Which statement best describes a third-degree AV block?

Which statement best describes a third-degree AV block?

• A. Atria and ventricles beat independently of each other
• B. P waves precede QRS with fixed PR interval
• C. QRS complexes are absent
• D. P waves are identical to QRS complexes

Answer: (A)

In a third-degree AV block, there is complete failure of conduction from the atria to the ventricles, so the atrial and ventricular activities run independently of each other. The atria keep their own rhythm (driven by the SA node), while the ventricles pick up a slower escape rhythm from a below‑the‑AV node focus. On the ECG this shows up as P waves occurring at a normal atrial rate and QRS complexes at a slower, regular ventricular rate, with no fixed relationship between a given P wave and the following QRS complex. The PR interval is not consistent because there is no conduction from atria to ventricles. This is why the description of atria and ventricles beating independently best describes third-degree AV block. The other ideas don’t fit because a fixed PR interval implies some conduction, QRS absent would mean no ventricular depolarization, and P waves being identical to QRS complexes would mix atrial and ventricular signals inappropriately.

In a third-degree AV block, there is complete failure of conduction from the atria to the ventricles, so the atrial and ventricular activities run independently of each other. The atria keep their own rhythm (driven by the SA node), while the ventricles pick up a slower escape rhythm from a below‑the‑AV node focus. On the ECG this shows up as P waves occurring at a normal atrial rate and QRS complexes at a slower, regular ventricular rate, with no fixed relationship between a given P wave and the following QRS complex. The PR interval is not consistent because there is no conduction from atria to ventricles.

This is why the description of atria and ventricles beating independently best describes third-degree AV block. The other ideas don’t fit because a fixed PR interval implies some conduction, QRS absent would mean no ventricular depolarization, and P waves being identical to QRS complexes would mix atrial and ventricular signals inappropriately.