI technique as described by D'Andrea et al. might provide a more practical means to detect electrical inhomogeneity related to ventricular dyssynchrony. Another important consequence of their work may be the detection of diastolic RV dysfunction in the setting of PR with the use of a relatively simple pulsed DTI measure (RV early diastolic peak velocity). RV diastolic dysfunction in patients operated for tetralogy of Fallot is one of the factors affecting long-term outcome. 14 However, the interpretation of the reduced tricuspid valve annulus early diastolic peak velocity, that has also been observed in the study by Weideman et al. in Fallot patients, is difficult. 12 We do not know how this velocity is affected by the presence of various factors including, among others, (a) a transannular patch, common after Fallot repair, (b) differences in loading conditions, as may occur with differences in amounts of residual PR or PS, (c) myocardial factors, such as the amount of fibrosis or (d) differences in fiber orientation that exist between normals and Fallot patients. In fact, caution should be observed in the interpretation of any DTI data. The direct quantification of myocardial velocities as measures of ventricular function has been questioned, since myocardial velocities quantify regional myocardial motion. Regional myocardial motion is influenced not only by actual contraction, but also by overall heart motion, cardiac rotation and motion induced by contraction in adjacent segments. 12 At present, 3 different approaches to analysis of pulsed TDI measurements are taken: (a) direct analysis of velocity data, as in the current study by D'Andrea et al., 11 (b) assessment of acceleration during isovolumic acceleration, and (c) calculation of strain rate and strain, as assessed from velocity measurements from different locations in the myocardium. Perhaps the best validated DTI derived parameter is acceleration during isovolumic contraction. 15 Even for this index, validated against invasive indices of RV contractility, questions remain as to the reproducibility in various patient groups and the validity of an index of ventricular contractility based on velocity data derived from a very limited part of the ventricular wall. Others have advocated the use of regional deformation, such as assessed with strain (rate) imaging, rather than regional velocity. This may best be performed in 3 dimensions, an option that is not currently available with echocardiography. 16 Nevertheless, both the use of contrast for RV image enhancement and the introduction of DTI in RV function assessment are promising new tools. Further studies, including serial follow-up data in patients and further validation of DTI measurements will tell us if echocardiography has really made its comeback for RV function assessment.