Digoxin Toxicity

Clinical meaning

Digoxin is a cardiac glycoside used therapeutically for heart failure with reduced ejection fraction (HFrEF) and ventricular rate control in atrial fibrillation. Its narrow therapeutic index (therapeutic level 0.5-2.0 ng/mL, with toxicity often occurring above 2.0 ng/mL) makes digoxin toxicity a common and potentially fatal clinical problem.

Mechanism of action and toxicity: Digoxin inhibits the sodium-potassium ATPase pump (Na+/K+-ATPase) on cardiac myocyte cell membranes. At therapeutic doses, this produces modest positive inotropic effect — inhibiting the Na+/K+-ATPase increases intracellular sodium, which reduces the sodium gradient driving the sodium-calcium exchanger (NCX). This causes intracellular calcium accumulation, enhancing cardiac contractility (positive inotropy). Digoxin also increases vagal (parasympathetic) tone, slowing AV nodal conduction and heart rate (negative chronotropy/dromotropy).

At TOXIC doses, excessive Na+/K+-ATPase inhibition causes dangerously high intracellular calcium, producing: (1) delayed afterdepolarizations (DADs) — spontaneous depolarizations during phase 4 of the action potential triggered by calcium overload of the sarcoplasmic reticulum, which can initiate ectopic beats and triggered automaticity (the mechanism of digoxin-induced arrhythmias); (2) enhanced vagal tone causing sinus bradycardia, sinoatrial block, and AV block; (3) enhanced automaticity with suppressed conduction — the combination that produces the classic 'regularized atrial fibrillation' (accelerated junctional rhythm with AV block) and bidirectional ventricular tachycardia.

The most critical predisposing factor is HYPOKALEMIA. Potassium and digoxin compete for the same binding site on the Na+/K+-ATPase pump. When potassium is low, more binding sites are available for digoxin → more pump inhibition → toxicity at LOWER serum digoxin levels. Therefore, a patient with hypokalemia can develop digoxin toxicity even with a 'therapeutic' serum digoxin level. Hypokalemia frequently results from concurrent diuretic therapy (loop and thiazide diuretics), which is extremely common in heart failure patients already taking digoxin.

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Diagnosis & workup

Diagnostics & workup: - Serum digoxin level: therapeutic range 0.5-2.0 ng/mL (levels >2.0 ng/mL are associated with toxicity, but toxicity can occur at 'therapeutic' levels in hypokalemia or hypomagnesemia); draw trough level at least 6 hours after the last dose (digoxin has a 6-8 hour distribution phase — levels drawn earlier are falsely elevated) - Serum potassium: CRITICAL — hypokalemia potentiates digoxin toxicity; hyperkalemia (K+ >5.5 mEq/L) in acute digoxin overdose is a marker of severe toxicity indicating massive Na+/K+-ATPase inhibition and is an indication for digoxin-specific antibody (Fab) fragments - Serum magnesium: hypomagnesemia worsens digoxin toxicity and often coexists with hypokalemia; correct magnesium first (hypokalemia is often refractory until magnesium is repleted) - ECG: classic findings of digoxin EFFECT (not toxicity): scooped ST-segment depression ('Salvador Dali mustache'), shortened QTc, PR prolongation; toxicity findings: any new arrhythmia, particularly PAT with block, bidirectional VT, accelerated junctional rhythm, high-degree AV block - Serum calcium: hypercalcemia potentiates digoxin toxicity - BUN/creatinine and GFR: assess renal function as declining clearance causes digoxin accumulation - Thyroid function: hypothyroidism reduces digoxin clearance; hyperthyroidism increases clearance (may require higher doses)

Risk factors: - Hypokalemia (MOST IMPORTANT): potassium and digoxin compete for the Na+/K+-ATPase binding site — low potassium increases digoxin binding and toxicity; commonly caused by concurrent loop/thiazide diuretics - Renal impairment: digoxin is 70% renally excreted; declining renal function (common in elderly HF patients) causes drug accumulation — dose must be adjusted for GFR - Advanced age: reduced renal clearance, reduced lean body mass (digoxin distributes to lean tissue), and increased sensitivity to digoxin effects - Hypomagnesemia: magnesium is a cofactor for the Na+/K+-ATPase pump; deficiency potentiates digoxin toxicity independent of potassium level (often coexists with hypokalemia in diuretic-treated patients) - Hypercalcemia: elevated calcium POTENTIATES digoxin's effect on intracellular calcium accumulation, increasing toxicity risk - Drug interactions that increase digoxin levels: amiodarone (increases level 70-100% — HALVE the digoxin dose when starting amiodarone), verapamil, quinidine, cyclosporine, macrolide antibiotics, propafenone - Hypothyroidism: reduces renal clearance and volume of distribution of digoxin, increasing serum levels - Low lean body mass (cachectic patients, elderly): digoxin distributes primarily to lean tissue — standard doses in low-muscle patients produce higher serum levels

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