All of the following regarding the atrioventricular (AV) node are true EXCEPT:

Explanation

## Atrioventricular Node: Anatomy and Conduction ### Correct Statements (Options A, B, D) **Key Point:** The AV node is a critical relay station in the cardiac conduction system, located in the Koch triangle with the slowest conduction velocity in the heart. ### Koch Triangle Anatomy (Option A — TRUE) **High-Yield:** The Koch triangle is a crucial landmark for AV node localization: - **Anterior boundary:** Tendon of Todaro (a fibrous band in the right atrium) - **Inferior boundary:** Septal leaflet of the tricuspid valve - **Base:** Coronary sinus ostium This triangle is clinically important in catheter ablation procedures for AV nodal reentrant tachycardia (AVNRT). ### AV Node Conduction Properties (Option B — TRUE) | Feature | Details | |---------|----------| | **Conduction Velocity** | Slowest in heart (~0.02–0.05 m/s) | | **PR Interval Delay** | Accounts for ~100 ms of the 120–200 ms PR interval | | **Purpose** | Allows atrial contraction to complete before ventricular activation | ### Functional Zones (Option D — TRUE) The AV node does contain three functional regions: - **AN (atrionodal) zone:** Transitional tissue between atrium and compact node - **N (nodal) zone:** The compact AV node proper — site of maximal delay - **NH (nodal-His) zone:** Transitional tissue between AV node and His bundle Conduction velocity progressively increases from N → NH → His bundle, making Option D essentially correct as stated. ### Why Option C Is INCORRECT **Warning:** Option C states the AV node receives dual blood supply from the **right coronary artery (RCA) and the left anterior descending artery (LAD)**. This is **FALSE**. The correct blood supply to the AV node is: - **Right coronary artery (RCA):** ~90% of individuals (right-dominant circulation) - **Left circumflex artery (LCx):** ~10% of individuals (left-dominant circulation) The **LAD does NOT supply the AV node**. The LAD primarily supplies the anterior interventricular septum, anterior left ventricle, and the bundle branches (left bundle branch and right bundle branch). Confusing the LCx with the LAD is a classic distractor in cardiac anatomy questions. **Clinical Pearl:** This is why inferior MI (RCA territory) commonly causes AV block — the AV nodal artery is a branch of the RCA in most individuals. In contrast, anterior MI (LAD territory) more commonly causes bundle branch blocks rather than AV nodal block. ### Conduction Velocity Hierarchy in the Heart | Tissue | Velocity | |--------|----------| | Purkinje fibers | 2–4 m/s (FASTEST) | | His bundle | 1.5 m/s | | Atrial muscle | 1.0 m/s | | Ventricular muscle | 0.3–0.4 m/s | | AV Node | 0.02–0.05 m/s (SLOWEST) | [cite: Clinically Oriented Anatomy 8e Ch 1; Guyton & Hall Medical Physiology 14e Ch 11; Gray's Anatomy 41e]