Which statement describes Third Degree AV Block (CHB)?

• A. Dropped QRS without PRI prolongation
• B. PRI lengthens until a QRS is dropped
• C. Ventricular rhythm shows partial dissociation
• D. P waves and QRS dissociate completely

Answer: (D)

Complete AV block means the electrical signal from the atria cannot reach the ventricles, so atrial and ventricular activities run independently. On the ECG you’ll see P waves at the normal atrial rate and QRS complexes at a slower ventricular rate, with no fixed relationship between the two. This complete dissociation of P waves and QRS complexes is what defines third-degree AV block.

That’s why the statement describing P waves and QRS dissociating completely is the best description. Other patterns fit different blocks: a dropped QRS without PRI change points to some second-degree block, a PRI prolonging until a QRS is dropped fits Wenckebach (Mobitz I), and partial dissociation suggests some retained relation between atrial and ventricular activity rather than complete independence.