digoxin × furosemide

Furosemide causes potassium and magnesium loss. Hypokalemia increases cardiomyocyte sensitivity to digoxin – digitalis arrhythmia (atrial tachycardia with AV block, ventricular bigeminy) can develop even at therapeutic concentrations.

Keep potassium above 4.0 mmol/L and magnesium in the upper-normal range. Check electrolytes every 1–3 months on stable therapy, more often when adjusting furosemide. For hypokalemia, use potassium supplements or potassium-sparing diuretics (spironolactone).