An 82-year-old woman on digoxin for heart failure presents with nausea and visual halos. ECG shows regular narrow-complex tachycardia at 140 bpm with more P waves than QRS complexes and some non-conducted P waves. Serum potassium is 4.0 mEq/L. Which arrhythmia is this, and why is it characteristic of her condition?

• A Atrial tachycardia with AV block, from digoxin-induced enhanced automaticity combined with nodal blockade ✓
• B Typical AVNRT, caused by dual nodal physiology unmasked by digoxin
• C Atrial flutter with 3:1 block, from digoxin shortening atrial refractoriness
• D Second-degree AV block Mobitz II, from direct His-Purkinje toxicity

Explanation

Paroxysmal atrial tachycardia with block is the classic digoxin toxicity rhythm: digoxin increases atrial automaticity while simultaneously prolonging AV nodal refractoriness, so rapid atrial discharges coexist with dropped beats. Visual disturbances (yellow-green halos) and gastrointestinal symptoms support toxicity despite a normal potassium. Flutter with 3:1 block is rare because flutter usually conducts 2:1 or 4:1, and Mobitz II implies structural His-Purkinje disease rather than a drug effect on nodal tissue.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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