The PR interval on ECG is prolonged in first-degree AV block. The normal delay at the AV node (rather than His-Purkinje) serves which physiological function?

• A Prevents retrograde conduction from ventricle to atria
• B Provides time for atrial repolarisation before the QRS complex
• C Ensures His bundle depolarisation precedes bundle branch activation
• D Allows ventricular filling to be completed before ventricular contraction begins ✓

Explanation

The physiological AV node delay (~100–120 ms of the normal PR interval of 120–200 ms) ensures that ventricular contraction does not begin until after atrial contraction has completed ventricular filling. The atrial 'kick' (atrial systole) contributes approximately 25–30% of ventricular end-diastolic volume, particularly important at fast heart rates. Without this delay, AV synchrony is lost — as in complete heart block or junctional rhythms — reducing cardiac output. The AV node delay is mediated by slow-response action potentials (Ca2+-dependent phase 0) with relatively slow conduction velocity (~0.05 m/s).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.