Description
Furosemide-induced hypokalemia and hypomagnesemia increase the risk of digoxin toxicity even when digoxin levels remain within the conventional therapeutic range.
Mechanism
Digoxin's cardiac glycoside effect is potentiated by low serum potassium and magnesium; furosemide causes renal losses of both electrolytes. Hypokalemia sensitises myocytes to digoxin-induced automaticity and conduction abnormalities.
Clinical Significance
Hypokalemia below 3.5 mEq/L markedly increases the risk of digoxin-induced arrhythmias (ventricular ectopy, AF acceleration, high-degree AV block).
Management
Monitor potassium and magnesium closely; supplement as needed to maintain K+ ≥ 4.0 mEq/L and Mg2+ ≥ 0.8 mmol/L in patients taking both drugs; add potassium-sparing diuretic or ACE inhibitor if persistent hypokalemia.