Question

Which of the following electrocardiographic changes occur during second and third degrees of atrio-ventricular heart block?
(A) In second degree atrio-ventricular heart block, ECG shows presence of 'P' wave but complete absence of 'QRS & T' wave in the block beat
(B) In second degree atrio-ventricular heart block, ECG shows complete absence of 'P, QRS & T' complex for one beat
(C) In third degree atrio-ventricular heart block, ECG shows slow and independent ventricular rate characterized by 'QRS' complex that are dissociated from faster 'P' wave
(D) In third degree atrio-ventricular heart block, ECG shows variations in 'P-P' interval with or without 'P-R' interval
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A) and (C) only.
• C. (A) and (D) only.
• D. (A) and (B) only.

Hint:

To easily identify AV blocks on an ECG: - 2nd-degree: Look for "dropped" QRS complexes after normal P waves. - 3rd-degree: Look for complete dissociation between P waves and QRS complexes.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Atrioventricular (AV) blocks are cardiac conduction disturbances where the electrical signal from the atria to the ventricles is delayed or completely blocked at the level of the AV node.

Step 2: Detailed Explanation:
Let us analyze each of the stated electrocardiographic changes:
- Statement (A): In second-degree AV block, some sinus impulses are not conducted to the ventricles. On the ECG, this is represented by a normal P wave (atrial depolarization occurs) that is not followed by a QRS-T complex (ventricular depolarization is blocked). This description is correct.
- Statement (B): The complete absence of P, QRS, and T waves for an entire beat is characteristic of sinoatrial (SA) block or sinus arrest, not AV block, because in AV block, the SA node still fires and produces a P wave. This statement is incorrect.
- Statement (C): Third-degree (complete) AV block is characterized by complete failure of conduction through the AV node. The atria and ventricles beat independently (AV dissociation). The atria beat at their normal fast sinus rate (regular P waves), while the ventricles beat at a slow escape rate driven by a subsidiary pacemaker (slow, independent QRS complexes). This statement is correct.
- Statement (D): In third-degree AV block, the P-P interval remains regular because the SA node is firing normally; there is no variation in the P-P interval. This statement is incorrect.
Thus, only statements (A) and (C) are correct.

Step 3: Final Answer:
The correct combination is (A) and (C) only, which corresponds to option (B).