The importance of the left atrium in cardiovascular performance has long been acknowledged. Quantitative assessment of left atrial (LA) function is laborious, requiring invasive pressure-volume loops and thus precluding its routine clinical use. In recent years, novel postprocessing imaging methodologies have emerged, providing a complementary approach for the assessment of the left atrium. Atrial strain and strain rate obtained using either Doppler tissue imaging or two-dimensional speckle-tracking echocardiography have proved to be feasible and reproducible techniques to evaluate LA mechanics. It is essential to fully understand the clinical applications, advantages, and limitations of LA strain and strain rate analysis. Furthermore, the technique’s prognostic value and utility in therapeutic decisions also need further elucidation. The aim of this review is to provide a critical appraisal of LA mechanics. The authors describe the fundamental concepts and methodology of LA strain and strain rate analysis, the reference values reported with different imaging techniques, and the clinical implications.
The left atrium has a pivotal role in the sequence of events that modulate left ventricular (LV) filling. This is accomplished by means of four basic functions involving the left atrium: phase 1, reservoir function (collection of pulmonary venous flow during LV systole); phase 2, conduit function (passage of blood to the left ventricle during early diastole); phase 3, active contractile pump function (15%–30% of LV filling in late diastole); and phase 4, suction force (the atrium refills itself in early systole). Left atrial (LA) relaxation, chamber stiffness, and contractility influence reservoir, conduit, and contractile function, respectively.
Until recently, the echocardiographic study of the left atrium was performed using two-dimensional (2D) measurements, extrapolation of phasic volumes, and Doppler flow assessment of the mitral valve and the pulmonary veins. These classic parameters improved understanding of the normal and diseased heart, but they had a number of limitations, such as foreshortening, lack of a gold-standard measurement of LA function, and difficulties with the echocardiographic window and with the timing of various atrial events. Moreover, errors were frequent because of the geometric assumption of a biplane volume calculation. Three-dimensional (3D) echocardiography significantly improved LA volume calculation because of automated border detection and the acquisition of a 3D data set at different phases of the cardiac cycle. However, the values obtained were heavily influenced by gain settings, resulting in large interobserver and test-retest variability, which hampered its daily application.
In the past decade, echocardiography-based automated techniques for sophisticated analysis of myocardial displacement have emerged, such as Doppler tissue imaging (DTI) or speckle-tracking (ST). They permit the quantification of parameters of regional myocardial function, such as displacement, velocity, strain (ε) and strain rate (SR). Myocardial mechanics have been validated with sonomicrometry and tagged magnetic resonance imaging. These new methodologies were initially used to study the LV myocardium and subsequently applied to the left atrium, supporting LA ε and SR in the assessment of LA active and passive deformation.
The aim of this review is to provide a critical appraisal of LA mechanics. We describe the fundamental concepts and the methodology of LA ε and SR assessment, reference values, and clinical implications, and we discuss its incremental clinical importance.
LA Mechanics
“LA remodeling” refers to a time-dependent adaptive regulation of cardiac myocytes to maintain homeostasis against external stressors. The type and extent of remodeling depend on the strength and duration of the exposure to these stressors. A hallmark of LA structural remodeling is dilatation, which is often accompanied by a change in LA performance. In healthy individuals, the left atrium is a highly expandable chamber with relatively low pressures, but in the presence of acute and chronic injury, the left atrium stretches and stiffens.
Myocardial ε and SR represent the magnitude and rate, respectively, of myocardial deformation. Thus, during ventricular systole and late ventricular diastole, atrial ε and SR reflect atrial distensibility (irrespective of the underlying rhythm) and atrial contractility (in the presence of sinus rhythm), respectively. Strain is a fractional change in the length of a myocardial segment. It is unitless and is usually expressed as a percentage. It can have positive or negative values, which reflect lengthening or shortening. SR is the rate of change in ε and corresponds to the speed at which myocardial deformation occurs, expressed per second.
LA ε and SR curves display the physiology of atrial function and closely follow LV dynamics during the cardiac cycle ( Figure 1 ). During the reservoir phase, corresponding to LV isovolumic contraction, ejection, and isovolumic relaxation, the left atrium is stretched as it fills with blood from the pulmonary veins. In this way, longitudinal atrial ε increases, reaching a positive peak at the end of atrial filling. This phase is also influenced by the downward movement of the mitral annulus toward the apex, as a result of LV contraction, just before the opening of the mitral valve. After mitral valve opening, the left atrium empties quickly. At this point, LA ε decreases to a plateau, corresponding to LA diastasis. Subsequently, the atrial wall shortens from a longitudinal perspective during the LA contraction phase, also referred as LA booster pump function, leading to a further decrease in LA ε. During the LA conduit and contraction phases, the LA ε curve inversely reflects the pattern of LV deformation. Therefore, LA mechanics seems to be influenced not only by LA stiffness but also by LV compliance during ventricular filling and by LV contraction through the descent of the base during LV systole.
Assessment
It is possible to assess LA ε and SR using either ST echocardiography or DTI modalities. A detailed description of myocardial mechanics, ST, and DTI can be found in a consensus statement from the American Society of Echocardiography and the European Association of Echocardiography.
Speckles are acoustic markers equally distributed within the myocardium that are seen in grayscale B-mode images. Two-dimensional ST-based echocardiography uses standard B-mode images to track blocks of speckles from frame to frame and measure lengthening and shortening relative to the baseline (Lagrangian ε). This provides local myocardial displacement information, from which velocity, ε, and SR can be derived. Two-dimensional ST was recently applied to study the myocardial mechanics of a thin-walled structure such as the left atrium. For the analysis, apical views are obtained using conventional 2D grayscale echocardiography, during a breath-hold, with a stable electrocardiographic recording. The frame rate is set between 60 and 80 frames/sec, and recordings are processed using acoustic tracking software. The LA mechanical indexes are calculated by averaging values observed in all LA segments (global ε) with a 15-segment (six equidistant regions in the apical four-chamber view, six in the two-chamber view, and three in the three-chambers views) or a 12-segment (six equidistant regions in the four-chamber view and six more in the two-chamber views) model. Recently, satisfactory agreement has been demonstrated for ST assessment using different software packages.
Doppler imaging uses the phase shift between consecutive echoes to calculate velocity. With DTI, a low-pass wall filter is used to display only low-velocity signals originating from moving tissue, excluding high-velocity signals originating from blood flow. By integrating the velocity over time, myocardial mechanical indexes can be calculated. In DTI mode, the imaging angle must be adjusted to ensure a parallel alignment of the sampling window with the myocardial segment of interest. This means that not all segments can be analyzed, for example, the atrial roof segments. Gain settings, filters, pulse repetition frequency, sector size, and depth should also be adjusted to optimize color saturation. The frame rate is adjusted to >100 frames/sec. Longitudinal ε and SR can be measured in the middle portions of the various segments of the LA wall (septal, lateral, posterior, anterior, and inferior) using apical two-chamber, three-chamber, and four-chamber views.
LA ε and SR
Irrespective of which methodology is used for image acquisition and graphic representation of LA mechanics, the software generates longitudinal ε and SR curves for each atrial segment. The radial deformation cannot be calculated, because the LA wall is thin and the spatial resolution is limited. It is possible to quantify LA ε in two different ways, which differ only by the choice of frame from which the software starts the processing. The first uses P-wave onset ( Figure 2 A) and the second the QRS complex ( Figure 2 B) as the first reference frame. Regardless of whether the P wave or the QRS complex serves as the first reference frame, the LA SR curve is triphasic ( Figure 2 C and 2D).
