Question

Which one of the following ECG findings represents a manifestation of digitalis toxicity?

• A. ST segment depression
• B. Atrial tachycardia with variable block
• C. Atrial flutter
• D. Shortening of PR interval

Hint:

Digitalis toxicity does two opposite things to the atrium and the AV node at the same time - think about what rhythm that combination creates.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks which ECG change is a marker of digitalis (digoxin) toxicity rather than just the drug's normal, expected pharmacological effect on the tracing.

Step 2: Key Concept or Approach:
Digitalis has two opposing electrical actions at toxic levels: it increases automaticity in atrial tissue (so ectopic atrial pacemakers fire faster) while simultaneously slowing conduction through the AV node through increased vagal tone. When both effects occur together, the result is a fast atrial rhythm that cannot fully conduct to the ventricles, giving atrial tachycardia with a variable degree of AV block. This combination of enhanced automaticity plus impaired conduction is considered a near-pathognomonic marker of digitalis toxicity.

Step 3: Working Through the Options:
ST segment depression with a characteristic sagging, 'scooped' appearance is simply the normal therapeutic digitalis effect seen at standard doses, not a sign of toxicity. Atrial flutter can occur for many unrelated reasons and is not a specific toxicity marker. Digitalis toxicity typically prolongs the PR interval by slowing AV conduction, it does not shorten it - a shortened PR interval instead points toward a pre-excitation syndrome. Atrial tachycardia with variable AV block combines the two hallmark toxic effects of digitalis (increased atrial automaticity and depressed AV conduction) and is the classic toxicity arrhythmia.

Step 4: Conclusion:
Atrial tachycardia with variable block is the ECG manifestation of digitalis toxicity.