Left Atrium Mechanics 

Journal of the American Society of Echocardiography 

Volume 24 Number 3 2011 

 

The left atrium performs four basic mechanical  functions: phase 1, reservoir (collection of pulmonary venous flow during LV systole); phase 2, conduit (passage of blood to the left ventricle during early diastole); phase 3, active contractile pump (15%–30% of LV filling in late diastole); and phase 4, suction force(the atrium refills itself in early systole). Through these functions, the left atrium modulates LV filling.  

 

LA dilation occurs in response to impaired LV filling and as a consequence of mitral disease and/or atrial fibrillation. LA function can be separated into a roughly exponential pressure volume relationship during the reservoir and conduit phases and a counterclockwise pressure-volume loop during atrial contraction and suction.  Any comprehensive assessment of LA function should require accurate LA pressure, which can only be indirectly estimated by echocardiography.

 

Complicating the situation further are the facts that (1) unlike the left ventricle, there is no true LA isovolumic phase (because the pulmonary vein orifices are always open), and (2) reservoir function is determined as much by LV function (descent of the mitral annulus during systole) as by primary LA properties. Passive and active LA properties can be characterized by combining 3D echocardiographic volumes with invasive pressure measurements in conjunction with changes in loading conditions.

 

By using this method, a reduction of LA systolic loop occurred during circumflex ligation (which induces LA ischemia) but not with left anterior descending coronary artery ligation (which affects only the left ventricle). On a regional basis, LA function can be fundamentally described in terms of stress-strain relationships. Although strain is becoming increasingly accessible by echocardiography, there is no way to estimate wall stress, even invasively. Fortunately, because the left atrium is thin walled, one can reasonably equate LA pressure with wall stress. 

 

Global and Regional LA Function:  

LA function is currently estimated by 2D measurements of LA volumes, by Doppler analysis of transmitral flow (peak and time velocity integral of a velocity, atrial filling fraction) and by pulmonary vein flow (peak and duration of atrial reverse velocity).  

 

Because 2D echocardiography is limited by the use of geometric models and by possible errors due to foreshortening, it may underestimate LA volume compared with cardiac magnetic resonance, while Doppler assessment of LA function and/or the use of the LA ejection force are indirect parameters.  

 

Three-dimensional LA volume measurements, which do not require geometric assumptions, can accurately estimate global LA function. Acoustic quantification, an automated border detection technique, provides online continuous LA area or volume over time, but the values obtained are heavily influenced by gain settings, resulting in large interobserver and testretest variability. 

 

Both DTI and 2D STE allow noninvasive assessment of global LA function and regional deformation of LA walls. Two-dimensional STE also successfully provides LA volume curves during one cardiac cycle, from which various LA mechanical indices can be obtained, and allows a direct assessment of LA endocardial contractility and passive deformation.  

 

Two different modalities have been proposed to quantify atrial deformation by STE (Figure 30). The first (total of 12 equidistant regions, six in the apical four-chamber view and six in the apical two-chamber view) takes as a reference point the QRS onset and measures the positive peak atrial longitudinal strain (corresponding to atrial reservoir). 

 

The second (total of 15 equidistant regions, six in the apical four-chamber view, six in the apical  two chamber view, and three in the inferoposterior wall in long axis) uses the P wave as the reference point, enabling the measurement of a first negative peak atrial longitudinal strain (corresponding to atrial systole), a second positive peak atrial strain (corresponding to LA conduit function), and their sum. 

 

Normal Values: 

 Two-dimensional speckle-tracking echocardiographic  normal values of LA strain have been recently reported. By using a 12-segment model and QRS onset as the reference point, the mean peak atrial longitudinal strain of 60 healthy individuals was 42.2 +- 6.1% (5th to 95th percentile range, 32.2%–53.2%) The average values of positive and negative peak strain were 23.2 +- 6.7%  and 14.6 +- 3.5%, respectively, in a 15-segment model, which used the P wave as the reference point (64 normal subjects). 

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Published Findings:  

Strain and SRs and early diastolic global strain were reported to be reduced in 12 atrial segments in patients withatrial septal occluder devices compared with control subjects.162 In patients in sinus rhythm who had undergone either cardioversion or catheter ablation for atrial fibrillation, colorDTI velocities and strain were lower compared with normal controls, but the ablation group had increased regional and global LA dysfunction. Conversely, LA strain was shown to be increased in patients with mitral regurgitation. Also, strain and SR were used for the evaluation of 

patients with atrial fibrillation to assess the risk for new atrial fibrillation after cardioversion.

 

Triplane 3D color DTI, which has the advantage  of simultaneously recording SRs in three views to minimize beat variation, was used to demonstrate significantly lower peak SRs in patients with hypertension compared with normal controls and athletes. 

 

Similar to DTI, STE-derived global LA motion analysis after percutaneous interatrial defect repair showed the expected absence of strain measured at the device site. 

 

 Decreased negative LA SR is also an independent predictor of episodes of paroxysmal atrial fibrillation in patients in sinus rhythm. 

 

Summary and Recommended Indications:  

Indications for 2D STE of the left atrium include regional LA assessment in patients with LV diastolic dysfunction, evaluation of LA properties after atrial fibrillation to predict the maintenance of sinus rhythm, and evaluation after percutaneous interatrial defect repair. In addition, LA regional strain appears potentially suitable to identify patients at risk for LA failure or arrhythmias and to assess LA characteristics in patients with LA dilation of undetermined cause. However, at the present time, 2D STE of the left atrium does not appear ready for clinical use.