Abstract
The atria are reservoirs, conduits, and pumps for blood traveling from the pulmonary and systemic veins into the ventricles. Normally functioning atria are compliant and active pumps with the ability to enhance ventricular filling and cardiac output. Abnormal atrial structure and function may be present in myocardial, valvular, heart rhythm, and congenital disorders. Thrombus and tumors may also present in the atria. In this chapter, the echocardiographic evaluation of atrial structure and function will be reviewed.
Keywords
atrial function, diastolic, echocardiography, left atrium, right atrium
Introduction
With each cardiac cycle, the left and right atria act as reservoirs, conduits, and pumps for blood traveling from the pulmonary and systemic veins into the ventricles. Normally functioning atria are compliant, with the ability to accommodate dynamic changes in intravascular volume without pathologic increases in pressure and are active pumps with the ability to enhance ventricular filling and cardiac output. Atrial structure and function can be characterized by echocardiography, which provides diagnostic and prognostic information.
Left Atrium
Structure
The left atrium is located superior to the left ventricle, posterolateral to the right atrium, posterior to the aortic root, and anterior to the esophagus. The left atrium receives the pulmonary veins, has an appendage, and directs blood into the left ventricle through the mitral valve. With conventional two-dimensional (2D) transthoracic echocardiography, the left atrium should be visualized from the parasternal, apical, and subcostal views. However, inherent to the limitations of 2D imaging, no single view completely characterizes the shape and size of the left atrium. Therefore it is recommended that multiple views from standard imaging planes be obtained to more completely visualize the left atrium with careful attention to focus on the structure of interest, optimize endocardial border definition, and avoid foreshortening.
Quantification of left atrial size by transthoracic echocardiography has advanced from M-mode and 2D linear measurements to 2D and three-dimensional (3D) volumes. Compared with linear measures, volumes more accurately quantify left atrial size and perform better as prognostic markers. Left atrial volumes obtained with 3D imaging are similar to those obtained from cardiac computed tomography (CT) or magnetic resonance imaging (MRI). However, the lack of widespread clinical availability, standardization, and normative data from 3D echocardiography has led the American Society of Echocardiography to currently recommend 2D left atrial volumetric assessment by transthoracic echocardiography. Characterization of left atrial size by transesophageal echocardiography is limited to semiquantitative assessment, because the entire left atrium usually does not fit within the image. However, transesophageal echocardiography is superior to transthoracic echocardiography for imaging of the left atrial appendage, interatrial septum, and pulmonary veins.
Quantification of 2D left atrial volume is performed from the transthoracic apical four- and two-chamber views. In each of these views, the left atrial endocardial borders are traced at end ventricular systole just before mitral valve opening when the left atrium is at its maximal size. A modified Simpson’s (method of disks) biplane volume can then be calculated ( Fig. 17.1 ). Alternatively, the area (cm 2 ) and lengths (cm) of the left atrium can be measured in each view with volume calculated as (0.85 × Area 4c × Area 2c )/length, where the shorter length of the major axis from the four- or two-chamber view is used. With either the biplane Simpson’s or area-length approach, it is recommended that left atrial volume be indexed to body surface area to account for gender differences, with the upper limit of normal for both men and women being less than 34 mL/m 2 ( Table 17.1 ). If volumetric assessment from the apical views cannot be obtained, then the 2D linear anterior-posterior length from the parasternal long-axis view can be used to size the left atrium. This measure is the distance between the posterior edge of the aortic root and the posterior wall of the left atrium, timed at the end of ventricular systole in the cardiac cycle.
| Normal | Mild | Moderate | Severe | |||
|---|---|---|---|---|---|---|
| Left atrium | Volume index (mL/m 2 ) | Men | 16 to <34 | 34 to <41 | 41 to ≤48 | >48 |
| Women | 16 to <34 | 34 to <41 | 41 to ≤48 | >48 | ||
| A-P diameter (cm) | Men | 3.0 to <4.0 | 4.0 to <4.6 | 4.6 to <5.2 | ≥5.2 | |
| Women | 2.7–3.8 | 3.8 to <4.2 | 4.2 to <4.7 | ≥4.7 | ||
| Right atrium a | Volume index (mL/m 2 ) | Men | 18 to <32 | — | — | — |
| Women | 15 to <27 | — | — | — |
a Threshold values for defining severity of right atrial enlargement have not been established.
Left atrial size is a barometer for left-sided filling pressures, with enlargement typically correlating with chronically elevated left ventricular end-diastolic pressure, and/or pulmonary capillary wedge pressure. Therefore left atrial enlargement (>34 mL/m 2 ) is a central component of the algorithm for the echocardiographic assessment of left ventricular filling pressures and diastolic dysfunction. However, left atrial enlargement also occurs in the setting of mitral valve disease, atrial fibrillation, and intracardiac shunts, such that the presence of these conditions must be considered when assessing left ventricular filling pressures and diastolic function. Left atrial enlargement has been consistently demonstrated to be a powerful predictor of adverse cardiovascular outcomes, including incident atrial fibrillation, stroke, heart failure, and death. Importantly, the prognostic information carried in left atrial size is independent of clinical factors, left ventricular size, and ejection fraction.
Function
While not commonly clinically reported on transthoracic echocardiography, quantification of left atrial function from conventional 2D and Doppler imaging as well as speckle tracking and 3D echocardiography is garnering increased attention. Atrial function is typically described in three phases: reservoir, conduit, and pump ( Fig. 17.2 ). The atrial reservoir phase corresponds to ventricular systole (atrial relaxation) when the mitral valve is closed and the left atrium expands due to venous return and descent of the mitral annulus towards the left ventricular apex. The conduit (passive emptying) phase begins with ventricular diastole when the mitral valve opens and blood flows through the atrium into the ventricle. The pump (active emptying) phase coincides with atrial contraction and is completed with the onset of ventricular systole, when the reservoir phase starts again. Overall left atrial function (emptying fraction) can be quantified from the largest and smallest left atrial volumes. Using 2D or 3D volumetric measurements, overall and phasic atrial function can be calculated with the following formulas:
Overall by emptying fraction: (LA max vol − LA min vol)/LA max vol
Conduit by passive emptying fraction: (LA max vol − LA pre A vol)/LA max vol.
Pump by active emptying fraction: (LA pre A vol − LA min vol)/LA pre A vol.
Reservoir by expansion index: (LA max vol − LA min vol)/LA min vol.
Blood flow Doppler can also be used to assess left atrial function. Early diastolic blood flow (conduit phase) from the left atrium into the left ventricle can be measured with pulse wave spectral Doppler at the tips of the mitral valve leaflets as the E wave. From this same Doppler tracing, atrial pump function can be quantified from the A wave peak velocity and velocity time integral, which coincides with the electrocardiographic P wave and atrial contraction ( Fig. 17.3 ). Pulse wave spectral Doppler of the pulmonary veins can also reveal an A wave, which can be used to characterize atrial contractile function. In contrast to blood flow Doppler, which is driven by pressure gradients between two chambers, pulse wave tissue Doppler of the mitral annulus provides information regarding the mechanical properties of the myocardium. The late diastolic A prime velocity of the mitral annular tissue Doppler signal is also a measure of left atrial contraction. In atrial fibrillation, the spectral Doppler and tissue Doppler A waves are absent.
Assessing left atrial function has informed understanding of thromboembolic risk in atrial fibrillation. Using Doppler imaging, electromechanical dissociation of the left atrium has been demonstrated to occur following electrical cardioversion of atrial fibrillation to sinus rhythm. Similarly, patients with amyloidosis may have markedly reduced atrial contractile function despite electrocardiographic sinus rhythm. Spectral Doppler has also been used to interrogate blood flow velocities in the left atrial appendage during transesophageal echocardiography, with values below 34 cm/s associated with greater risk of atrial fibrillation recurrence and stroke ( Fig. 17.4 ).
Recently, speckle tracking echocardiography has been applied to study left atrial function ( Fig. 17.5 ). Patients with a history of paroxysmal atrial fibrillation but sinus rhythm during echocardiography have impaired left atrial function, suggesting a more chronic atrial myopathy. Furthermore, despite the correlation between left atrial size and function, recent studies demonstrate that left atrial function can be impaired even in the absence of left atrial enlargement. In small studies, functional impairments of the left atrium appear to offer additional prognostic information beyond left atrial size for the prediction of adverse cardiovascular outcomes.
