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 characterized by a delay in the electrical conduction from the atria to the ventricles, not a complete block.
• The hallmark on an ECG is a PR interval that is fixed (constant) and prolonged beyond the normal upper limit of 0.20 seconds (five small squares).
• A crucial diagnostic feature is that every P wave is followed by a QRS complex, indicating a 1:1 conduction ratio with no dropped or missed beats.
• The question's findings of a constant 0.24-second PR interval with 1:1 conduction perfectly match the definition of a first-degree AV block.

Why Other Options Were Wrong

• Option A: This rhythm involves a progressive lengthening of the PR interval with each beat until a QRS complex is eventually dropped. The question specifies a 'consistently prolonged' interval, not a progressively lengthening one.
• Option B: This involves a complete dissociation between atrial and ventricular activity. The P waves and QRS complexes occur independently of each other. The question states that 'all P waves are followed by a QRS complex,' which rules out a complete block.
• Option D: This rhythm is characterized by the absence of discernible P waves, which are replaced by chaotic fibrillatory waves, and an irregularly irregular ventricular response. The question clearly mentions the presence of P waves.

Related Visual

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.
• First-degree AV block is often benign and asymptomatic, but it's a crucial finding for nurses to document and report as it can be an early sign of underlying heart disease or a side effect of medications (e.g., beta-blockers, digoxin).
• The primary nursing responsibility is to monitor the patient's ECG for any progression to higher-degree blocks (e.g., Mobitz I or II), which are more hemodynamically significant and may require intervention.
• What if? If the ECG showed a progressively lengthening PR interval followed by a P wave with no QRS complex, the correct identification would be Second-degree AV block, Mobitz type I (Wenckebach).

How to Approach the Question

• First, identify the key parameters given in the question: the PR interval is 0.24 seconds and is 'consistently prolonged,' and 'all P waves are followed by a QRS complex.'
• Analyze the PR interval. A normal PR interval is 0.12-0.20 seconds. At 0.24 seconds, it is prolonged. This immediately points toward a type of AV block.
• Analyze the relationship between P waves and QRS complexes. The statement 'all P waves are followed by a QRS complex' means there is a 1:1 conduction ratio and no 'dropped' or 'missed' beats.
• Synthesize these two findings. A prolonged but constant PR interval with no dropped beats is the specific definition of a First-degree AV block.
• Systematically rule out the other options based on this definition. Mobitz I involves a changing PR interval and dropped beats. Third-degree block involves complete dissociation (no relationship between P and QRS). Atrial fibrillation lacks P waves entirely.

Concept Tested & Keywords

• Concept Tested: ECG Interpretation of Atrioventricular (AV) Blocks
• Stem keywords: electrocardiogram (ECG), PR interval, prolonged, 0.24 seconds, P waves, QRS complex
• Lead-in keywords: identify
• Clinical cues: PR interval consistently prolonged at 0.24 seconds: This value is greater than the normal upper limit of 0.20 seconds, indicating a conduction delay.
• Clinical cues: All P waves are followed by a QRS complex: This signifies a 1:1 atrioventricular conduction, ruling out second or third-degree blocks where beats are dropped.

Question ID

QPBPbFnWyTs4_nFOzAmqj8

Copy ID

Reference Book

E6 Medicine Harrison 22e Part 1 p. 1966-1968

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