A nurse is analyzing an electrocardiogram (ECG) strip and observes that the PR interval is consistently prolonged at 0.24 seconds, but all P waves are followed by a QRS complex. Which cardiac arrhythmia should the nurse identify?

1. A

   Second-degree AV block, Mobitz type I

2. B

   Third-degree (complete) AV block

3. C

   First-degree AV block

4. D

   Atrial fibrillation

Appeared in: NORCET 9 Prelims-2025

Explanation

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• First-degree AV block is defined by a delay in conduction from the atria to the ventricles, not a complete block.
• The hallmark ECG finding is a PR interval that is consistently prolonged beyond 0.20 seconds.
• A crucial feature is that every P wave is followed by a QRS complex, indicating a stable 1:1 relationship between atrial and ventricular contractions.
• The question describes a PR interval of 0.24 seconds (which is greater than 0.20 seconds) and consistent 1:1 conduction, matching the definition perfectly.

Why Other Options Were Wrong

• Option A: Second-degree AV block, Mobitz type I (Wenckebach) involves a progressive lengthening of the PR interval until a beat is dropped. The question states the PR interval is consistently prolonged.
• Option B: Third-degree AV block involves a complete dissociation between P waves and QRS complexes. The question specifies that all P waves are followed by a QRS, which contradicts this diagnosis.
• Option D: Atrial fibrillation is characterized by the absence of discernible P waves and an irregularly irregular rhythm. The question clearly states that P waves are present and followed by QRS complexes.

Related Visual

• Visual 1: ECG Strip Comparison. An infographic showing side-by-side ECG strips for Normal Sinus Rhythm, First-Degree AV Block, Second-Degree AV Block (Types I & II), and Third-Degree AV Block. This allows for direct visual comparison of the PR interval and P:QRS relationship in each condition.

Clinical Relevance

• Nursing practice connection: Use the key finding related to ECG Interpretation of Atrioventricular (AV) Blocks to guide bedside assessment, documentation, and the next nursing action.
• While often benign, the nurse must assess for potential causes of First-Degree AV Block, such as medications (beta-blockers, digoxin), electrolyte imbalances, or myocardial ischemia.
• The primary nursing responsibility is to monitor the patient's ECG for any signs of progression to a more severe, hemodynamically unstable block (like Mobitz II or Third-Degree).
• Patient education is important, especially if the block is new or related to medication. The nurse should teach the patient to report symptoms like dizziness, fainting, or shortness of breath.

How to Approach the Question

• First, analyze the key parameters given in the question: PR interval is 'consistently prolonged at 0.24 seconds' and 'all P waves are followed by a QRS complex'.
• Recall the normal PR interval range (0.12-0.20 seconds). A value of 0.24 seconds is clearly prolonged.
• Focus on the relationship between P waves and QRS complexes. The phrase 'all P waves are followed by a QRS' signifies a 1:1 conduction ratio.
• Evaluate the options based on these two findings: a prolonged but constant PR interval and a 1:1 P:QRS ratio.
• Eliminate options that don't fit. Mobitz I has a progressively lengthening PR. Third-degree has AV dissociation (no 1:1 ratio). Atrial fibrillation has no P waves.
• Conclude that the only diagnosis matching a consistently prolonged PR interval with 1:1 conduction is First-degree AV block.

Concept Tested & Keywords

• Concept Tested: ECG Interpretation of Atrioventricular (AV) Blocks
• Stem keywords: electrocardiogram (ECG), PR interval, prolonged, 0.24 seconds, P waves followed by a QRS complex
• Lead-in keywords: identify
• Clinical cues: PR interval 0.24 seconds (prolonged)
• Clinical cues: All P waves are followed by a QRS complex (1:1 conduction)

Question ID

QPBPbFnWyTs4_nFOzAmqj8

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