A patient presents with third-degree heart block on ECG. Which drug is contraindicated?

GMCH Nursing Officer-2025

Pharmacology

Medium

Appeared in: GMCH Nursing Officer-2025

Explanation

Digoxin is contraindicated in patients with third-degree (complete) heart block.
Its primary mechanism involves slowing conduction through the atrioventricular (AV) node by increasing vagal tone.
In complete heart block, where atrial impulses already fail to reach the ventricles, administering digoxin can worsen the block, leading to severe, life-threatening bradycardia or ventricular asystole.
The definitive treatment for third-degree heart block is cardiac pacing, and drugs that depress conduction are strictly avoided.

Option A: Epinephrine is a sympathomimetic drug that stimulates beta-receptors, increasing heart rate, contractility, and AV conduction.
Option B: Isoproterenol is a potent non-selective beta-agonist that significantly increases heart rate and facilitates AV conduction.
Option C: Atropine is an anticholinergic drug that blocks vagal influence on the SA and AV nodes, thereby increasing heart rate.

Visual 1: ECG Strip - An ECG showing third-degree heart block. The key feature to illustrate is the complete dissociation between P waves (atrial activity) and QRS complexes (ventricular activity), with P waves and QRS complexes occurring at their own independent, regular rates.
Visual 2: Diagram - A diagram of the heart's electrical conduction system. It should highlight the SA node, AV node, and Bundle of His, with a clear visual block shown at the AV node to represent the pathophysiology of third-degree heart block.

Clinical Relevance

Nursing practice connection: Use the key finding related to Pharmacological contraindications in third-degree atrioventricular (AV) block to guide bedside assessment, documentation, and the next nursing action.
A nurse's primary responsibility upon identifying third-degree heart block on a monitor is to assess the patient's hemodynamic stability (BP, mental status, perfusion) and immediately notify the physician.
It is critical to review the patient's medication administration record (MAR) and withhold any drugs that can slow AV conduction, such as digoxin, beta-blockers (e.g., metoprolol), and certain calcium channel blockers (e.g., verapamil, diltiazem).
The nurse should anticipate and prepare for emergency intervention, which is typically transcutaneous or transvenous pacing. Ensure the crash cart and pacing equipment are readily available.

How to Approach the Question

First, identify the core clinical condition from the stem: 'third-degree heart block'.
Recall the pathophysiology of this condition: a complete electrical dissociation between the atria and ventricles due to a block in the AV node or below.
Analyze the question's specific requirement: identify a 'contraindicated' drug. This means you are looking for a medication that would worsen the condition.
Evaluate each option based on its mechanism of action on the heart's conduction system.
Epinephrine, Isoproterenol, and Atropine all act to increase heart rate or improve AV conduction. They are treatments, not contraindications.
Digoxin acts to slow AV conduction. Applying a drug that slows conduction to a heart that already has a complete block is dangerous and therefore contraindicated.

Concept Tested & Keywords

Concept Tested: Pharmacological contraindications in third-degree atrioventricular (AV) block.
Stem keywords: third-degree heart block, ECG, drug, contraindicated
Lead-in keywords: Which drug is contraindicated?

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