Left Atrial Anatomy Revisited

Abstract

R ecent decades have seen rapid developments in arrhythmia treatment, especially the use of catheter ablation. Although the substrates of atrial fibrillation, its initiation and maintenance, remain to be fully elucidated, catheter ablation in the left atrium has become a therapeutic option for patients with this arrhythmia. With ablation techniques, various isolation lines and focal targets are deployed; the majority of these are anatomic approaches. It has been over a decade since we published our first article on the anatomy of the left atrium relevant to interventional electrophysiologists. 1 Our aim then, as now, was to increase awareness of anatomic structures inside the left atrium. In this review of anatomy, we revisit the left atrium, inside as well as outside, for a better understanding of the atrial component parts and the spatial relationships of specific structures. Location and Atrial WallsViewed from the frontal aspect of the chest, the left atrium is the most posteriorly situated of the cardiac chambers. Owing to the obliquity of the plane of the atrial septum and the different levels of the orifices of the mitral and tricuspid valves, the left atrial chamber is more posteriorly and superiorly situated relative to the right atrial chamber. The pulmonary veins enter the posterior part of the left atrium with the left veins located more superior than the right veins. The transverse pericardial sinus lies anterior to the left atrium, and in front of the sinus is the root of the aorta. The tracheal bifurcation, the esophagus, and descending thoracic aorta are immediately behind the pericardium overlying the posterior wall of the left atrium. Further behind is the vertebral column.Following the direction of blood flow, the atrial chamber begins at the pulmonary veno-atrial junctions and terminates at the fibro-fatty tissue plane that marks the atrioventricular junction at the mitral orifice. The walls of the left atrium are muscular and can be described as superior, posterior, left lateral, septal (or medial), and anterior, as suggested by McAlpine, 2 who drew attention to the importance of describing the heart in its anatomic position in the chest, in the orientation he termed "attitudinal," which is the appropriate terminology for cardiac interventionists. 3 The left atrium is relatively smooth-walled on its internal aspect ( Figure 1A), but its walls are not uniform in thickness. The roof or superior wall is in close proximity to the bifurcation of the pulmonary trunk and the right pulmonary artery. The thickness of its muscle component measured transmurally ranges from 3.5 to 6.5 mm (mean, 4.5Ϯ0.6 mm) in formalin-fixed heart specimens. 4 The thickness of the lateral wall ranges between 2.5 and 4.9 mm (mean, 3.9Ϯ0.7 mm). The anterior wall, related to the aortic root, ranges from 1.5 to 4.8 mm (mean, 3.3Ϯ1.2 mm) thick, but it can become very thin at the area near the vestibule ( Figure 1B) of the mitral annulus, diminishing to an average thickness of 2 mm. Importantly, there is an area of the anterio...

Three‐dimensional Integrated Functional, Structural, and Computational Mapping to Define the Structural “Fingerprints” of Heart‐Specific Atrial Fibrillation Drivers in Human Heart Ex Vivo

“…The LA subendocardium contained 61.9% fibrotic tissue concentrated in the PLA, LA roof, and floor, while the RA subendocardium contained only 25.7% fibrotic tissue, mainly in the LRA (Figure 3A). Our CE‐MRI and other studies30 indicate a thin endocardial layer (0.1–0.7 mm) of predominantly fibrous tissue in the left atria, which explains why the highest percent of fibrosis was observed in the LA endocardium. The functional effect of this endocardial layer of fibrous tissue needs to be evaluated in future studies.…”

“…The fixation process of the atria may affect size and thickness variation measurements derived from ex vivo CE‐MRI by ≈5% as shown by other studies 49. The computed atrial wall thickness in the left atrial appendage was found to be affected by the extremely complex and trabecular atrial anatomy,30 and thus wall thickness of the left atrial appendage was not reported. The atrial cellular activation model is based on a simplified Fenton‐Karma model, not biophysics‐based cellular models10, 12 for the sake of computational efficiency, although it was validated by optical mapping data in this study and widely used by us15, 27 and others 28.…”

“…Early knowledge on atrial fiber orientations has been obtained by Ho and colleagues using photography and tracings of visually observed myofiber tracts after manually dissecting human hearts 30. The first complete quantitative description of whole intact sheep atria myofiber architecture was done using serial surface imaging and 3D structure tensor analysis 15, 27.…”

EHRA/HRS/APHRS/SOLAECE expert consensus on atrial cardiomyopathies: definition, characterization, and clinical implication

“…The LA is situated more posteriorly and superiorly than the right atrium separated by the obliquity of the plane of the IAS. 21 The LA appendage (LAA) is smaller than the right atrium appendage (RAA). Narrower and with different shapes has a distinct opening to the atrial body and overlies the left circumflex coronary artery.…”

Left atrial appendage as a vantage point for mapping and ablating premature ventricular contractions originating in the epicardial left ventricular summit

“…The left atrial appendage has many anatomical variants, but it often rests on the lateral aspect of the LV and its tip can span all the way to reach the epicardial LV summit and even touch the RVOT, where the two ventricles meet 4.…”