Assessment of Left Atrial Function

Left Atrial Function

The principal role of the LA is to modulate left ventricular (LV) filling and cardiovascular performance. This is accomplished by its roles as a reservoir for pulmonary venous return during ventricular systole, a conduit for pulmonary venous return during early ventricular diastole, and a booster pump for ventricular filling during late ventricular diastole. In normal patients, the atrial contribution to ventricular stroke volume by the reservoir, conduit, and booster phases are 40%, 35%, and 25%, respectively. The interplay between these atrial functions and ventricular performance throughout the cardiac cycle is fundamental to understanding changes in the LA functional indices. For example, although reservoir function is governed by atrial compliance during ventricular systole, reservoir capacity is influenced by atrial contractility and relaxation, systolic descent of the LV base, and the LV end-systolic volume. Conduit function is influenced by atrial compliance and is reciprocally related to reservoir function, but because the mitral valve is opened, it is closely related to LV relaxation and compliance. Finally, atrial booster pump function reflects the magnitude and timing of atrial contractility but is dependent on the degree of venous return (atrial preload), left ventricular end-diastolic pressures (atrial afterload), and LV systolic reserve.

LA function is most often assessed by echocardiography using LA volumetric analysis; spectral Doppler of transmitral, pulmonary venous, and left atrial appendage (LAA) flow; and tissue Doppler and deformation analysis (strain and strain rate imaging) of the LA body ( Table 39.1 and Fig. 39.1 ). LA volumetric and Doppler methods were the earliest methods used to assess LA function but have been overtaken by speckle-tracking deformation analysis as the primary method used. It must be noted that results obtained from deformation and volumetric analysis are not directly comparable. Although atrial pressure–volume loops can be generated in humans using invasive and semi-invasive means, these methods are cumbersome, time consuming, and difficult to apply.

Generally, LA volumetric and speckle-tracking deformation analyses are obtained from two-dimensional (2D) echocardiograms using two- and four-chamber focused views of the LA when the LA is not foreshortened. However, obtaining LA functional data from three-dimensional (3D) echocardiography is preferred because it is more accurate because of visualization of all the LA walls in a dataset ( Fig. 39.2 ). Although there have been concerns in the past regarding the time and expertise required to analyze 3D LA datasets, advancements in semi- or fully automated analysis programs are resolving this issue. CCT and CMRI have also been used to assess volumetric LA functions but are not as popular because of radiation exposure and accessibility issues.

Volumetric Methods

Volumetric assessment of LA reservoir, conduit, and booster pump function can be obtained from LA volumes at their maximum (at end-systole, just before mitral valve opening), minimum (at end-diastole, when the mitral valve closes), and immediately before atrial systole (before the electrocardiographic P wave). From these volumes, total, passive, and active ejection or emptying fractions, representative of reservoir, conduit, and booster pump function, respectively, can be calculated (see Fig. 39.1 and Table 39.1 ). The expansion index, which normalizes total LA emptying volume to minimum LA volume is an index that may be more closely related to reservoir function than LA total ejection fraction (LAEF). The LA functional index (LAFI) is a novel measure that incorporates the LAEF, the LV outflow tract velocity time integral (LVOTvti), and the maximum LA volume indexed to body surface area (LAVi): [LAFI = (LAEF × LVOTvti)/LAVi]. Although the passive ejection fraction is used as a surrogate of conduit function, conduit volume is actually the volume of blood that passes through the LA that cannot be accounted for by reservoir or booster pump functions. This volume of blood requires simultaneous measurement of LV and LA volumes: [LV stroke volume − (LA _max − LA _min )].