Mobitz type-2 second-degree heart block ECG is characterised by:

Appeared in: DSSSB 13 August 2024

Explanation

Mobitz Type II block is identified by a consistent and unchanging PR interval in the beats preceding a suddenly dropped QRS complex.
The key feature is the intermittent, non-conducted P wave that occurs without any preceding change in the PR interval.
This rhythm indicates a problem in the His-Purkinje system (infranodal), which is considered more unstable and serious than a block in the AV node itself.

Option B: This describes the classic pattern of Mobitz Type I (Wenckebach) second-degree AV block.
Option C: A prolonged PR interval alone, without any dropped beats, is characteristic of a first-degree AV block.
Option D: This describes a third-degree (complete) heart block, where there is a complete failure of communication between the atria and ventricles.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain ECG Interpretation of Atrioventricular (AV) Blocks as background academic context rather than a clinical decision trigger.
Mobitz Type II is a high-grade block with a significant risk of progressing to a complete (third-degree) heart block, which can cause syncope, hemodynamic collapse, and sudden cardiac arrest.
Due to this high risk, Mobitz Type II is a Class I indication for a permanent pacemaker, even if the patient is asymptomatic, to prevent life-threatening bradycardia.
What if? If the ECG showed a progressively lengthening PR interval before the dropped beat, the diagnosis would be Mobitz Type I (Wenckebach). This is a lower-risk block, often located in the AV node, and is typically managed with observation unless the patient is symptomatic (e.g., dizzy, fainting).

How to Approach the Question

First, analyze the overall rhythm. Identify the P waves and QRS complexes.
Determine if there is a 1:1 relationship between P waves and QRS complexes. If not, a block is likely present.
Look for 'dropped beats' (a P wave that is not followed by a QRS complex). If you see them, it's at least a second-degree block.
Next, measure the PR interval of the conducted beats before the dropped beat.
If the PR interval is constant and unchanging before the drop, it's Mobitz Type II.
If the PR interval progressively gets longer before the drop, it's Mobitz Type I (Wenckebach).

Concept Tested & Keywords

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E6 Medicine Harrison 22e Part 1 pp. 1966-1968, 1967-1969

E6 Nelson Textbook of Pediatrics(2024) — Volume 2 p. 647-649

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