Which statement describes Third Degree AV Block (CHB)?

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.

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.