Evaluation of Right Ventricular Diastolic Function

Introduction

Echocardiography plays an important role in the evaluation of RV function. Importantly, noninvasive indices of RV diastolic function are associated with worse outcomes in patients with pulmonary hypertension (HTN) and chronic heart failure. There are several diseases associated with RV diastolic dysfunction, including group I pulmonary HTN, congenital heart disease, pulmonary embolism, right ventricular ischemia or infarction, and cardiomyopathies. Echocardiographic assessment of RV diastolic function relies on several two-dimensional (2-D) and Doppler variables ( Box 14.1 ). This chapter reviews the noninvasive indices of RV diastolic function, including their correlation with RV filling pressures.

Invasive Assessment of RV Diastolic Function

The right ventricle ejects blood into the low-pressure and high-compliance pulmonary circulation. In healthy subjects, the right ventricle has brief isovolumic contraction and relaxation periods. RV filling during early diastole occurs as RV relaxation leads to a positive pressure gradient between the right atrium and the right ventricle. RV filling depends on active relaxation, chamber stiffness, and transtricuspid pressure gradient. Active relaxation is among the important determinants of RV diastolic function and is dependent on calcium reuptake by the sarcoplasmic reticulum, intrinsic contractility, uniformity of relaxation, and the load-dependent properties of relaxation. Load-dependent parameters of RV function can be evaluated invasively. Insights into impaired RV relaxation can be appreciated using high-fidelity pressure catheters to measure peak rate of RV pressure decline (−dP/dt) and the time constant of pressure decay during isovolumic relaxation (tau [τ]). Abnormal RV relaxation occurs in patients with coronary artery disease with RV involvement, pulmonary HTN, hypertrophic cardiomyopathy, and heart failure with preserved ejection fraction (HFpEF).

During late diastole, RV stiffness and RA contraction are the important determinants of RV filling. RV chamber stiffness is dependent on myocardial stiffness, RV volume and mass, ventricular interdependence, and pericardial and pleural constraints. There is a curvilinear relationship between RV diastolic volume and pressure. RV chamber stiffness can be characterized by the end-diastolic pressure-volume relationship. RV stiffness constant (beta [β]) is measured using high-fidelity conductance catheters. Increased RV stiffness contributes to the increased RV filling pressure in patients with HFpEF. In patients with pulmonary HTN, preserved RA booster pump function is associated with lower likelihood of progression to heart failure.

Throughout diastole, tissue mechanics, fluid dynamics, and intracavitary vorticity provide insight into diastolic function. Animal studies using three-dimensional (3-D) real-time echocardiography have revealed the formation of vortices during early diastolic RV filling, which is impaired with chamber dilatation. Recently, using four-dimensional flow cardiac magnetic resonance imaging (MRI), a correlation of vorticity with echocardiographic indices of RV diastolic function was shown in patients with pulmonary HTN.

Right Atrial Size

RA size is related to RV systolic and diastolic function. Accordingly, RA enlargement is a predictor of adverse outcomes in patients with pulmonary arterial HTN and chronic heart failure. Traditionally, evaluation of RA size includes measurement of RA diameters (minor and major) and RA area using 2-D imaging in the apical four-chamber view ( Fig. 14.1 ). The upper limits of normal are 4.4 cm and 5.3 cm, respectively, for RA minor axis and major axis dimensions. For RA area, the upper limit of normal is 18 cm ² . Similar to the relation between the left atrium and the left ventricle, RA volume is usually increased in patients with RV diastolic dysfunction. It is reasonable to consider RA volumes when drawing conclusions about RA pressure in patients with equivocal findings with other signals. RA maximum volume is measured at the end of RV systole before tricuspid valve opening, whereas RA minimum volume is measured at end diastole after RA contraction. Similar to the left atrium, RA emptying fraction can be computed as the difference between RA maximum and minimum volumes/RA maximum volume. For RA volume measurements, RA volume can be obtained using single-plane area length or disk summation techniques in a dedicated apical four-chamber view ( Fig. 14.2 ), and normal reference ranges have been published.

In patients with increased mean RA pressure, RA maximum and minimum volumes are increased, whereas RA emptying fraction is decreased. While statistically significant, the correlation of RA volumes with RA pressure is weak and is affected by RA stiffness and contractility. In addition, RA volumes may be increased for reasons other than diastolic dysfunction, such as hyperdynamic state, athletic training, atrial fibrillation, and tricuspid valve disorders.

More recently, it has been recognized that 2-D echocardiography underestimates true RA volumes as measured by cardiac MRI. Nowadays, 3-D echocardiography can be utilized in daily clinical laboratory practice ( Fig. 14.3 ). 3-D RA volumes by transthoracic echocardiography (TTE) have a stronger correlation with RA volumes by cardiac MRI, as well as lower bias than 2-D RA volumes versus cardiac MRI. Real-time RA volumes by 3-D echocardiography have satisfactory reproducibility. While promising, only few studies have reported on the normal reference ranges for 3-D RA volumes. More importantly using 3-D echocardiography, investigators reported that in patients with acutely decompensated heart failure, 3-D RA maximum volume index, in conjunction with IVC parameters, has a high accuracy in the detection of elevated RA pressure.

Right Atrial Function

The RA serves as a reservoir during systole, as a conduit during early diastole, and as a pump during late diastole. RA volume changes and phasic function can be quantified using maximum, minimum, and pre-A RA volumes as recorded by 2-D and 3-D echocardiography. RA ejection fraction provides assessment of RA systolic function and can be obtained as the difference between RA pre-A volume and RA minimum volume divided by RA pre-A volume. While RA ejection fraction is an index of RA systolic function, RA emptying fraction is an index of RA reservoir function. Reduced right atrial emptying fraction is associated with increased risk of death in patients with pulmonary arterial HTN.

In addition to 2-D–derived RA volumes, RA myocardial mechanics can be evaluated using speckle tracking ( Fig. 14.4 ). There are emerging data on normal values for RA longitudinal strain as well as phasic 3-D RA volumes and ejection fraction in healthy individuals. RA longitudinal systolic strain and early diastolic longitudinal strain rate, representing RA reservoir and conduit functions, respectively, are reduced in patients with pulmonary arterial HTN when compared to controls. Likewise, abnormal RA strain has been reported in patients with right ventricular infarction and inferior wall myocardial infarction when compared with patients with inferior wall myocardial infarction but without RV involvement. In addition, investigators have shown that reduced RA strain is predictive of abnormal hemodynamics and adverse clinical outcomes in patients with pulmonary arterial HTN. It remains to be seen whether RA strain can add to the noninvasive assessment of RV filling pressures.

RV Morphology: size and wall thickness

The right ventricle has a complex geometry. Current guidelines recommend the use of multiple acoustic windows to assess RV size with 2-D echocardiography. The RV-focused apical four-chamber view is needed to determine RV end-diastolic area, end-systolic area, and fractional area change. Notwithstanding, the left parasternal long and short axis views, the left parasternal RV inflow view, and subcostal views can also be used to gain insight into RV size. An RV end-diastolic diameter greater than 41 mm at the base and greater than 35 mm at the midlevel in the RV-focused view denotes RV enlargement.

3-D echocardiography overcomes the limitations of 2-D imaging. 3-D echocardiography has been compared with cardiac MRI. A good correlation is observed between 3-D echocardiographic and cardiac MRI volumes; values obtained by 3-D echocardiography tend to be smaller. RV end-diastolic volumes of 87 mL/m ² in men and 74 mL/m ² in women, and RV end-systolic volumes of 44 mL/m ² for men and 36 mL/m ² for women, have been recommended as the upper limits of the corresponding normal ranges. Normative ranges for RV size based on age, body size, and gender have been published. Importantly, investigators have shown that the use of contrast enhancement to assess RV size improves reproducibility and accuracy. More recently, investigators evaluated 3-D echocardiographic values for RV regional curvature indices from 3-D endocardial surfaces in a large group of individuals with a normal right ventricle. They depicted some age-related differences in the RV shape, which may be due to increased chamber stiffness. These newly described indices could be of value in the evaluation of RV diastolic function. In addition, RV free wall thickness greater than 5 mm is consistent with RV hypertrophy, which is also associated with diastolic dysfunction. RV free wall thickness can be measured using either M mode or 2-D imaging in the subcostal view.