Quantification of the relative contribution of the different right ventricular wall motion components to right ventricular ejection fraction: the ReVISION method

Echocardiography is a widely used, non-invasive imaging modality to describe cardiac morphology and function. However, ultrasonic assessment of the RV is challenging due to the complex anatomy of the chamber. Conventional two-dimensional acquisition protocols require to obtain several imaging views using the parasternal, apical and subcostal approaches. The most commonly used echocardiographic measures to characterize RV morphology are simple linear diameters, while functional aspects are mainly investigated by measuring the tricuspid annular plane systolic excursion (TAPSE) and the RV fractional area change (FAC) [11]. The latter parameters have shown their correlation with RV ejection fraction measured by cardiac magnetic resonance imaging (MRI) [12] and proven their prognostic value as well [13‐16]. Nevertheless, several limitations have to be taken into consideration, especially in certain pathological conditions. TAPSE is an easy-to-obtain M-mode parameter of RV function, referring solely to longitudinal motion of RV free wall (Fig. 5). However, since the reference is outside of the heart, TAPSE measures not only the shortening of the RV free wall but also the traction of the RV resulting from left ventricular contraction and the effects of the heart translation within the chest [4]. FAC incorporates both the radial displacement of the RV free wall and the longitudinal motion of the tricuspid annulus toward the apex, assessed on a single tomographic apical four-chamber view and therefore, suffers from the inherent limitations of the limited RV myocardial mass included in this measurement (Fig. 6). These shortcomings of TAPSE and FAC can be overcome by the acquisition of a 3D data set including the whole RV using echocardiography.

Using 3D echocardiographic datasets, RV volumes and global ejection fraction can be measured with a good correlation with MRI results [8, 9]. However, we still lack data about the relative contribution of longitudinal, radial and anteroposterior component of RV wall motion to global ejection. During physiological conditions, the longitudinal shortening was suggested to account for the majority of RV pump function, however, larger normative studies are needed to characterize potential age and gender related alterations [3]. Moreover, there are several diseases and clinical scenarios, where the normal ratio between the different mechanisms can change. Clinical interest includes post-cardiac surgery patients, heart transplanted patients (with evidently reduced TAPSE values without signs and symptoms of right heart failure; Fig. 7), pulmonary hypertension patients (with often severe symptoms and dilated RV along with preserved TAPSE; Fig. 8), congenital heart diseases, elite athletes, etc. Evidence suggests that RV longitudinal component measured by TAPSE is markedly reduced in post-cardiac surgery patients despite the absence of global RV ejection fraction decrease [17]. Recent studies with MRI suggest that RV function is reflected better by radial rather than longitudinal wall displacement in pulmonary hypertension patients [18, 19]. Pettersen and coworkers demonstrated that, after the Senning procedure (which transforms the RV to the systemic pump of the circulation), the patients have more pronounced radial component of RV function [20]. As a special case of RV overload, physiological or even detrimental effects of regular and/or acute exhaustive physical exercise may be of clinical interest [21, 22]. After an endurance race, conventional parameters suggest a less pronounced decrease of longitudinal shortening compared to ejection fraction, which also refers to a post-race functional shift [23]. Moreover, there is a growing interest in separated evaluation of longitudinal and radial RV function in pediatric cardiology patients [24]. These studies have shown the importance of the varying contribution of longitudinal and radial component in certain conditions, but did not provide distinct quantitative assessment for these phenomena. The ReVISION method may be the first to answer these questions by quantifying all major mechanisms using a single 3D dataset obtained by the easily accessible and non-invasive transthoracic echocardiography.

The underlying causes of the varying relative contribution of these mechanisms to global RV function in different cardiac conditions remain to be clarified. Beyond the mechanical effects of volume and/or pressure overload, there are several other and often overlapping factors that may be responsible for these findings. Several research groups hypothesized that the myocardial fiber architecture of the RV can change in certain conditions. Classical anatomical dissections revealed that in patients with congenital heart malformations such as Tetralogy of Fallot, pulmonary or tricuspid atresia, significant changes can be observed in the myocardial fiber arrangement within the RV wall. These changes include the more oblique orientation of the longitudinal layer, and also the presence of a middle circumferential layer which is absent from physiological conditions [25, 26]. Pressure overload has been proven to induce changes in RV fiber orientation as well, with a relative dominance of the circumferential fiber direction within the RV wall [27]. These effects may manifest in changes of RV shape, which can be severely deformed in pulmonary hypertension patients [28] and may also induce functional remodeling. Radial shortening, which is usually generated by the circumferential fibers in the subepicardial layer of the free wall, may reflect RV pump function better in this scenario than longitudinal shortening [18]. Considering that RV function has shown to be a powerful predictor of the prognosis of pulmonary hypertension [29], the possibility of a distinct quantification of the radial and longitudinal contribution to global RV ejection fraction may provide valuable information about the early detection of impaired RV myocardial function (e.g. when global ejection fraction is still normal) and RV functional remodeling during the follow-up of these patients. Moreover, there are very limited data on the subclinical stages of the disease, and distinct quantification of the radial and longitudinal contribution to global RV ejection fraction may serve as a screening method to detect initial RV involvement in patients with systemic sclerosis, idiopathic pulmonary fibrosis, etc. The function of the interventricular septum accounts for a significant part of global RV ejection fraction, mainly by its longitudinal shortening [3]. Some authors emphasize the detrimental effect of interventricular septal abnormal motion, which occurs in cardiothoracic operations that involve cardiopulmonary bypass [30, 31]. Pericardial constraint and ventricular interdependence can be hampered by the opening of the pericardial sac, which may result in RV functional alterations [32, 33]. However, left ventricular contraction contributes to RV function also by stretching the RV free wall over the interventricular septum. Importantly, this component may be at least partly characterized by the third (e.g. the anteroposterior) component of RV wall motion. No data exist on the functional significance of RV wall anteroposterior motion, which can be quantified by our novel method as well. Loss of innervation may be another important factor which may affect global RV function in patients undergoing heart transplantation. Evidence suggests that the transplanted heart can be reinnervated by both sympathetic and parasympathetic fibers [34, 35], and the re-gain of autonomic nervous control can result in the recovery of RV longitudinal shortening over time.

Current literature is scarce about the clinical value of RV peak ejection rate and peak filling rate due to their previously complicated measurements with radionuclide ventriculography [36, 37]. 3D echocardiography offers an easier way to measure them, allowing also the decomposition of the different contributions of the different RV wall motion directions that may provide additional physiological and pathological insights into the systolic and also diastolic dynamics of the RV. Considering that the fiber orientation of the RV varies in different regions and also transmurally [1], this method may be a marker of heterogeneous contraction and relaxation pattern in certain cardiovascular conditions.