In third-degree (complete) AV block, which statement is true about the relationship between P waves and QRS complexes?

In third-degree (complete) AV block, which statement is true about the relationship between P waves and QRS complexes?

• A. P waves and QRS have a fixed relationship
• B. P waves are absent
• C. P waves and QRS have no fixed relationship; an escape rhythm maintains the ventricular rate
• D. QRS always follows P with a constant PR

Answer: (C)

In complete AV block, there is no conduction from the atria to the ventricles, so the atrial and ventricular activities run independently. The atria are paced by the SA node, producing P waves at their own rate, while the ventricles are driven by a secondary (escape) pacemaker below the block, creating QRS complexes at a slower, regular rate. Because these two pacemakers act independently, there is no fixed timing between a P wave and the subsequent QRS complex. The PR interval is not constant, and P waves may occur at any point relative to the QRS complexes. The ventricular rate is governed by the escape rhythm, not by the atrial impulses. This is why the statement that P waves and QRS have no fixed relationship and that an escape rhythm maintains the ventricular rate is the correct description.

In complete AV block, there is no conduction from the atria to the ventricles, so the atrial and ventricular activities run independently. The atria are paced by the SA node, producing P waves at their own rate, while the ventricles are driven by a secondary (escape) pacemaker below the block, creating QRS complexes at a slower, regular rate.

Because these two pacemakers act independently, there is no fixed timing between a P wave and the subsequent QRS complex. The PR interval is not constant, and P waves may occur at any point relative to the QRS complexes. The ventricular rate is governed by the escape rhythm, not by the atrial impulses. This is why the statement that P waves and QRS have no fixed relationship and that an escape rhythm maintains the ventricular rate is the correct description.