Anatomy of left atrium

Anatomy of left atrium

Left atrium is the left upper chamber of the heart which receives oxygenated blood from the lungs through the four pulmonary veins. They are the right superior, right inferior, left superior and left inferior pulmonary veins. Pulmonary veins join the posterior part of the left atrium which is smooth and formed by the incorporation of the embryonic single pulmonary vein and its branches into the left atrium. Left atrial appendage is a finger like outpouching of the left atrium [1], which is an important source for clots in thromboembolism in atrial fibrillation. When it is enlarged, it forms a bulge along the left upper border, known as the third mogul. Interatrial septum separates the left atrium from the right atrium. There could be a patent foramen ovale in the interatrial septum which may cause phasic right to left shunt.

Left atrium is the posterior most cardiac chamber which lies in close proximity to the oesophagus. That is why left atrial clot can be visualized well in transesophageal echocardiography. For the same reason, catheter ablation within the left atrium for atrial fibrillation has a potential risk of atrioesophageal fistula, a dreaded complication. Fortunately it is rare with esophageal temperature monitoring and other meticulous precautions during catheter ablation in left atrium. Proximity to the esophagus also explains the mechanism of odynophagia or painful swallowing which can occur in pericarditis.

Other important structures immediately behind the left atrium are the tracheal bifurcation and descending thoracic aorta. Proximity to the tracheobronchial tree explains the left bronchus elevation noted on chest X-ray in left atrial enlargement. Vertebral column is located still posterior to it. Oblique sinus of pericardium is posterior to the left atrium. Transverse sinus of pericardium is anterior to the left atrium, between the left atrium and the root of aorta. Aortic cross clamp is applied through the transverse sinus of pericardium during open heart surgery.

The outlet region of left atrium near the mitral orifice is known as the vestibule. Mitral isthmus is the region of left atrium between the orifice of the left inferior pulmonary vein and the mitral hinge line. It is an important area from the point of view of cardiac electrophysiologist. Mitral isthmus ablation is used to treat perimitral atrial flutter, similar to cavotricuspid isthmus ablation used to treat atrial flutter with circuits in the right atrium. In mitral isthmus ablation, an ablation line is created from the left inferior pulmonary vein to the lateral mitral annulus [2].

Epicardial aspect of the left atrium is covered by pericardial fat pads, especially in the venoatrial junctions, interatrial grooves, along the coronary sinus and the roof. Ganglionated plexi are located in the subepicardium, while the preganglionic parasympathetic and postganglionic sympathetic fibres are there in the fat pads. Ablation of ganglionated plexi is an important adjunct procedure in the ablation of atrial fibrillation. Ganglionated plexi are a conglomeration of autonomic ganglia on the surface of the heart [3].

References

1. Ho SY, Cabrera JA, Sanchez-Quintana D. Left atrial anatomy revisited. Circ Arrhythm Electrophysiol. 2012 Feb;5(1):220-8. doi: 10.1161/CIRCEP.111.962720. PMID: 22334429.
2. Pathik B, Choudry S, Whang W, D’Avila A, Koruth J, Sofi A, Miller MA, Dukkipati S, Reddy VY. Mitral isthmus ablation: A hierarchical approach guided by electroanatomic correlation. Heart Rhythm. 2019 Apr;16(4):632-637. doi: 10.1016/j.hrthm.2018.10.005. Epub 2018 Oct 10. PMID: 30312756.
3. Stavrakis S, Po S. Ganglionated Plexi Ablation: Physiology and Clinical Applications. Arrhythm Electrophysiol Rev. 2017 Dec;6(4):186-190. doi: 10.15420/aer2017.26.1. PMID: 29326833; PMCID: PMC5739885.