What defines third-degree AV block on ECG?

• A. Atria and ventricles beat independently; P-P and R-R intervals regular; no AV conduction; QRS may be wide if escape rhythm below the AV node.
• B. AV block with occasional conducted beats and variable PR.
• C. Only P waves present with no QRS.
• D. Very fast ventricular rate with regular P waves.

Answer: (A)

Third-degree AV block is complete AV dissociation: the atrial impulses and ventricular impulses beat independently because no signal gets through the AV node or below. On the ECG you’ll see P waves marching at a steady rate and QRS complexes marching at their own steady rate, with a fixed P-P interval and a fixed R-R interval but no fixed relationship between a P wave and the following QRS. That lack of AV conduction means the ventricles must rely on an escape rhythm below the block, which explains the QRS appearance: if the escape focus is near the AV node or in the His bundle, the QRS is narrow; if the escape rhythm is below the AV node (in the ventricles), the QRS is wide. Ventricular rates in complete block are typically slow, reflecting the escape rhythm pace.