Third Degree Heart Block (Complete Heart Block) is typically characterized by which combination of features?

• A. Very slow rate and an abnormal pacemaker site
• B. Rapid rate with narrow QRS
• C. Normal rate and normal conduction
• D. Frequent PACs

Answer: (A)

Third-degree heart block means complete AV dissociation: the atria keep beating at their normal rate, but no impulses reach the ventricles. The ventricles then follow an escape rhythm from a slower, abnormal pacemaker below the block (either a junctional or ventricular focus). This combination produces a very slow ventricular rate because the ventricular rhythm isn’t coming from the usual conduction system. The pacemaker site is abnormal because the ventricles are driven by an ectopic focus below the block rather than by the SA node through the AV node. This differs from a rapid rate with a narrow QRS (a supraventricular tachycardia with preserved conduction), normal rate with normal conduction (no block), or frequent PACs (premature atrial beats) which don’t create complete AV dissociation.