Left atrium

Atrium sinistrum

Also known as: LA

The left atrium is the most posterior chamber of the heart and forms most of its base. It receives oxygenated blood from four pulmonary veins and passes it through the mitral valve to the left ventricle.

Overview

Mean pressure≈6–12 mmHg (≈ PCWP)

Pulmonary veins enteringUsually 4

Structure. Most of the left atrial wall is smooth. This part comes from the pulmonary veins, which were absorbed into the atrium during development. Pectinate muscles are found only in the left auricle. The auricle is a narrow, hooked, finger-like pouch that curls forward around the left side of the pulmonary trunk.

Openings. Four valveless pulmonary veins (right and left, superior and inferior) open into the posterior wall. The mitral orifice lies anteroinferiorly. On the septal wall is a crescentic flap of septum primum, the valve of the foramen ovale.

Relations. The oesophagus lies directly behind the left atrium, separated from it only by pericardium (the oblique sinus). This is why transoesophageal echocardiography gives excellent views of the atrium, the left atrial appendage and the mitral valve. The left atrium also sits just below the tracheal bifurcation, so marked enlargement widens the angle of the carina.

Location

Posterior surface (base) of the heart, at about the T6–T8 vertebral levels. It lies behind the right atrium and in front of the oesophagus and descending aorta.

Parts & subdivisions

Smooth-walled body

Receives the pulmonary veins.

Left auricle (left atrial appendage)

Trabeculated with pectinate muscles. It is the main site of thrombus formation in atrial fibrillation.

Valve of the foramen ovale

Remnant of the septum primum on the septal wall.

Mitral orifice

Outlet to the left ventricle.

Anatomy

Arterial supply

Left atrial branches of the circumflex artery. The right coronary artery contributes near the interatrial septum.

Venous drainage

Oblique vein of the left atrium (vein of Marshall) to the coronary sinus, and venae cordis minimae.

Relations

Behind: the oesophagus, in direct contact through the pericardium, and the descending aorta. Above: the tracheal bifurcation and the right pulmonary artery. In front: the ascending aorta and pulmonary trunk.

Function

Receives oxygenated blood returning from the lungs.

Acts as a reservoir, a conduit and a booster pump. Atrial contraction adds about 20% of left ventricular filling at rest.

Releases ANP when stretched.

Physiology

Mean left atrial pressure is about 6–12 mmHg. It is estimated clinically by the pulmonary capillary wedge pressure, measured with a Swan–Ganz catheter.

Clinical significance

Atrial fibrillation and the left atrial appendage

In atrial fibrillation not caused by valve disease, about 90% of atrial thrombi form in the left atrial appendage. This is why patients are anticoagulated, and why devices that close off the appendage are an alternative.

Left atrial enlargement

Mitral stenosis or regurgitation enlarges the left atrium. A large atrium predisposes to atrial fibrillation. It can also press on the oesophagus (causing dysphagia), stretch the left recurrent laryngeal nerve (causing hoarseness, Ortner syndrome) and widen the carina. On the ECG the P wave becomes broad and notched (P mitrale).

Left atrial myxoma

Left atrial myxoma is the most common primary cardiac tumour. It usually hangs on a stalk from the region of the fossa ovalis. It can obstruct the mitral valve (producing a "tumour plop"), embolise, or cause fever and weight loss.