Digoxin: Mechanism of Action & Treatment of Toxicity

Mechanism of Action

1. Primary Mechanism — Na⁺/K⁺-ATPase Inhibition

• Intracellular Na⁺ accumulates → Na⁺/Ca²⁺ exchanger (NCX) is less able to extrude Ca²⁺ (because it normally drives Ca²⁺ out by importing Na⁺) → intracellular Ca²⁺ rises
• Elevated intracellular Ca²⁺ → increased myocardial contractility (positive inotropy)

"Digitalis glycosides selectively bind to the alpha-subunit of the sarcolemmal Na⁺-K⁺-ATPase on its extracellular surface and reversibly inhibit the enzyme." — Barash's Clinical Anesthesia, 9e

Important note: An increase in extracellular K⁺ partially inhibits digoxin-Na⁺-K⁺-ATPase binding — this is why potassium administration can reverse digoxin toxicity from hypokalemia.

2. Autonomic (Neurohormonal) Effects

• Enhances parasympathetic (vagal) tone → slows SA node automaticity and AV nodal conduction (negative chronotropy, negative dromotropy)
• Reduces sympathetic nervous system activity → lowers norepinephrine, reduces LV afterload
• Inhibits renin release — additional neurohormonal benefit in heart failure
• Acts on cardiac baroreceptors, further modulating autonomic balance

"Digoxin is thought to enhance parasympathetic and reduce sympathetic nervous activity as well as to inhibit renin release." — Goldman-Cecil Medicine

3. Electrophysiologic Effects

4. Hemodynamic Effects (Heart Failure)

• Improved cardiac output → reflex reduction in LV preload and afterload
• Reduced LV wall tension and myocardial oxygen demand
• Used as oral inotrope in HFrEF; the DIG trial showed a 28% reduction in HF hospitalizations with no mortality benefit

Digoxin Toxicity

Features of Toxicity

• Cardiac (most dangerous): Virtually any arrhythmia except rapidly conducted atrial arrhythmias
  • Most common: premature ventricular contractions (PVCs) and bradycardic rhythms
  • Pathognomonic (rare): bidirectional ventricular tachycardia
  • AV blocks (1st, 2nd, 3rd degree), ventricular tachycardia/fibrillation
• Non-cardiac: Nausea, vomiting, confusion, visual disturbances (yellow-green halos), xanthopsia
• Metabolic: Hyperkalemia (acute poisoning — from Na⁺/K⁺-ATPase pump paralysis releasing intracellular K⁺)

ECG Findings (Therapeutic — Not Toxicity Signs)

• "Scooped" ST segment depression
• T-wave flattening or inversion
• QT interval shortening
• Increased U-wave amplitude

Predisposing Factors

• Hypokalemia (most important — potentiates digoxin binding)
• Hypomagnesemia
• Hypercalcemia
• Renal insufficiency (digoxin is renally cleared)

Therapeutic levels: 0.5–2.0 ng/mL | Toxic: >2.5 ng/mL (levels most reliable ≥6 hours post-ingestion)

Treatment of Digoxin Toxicity

Step 1 — General Supportive Care

• Continuous cardiac monitoring + IV access
• Correct hypoxia, hypoglycemia, hypovolemia
• Monitor serum electrolytes

Step 2 — GI Decontamination

• Activated charcoal (1 g/kg PO) — only if awake, alert, cooperative, and presenting within 1 hour of ingestion
• Gastric lavage is NOT recommended (can trigger vagal asystole)
• Hemodialysis, hemoperfusion, forced diuresis — NO role (digoxin has large volume of distribution)

Step 3 — Correct Electrolytes

• Potassium (K⁺): Correct hypokalemia (predisposes to toxicity); K⁺ administration also competitively inhibits digoxin binding to Na⁺-K⁺-ATPase
• Magnesium: IV magnesium sulfate counteracts ventricular irritability
• Calcium: Avoid in recognized digoxin toxicity (controversial — may worsen ventricular arrhythmias; hyperkalemia from digoxin is not the main cause of death)

Step 4 — Arrhythmia Management

⭐ Definitive Antidote: Digoxin-Specific Antibody Fragments (Digoxin-Fab)

1. Life-threatening arrhythmias (hemodynamically significant)
2. Hyperkalemia >5.0 mEq/L in acute poisoning
3. Cardiac arrest

• Rapid hypokalemia (as toxicity reverses and K⁺ re-enters cells)
• ↑ ventricular response in atrial fibrillation
• Cardiogenic shock (in patients dependent on digoxin for inotropic support)
• Rare: hypersensitivity (rash, flushing)