Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Cardiovascular Magnetic Resonance Assessment of Right Ventricular Anatomy and Function
Accurate noninvasive assessment of right ventricular (RV) mass and systolic function is important in several pathologies, such as grown-up congenital heart disease, pulmonary hypertension, interstitial lung disease, valvular heart disease, and arrhythmogenic RV cardiomyopathy. Right ventricular function is also a prognostic factor in coronary artery disease and heart failure, even after cardiac resynchronization therapy. This chapter aims to summarize the features of the normal right ventricle, briefly describe cardiovascular magnetic resonance (CMR) techniques for assessing RV dimensions and function, and give reference values for the assessment of the right ventricle.
Normal Right Ventricular Anatomy
The right ventricle is a thin, highly trabeculated structure that is triangular in form and, on gross inspection, appears to be wrapped around the left ventricle. It has three well-differentiated components with specific structure and function. The inlet portion extends from the tricuspid valve to the insertions of the papillary muscles on the ventricular wall, the trabecular portion involves the RV body and apex and is the main pumping component, and the outlet portion or infundibulum extends up to the pulmonary valve and has a thin, nontrabeculated wall ( Fig. 39.1 ). The anterosuperior wall of the right ventricle is rounded and convex, its inferior surface is flattened and forms a small part of the diaphragmatic surface of the heart, and its posterior wall is formed by the ventricular septum, which bulges into the right ventricle, owing to the much greater left ventricular (LV) systolic pressure, so a transverse section of the cavity presents a semilunar outline. The right ventricle has a continuum of muscle bands that rotate by approximately 160 degrees from the epicardium to the endocardium. The principal axis of these fibers is oblique to the long axis of the right ventricle. RV contraction is then more dependent on longitudinal shortening than that of the LV. In the normal adult, the total RV free wall mass is 21 ± 13 g/m 2 .
The right ventricle has several distinctive features. In its upper left portion, there is a conical pouch called the conus arteriosus or infundibulum, from which the pulmonary artery arises. A tendinous band connects the posterior surface of the conus arteriosus to the aorta. Also, as a subpulmonary ventricle, the RV has a lower ejection fraction and thinner walls than the left ventricle. RV wall thickness is usually 3 to 5 mm, the proportion between right ventricle and left ventricle being as 1 to 3 ; it is thickest at the base and gradually becomes thinner toward the apex. The whole inner surface except the conus arteriosus is covered by more or less prominent muscular columns called trabeculae carneae and from some of them (papillary muscles), the chordae tendineae connect the myocardium to the tricuspid valve, with its septal leaflet more apically placed than the septal leaflet of the mitral valve. The anterior tricuspid valve leaflet is usually the largest and extends from the infundibular region anteriorly to the inferolateral wall posteriorly; the septal leaflet extends from the interventricular septum to the posterior ventricular border; the posterior leaflet attaches along the posterior margin of the annulus from the septum to the inferolateral wall. Finally, a muscular band frequently extends from the base of the anterior papillary muscle to the ventricular septum. This band is considered to prevent overdistension of the ventricle and is called the moderator band.
The depictions of the moderator band, the infundibulum, and the different levels of insertion of the tricuspid and mitral septal leaflets are important diagnostic features for identification of the right ventricle, which can be difficult in some congenital cardiomyopathies.
Importance of Assessing Right Ventricular Dimensions and Function
The measurement of RV dimensions, morphology, and function is important in several situations, such as congenital heart disease, LV heart failure, pulmonary hypertension, pulmonary embolism, valvular heart disease, lung disease, and arrhythmogenic RV cardiomyopathy.
RV failure may result from conditions that lead to impaired RV contractility, such as RV infarction, right-sided cardiomyopathies, valvular heart disease or severe sepsis; to RV pressure overload, including pulmonic stenosis, pulmonary primary hypertension, and pulmonary hypertension secondary to left heart disease, lung disease, or thromboembolic disease; and to RV volume overload, for instance, tricuspid or pulmonary regurgitation. Many disorders, such as corrected and uncorrected adult congenital heart disease and intracardiac shunts, may result in right ventricle failure through a combination of pathophysiologic mechanisms. Also, decompensated right ventricle (both acute and acute-on-chronic) is increasing as the prevalence of predisposing conditions grows.
The prognostic value of RV function has been shown in several conditions such as LV heart failure, valvular heart disease, pulmonary hypertension, congenital heart disease, or myocardial infarction (MI). Thus the early detection of RV dysfunction can have an impact on therapeutic decision making and on prognosis. Finally, improved understanding of the RV response to pressure and volume overload might lead to more optimal surgical and medical treatments.
Techniques for Assessing Right Ventricular Dimensions and Function
Global RV function has been traditionally difficult to assess, given its irregular shape, which cannot be assumed to any geometrical model. Several imaging techniques have been used in the past to assess RV dimensions and function. The chest radiograph is a simple method to assess RV size with the cardiothoracic ratio, but this may be misleading because an enlarged RV may compress the LV, resulting in a normal cardiothoracic ratio. Angiography used to be the gold standard for assessment of RV volumes and regional and global function. But this technique is invasive, involves ionizing radiation and the use of potentially nephrotoxic contrast, and is not as accurate as CMR. Echocardiography and radionuclide ventriculography have been used for the assessment of RV dimensions and function. More recently, “nongeometric” techniques such as three-dimensional (3D) echocardiography, CMR, and multidetector-row computed tomography (CT) permit accurate assessment of RV volumes, function, and mass.
Echocardiography
Echocardiography is the most frequently used technique for assessing the right ventricle; it is cheap, widely available, and can be used bedside in very ill patients. It provides information about RV morphology, dimensions, septum convexity, function, tricuspid regurgitation, and estimates of pulmonary arterial pressure and RV pressure. But the assessment of the right ventricle with echocardiography has several limitations. First, the location of the right ventricle behind the sternum restricts the window that can be accessed by the ultrasound beam. Second, the complex shape and thin walls of the right ventricle make it necessary to image the right ventricle from several projections, although the short-axis view is usually the most helpful. Third, the thick trabeculations in the chamber may be confused with a thrombus, a tumor, or hypertrophic cardiomyopathy. Finally, there is a lack of accurate mathematical models to quantify RV mass and volumes with M-mode or two-dimensional echocardiography (2DE) because quantitative measurements are based on geometric assumptions that do not apply to the right ventricle. A qualitative approach for RV volume assessment is usually applied, with the RV size being described as either normal or mildly, moderately, or severely enlarged. If the RV is the same size as the LV, it is usually characterized as moderately enlarged, and if larger than the LV, it is severely enlarged. Also, qualitative evaluation of RV function is usually applied with RV characterized as normal or mildly, moderately, or severely dysfunctional. An approximation to quantification has been introduced with M-mode measurement of the tricuspid annular plane systolic excursion (TAPSE). This parameter provides a rough estimate of RV function but it is not angle independent: it only takes into account the longitudinal function of the RV, which is the predominant, but not unique, component of RV systolic function, and, because it is measured at the basal segment, the presence of regional wall motion abnormalities will affect its accuracy. The fractional area change in the four-chamber view has also been used but, again, this takes into account only the lateral free wall and the presence of regional wall motion abnormalities or RV dilatation might affect its accuracy ( Fig. 39.2 ). Other indicators of RV function are flow Doppler-derived indices such as the myocardial performance index, tissue Doppler measurements of myocardial tricuspid annular myocardial velocities and time intervals, and strain and strain rate measurements of contractility whether with tissue Doppler or speckle tracking imaging (STI). Tissue Doppler is an angle-dependent technique, and incorrect alignment with the ultrasound beam, poor signal-to-noise ratio (SNR) and variability because of placement of the region of interest may affect the results. Speckle tracking imaging is a promising technique that has been applied to the right ventricle to measure longitudinal strain in the six segments in which the right ventricle is classically divided. This technique is useful for the diagnosis of various right heart diseases, but it may have problems in the case of thin wall or poor image quality in the RV lateral wall, and there is variability across vendors. Doppler transesophageal echocardiography is another echocardiographic method of RV assessment, but it is semiinvasive, is not well suited for evaluation of anteriorly positioned right ventricles, and requires special skills. Three-dimensional echocardiography (3DE) has emerged as a more accurate and reproducible approach to ventricular quantitation, mainly by avoiding the use of geometric assumptions of the ventricular shape. Real-time 3DE is an online acquisition of a 3D dataset of the heart without the need for electrocardiographic and respiratory gating. It calculates right ventricular ejection fraction (RVEF) using the volumetric semiautomated border detection method, which needs to be manually adjusted, and after acquisition and display of end-diastolic and end-systolic frame, long-axis, planes, and volumetric data of the right ventricle are analyzed offline. Eventually, curves of regional and global RVEF are produced and analyzed. However, there are practical problems, such as full cardiac visualization, good-quality endocardial border recognition for manual endocardial tracing, and time consumption. Also, these methods need a stable cardiac rhythm and constant cardiac function during image acquisition, although new single-beat 3DE techniques allow a fast acquisition in one short breath-hold. 3DE has been compared with CMR for the evaluation of RV function, and improved results in comparison with 2D echocardiography have been obtained, both with the usual technique and with the new single-beat 3DE, with consistent underestimation of RV volumes and ejection fraction with 3DE. Although 3DE has been used mainly for the left ventricle, assessment of the right ventricle with 3DE is feasible during routine standard echocardiography.
Radionuclide Angiography
This technique provides a reliable quantitative measurement of ventricular function not based on geometric assumptions with good agreement with CMR. Although echocardiography and CMR are the two most commonly used imaging techniques for noninvasive assessment of the right ventricle, nuclear imaging provides new opportunities for comprehensive evaluation of the right ventricle from a single study, because it can assess RV perfusion and metabolism as well as morphology and ejection fraction. Some years ago this technique did not work well for the right ventricle, owing to problems such as the limited count numbers in this chamber and the overlap of other cardiac chambers. Currently, gated positron emission tomography (PET) has shown moderate-to-high correlation with CMR and CT in the assessments of RV volume and ejection fraction simultaneously with quantification of myocardial glucose metabolism in conditions such as pulmonary hypertension. Gated blood-pool single-photon emission computed tomography (SPECT) appears a promising technique because it does not require geometric assumptions and provides both global and regional RV function quantification with good diagnostic accuracy compared with CMR. Still, these techniques have disadvantages compared with other imaging modalities, such as poor resolution, the use of ionizing radiation, the need for an adequate bolus injection for first-pass studies and a regular rhythm, and the lack of clinical experience. Therefore they have been of limited use for the study of the right ventricle so far.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here
