A 62-year-old man on digoxin and furosemide for HFrEF presents with nausea, visual disturbances described as yellow halos around lights, and ventricular bigeminy. Serum potassium is 3.0 mEq/L. The most likely precipitating mechanism is:

• A Reduced digoxin binding to Na+/K+ ATPase
• B Diuretic-induced hypokalemia increasing digoxin binding to cardiac Na+/K+ ATPase ✓
• C Furosemide inhibiting hepatic metabolism of digoxin
• D Volume depletion reducing renal clearance of furosemide

Explanation

Digoxin competes with potassium for binding on the extracellular face of Na+/K+ ATPase. Hypokalemia permits greater digoxin binding at a given serum level, producing toxicity even at therapeutic concentrations. Loop diuretics are the commonest culprit in heart failure patients. Hyperkalemia would reduce binding, which is why option A describes the opposite direction. Management includes correcting potassium and giving digoxin-specific antibody fragments if arrhythmias are severe.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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