The triangle of Koch on the right atrial endocardium demarcates the location of the AV node. The three boundaries of the triangle of Koch are:

• A Tendon of Todaro (anteriorly), septal leaflet of tricuspid valve (anteroinferiorly), and coronary sinus os (posteriorly) ✓
• B Fossa ovalis, crista terminalis, and pectinate muscles
• C Bundle of His, right coronary artery, and mitral valve annulus
• D SA node, anterior interatrial bundle, and inferior vena cava os

Explanation

The triangle of Koch is formed by three landmarks: (1) the tendon of Todaro (a fibrous ridge running from the inferior vena cava to the central fibrous body), (2) the attachment of the septal leaflet of the tricuspid valve (anteroinferiorly), and (3) the coronary sinus orifice (marking the apex posteriorly). The AV node is located at the apex of this triangle, just above the coronary sinus os in the interatrial septum. This triangle is the key landmark for electrophysiologists performing ablation of AV nodal re-entrant tachycardia (AVNRT), where slow pathway ablation is performed posteroinferiorly near the coronary sinus os.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.