Atrial septum

Last revised by Dr Joachim Feger on 05 Sep 2021

The atrial or interatrial septum (IAS) is a fibromuscular anatomical structure dividing the left and right atrium and is of substantial importance for inter and interatrial conduction.

The true atrial septum is defined by the septal area which could be pierced or crossed without exiting the heart consists of the flap-like structure within the oval fossa and the surrounding muscular rim, in particular, the anteroinferior rim 1-4.

The remainder essentially is made up of the interatrial groove a result of an infolding of the right atrial wall which separates the superior vena cava from the right pulmonary veins superoposteriorly. Inferiorly the interatrial groove runs adjacent to the inferior vena cava and the coronary sinus and connects to the inferior pyramidal space. The anterosuperior part of the interatrial groove usually holds the sinus node artery.

The interatrial groove is immediately adjacent to the atrial septum superiorly, posteriorly and inferiorly. Additional bordering structures include the following 1,2:

The arterial supply is very variable and usually includes a network of several small branches, usually originating from the right coronary artery, the right posterolateral artery or the circumflex artery depending on coronary arterial dominance. Rarely the interatrial septum is supplied by Kugel’s artery if the latter is present 2,5.

Innervation of the interatrial septum includes extrinsic and intrinsic ganglia 2.

The atrial septum can be visualized on four-chamber and short-axis views on echocardiography, cardiac CT or cardiac MRI.

The interatrial septum is characterized by an echogenic appearance 2.

The atrial septum can be best visualized on ECG-gated cardiac CT with a triphasic bolus injection technique suitable for additional opacification of the right-sided cardiac chambers.

The atrial septum can be best easily visualized and evaluated on cardiac MRI on four-chamber and short-axis views including the oval fossa.

The development of the atrial septum is a complicated process that initiates at around the 4th week and ends after birth 3,4:

  • 4th week: the septum primum, a muscular ridge with a mesenchymal cap originates at the posterior and superior atrial roof and grows towards the atrioventricular canal, lined by endocardial cushions; the ostium primum between the mesenchymal cap of the septum primum and the atrioventricular canal shrinks in this process.
  • 5th week: secondary perforations of the septum primum develop, just before the closure of the ostium primum at the time when the mesenchymal cap and vestibular spine fuse with the atrioventricular endocardial cushions; later the perforations expand and form the ostium secundum.
  • 8th-12th week: the mesenchymal cap and the vestibular spine differentiate into a myocardial rim or buttress at the anteroinferior border of the foramen ovale; an atrial infolding develops at the posterosuperior atrial roof just to the right atrial side of the septum primum.
  • after birth: due to the increase of left atrial pressure, the flap-like septum primum is pushed against the posterosuperior right atrial fold and fuses with it.

The interatrial septum is a target area for catheter ablation procedures. The atrioventricular node is located near the central fibrous body of the membranous atrioventricular septum just beneath the apex of Koch's triangle. Moreover, structural changes of the interatrial septum have been shown to pose a potential source of atrial fibrillation and other atrial tachyarrhythmias 2.

Pathological conditions of the interatrial septum include 1-3:

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