What is the hallmark of third-degree (complete) atrioventricular block?

• A. Irregular rhythm with no AV dissociation
• B. Narrow QRS tachycardia with normal AV conduction
• C. P waves preceding each QRS with regular rhythm
• D. Slow heart rate and no coordination between atria and ventricles

Answer: (D)

Complete AV block occurs when no atrial impulse reaches the ventricles, so the atria beat independently of the ventricles. The hallmark is AV dissociation: P waves occur at the SA node rate, while the ventricles fire on a slower escape rhythm with no fixed relationship to the P waves. This produces a slow overall heart rate and a lack of coordination between atrial and ventricular activity. That combination—slow rate plus no coordination between atria and ventricles—best describes the condition. The other patterns imply some maintained conduction or an organized atrioventricular relationship, not complete block.