Digoxin-specific antibody ✅

🚨 Digoxin-specific antibody fragments, also called digoxin immune Fab or DigiFab®, are the antidote for life-threatening digoxin or cardiac glycoside toxicity. They bind free digoxin in the blood, forming complexes that are then renally cleared. Always check the BNF digoxin-specific antibody fragments monograph, local policy and TOXBASE/NPIS before prescribing where possible.

ℹ️ About

• Drug: digoxin-specific antibody fragments.
• Brand: DigiFab®.
• Class: antidote; ovine Fab immunoglobulin fragments.
• Use: treatment of known or strongly suspected life-threatening digoxin or cardiac glycoside toxicity.
• Formulation: 40 mg vial for solution for infusion.
• Key fact: one 40 mg vial binds approximately 0.5 mg digoxin.
• Specialist advice: discuss with TOXBASE/NPIS, senior clinician, cardiology, ICU or toxicology as appropriate.

🧬 Mode of Action

• Digoxin inhibits the sodium-potassium ATPase pump, increasing intracellular sodium and calcium.
• This increases vagal tone and myocardial automaticity, causing bradyarrhythmias, AV block and ventricular arrhythmias.
• Digoxin-specific Fab fragments bind free digoxin with high affinity.
• The bound digoxin becomes pharmacologically inactive and is eliminated mainly by the kidneys.
• Clinical improvement is often rapid, sometimes within 30–60 minutes, but monitoring must continue because rebound toxicity can occur.

⚠️ When to Suspect Severe Digoxin Toxicity

• 🤢 Nausea, vomiting, abdominal pain, anorexia or diarrhoea.
• 👁 Visual symptoms: yellow-green vision, blurred vision or halos.
• 🧠 Confusion, delirium, weakness or dizziness.
• 🐢 Bradycardia, AV block or slow atrial fibrillation.
• ⚡ Ventricular ectopics, bidirectional VT, VT or VF.
• 🧪 Hyperkalaemia, especially in acute poisoning.
• 🫘 Risk factors: renal impairment, older age, dehydration, hypokalaemia, hypercalcaemia, interacting drugs.

🚨 Indications for Digoxin-Specific Antibody Fragments

• Cardiac arrest or peri-arrest rhythm due to suspected digoxin/cardiac glycoside toxicity.
• Life-threatening ventricular arrhythmias, including VT or VF.
• Severe bradyarrhythmia, severe sinus bradycardia, or second-/third-degree AV block not responding to atropine.
• Haemodynamic instability with suspected digoxin toxicity.
• Hyperkalaemia in acute digoxin toxicity, especially potassium >5.0 mmol/L.
• Very high serum digoxin concentration after acute ingestion or in chronic toxicity, interpreted with timing, renal function and clinical features.
• Large acute ingestion, especially in adults, children, renal impairment or underlying cardiac disease.

⚠️ Do not treat the number alone. Serum digoxin concentrations are difficult to interpret if taken too early after ingestion, because distribution into tissues takes time. A result taken before at least 6 hours post-ingestion may overestimate clinically relevant tissue toxicity. The patient’s ECG, potassium, renal function, haemodynamics and symptoms are often more important than the serum concentration alone.

💊 Dose - Always Check BNF / TOXBASE / Local Policy

🧮 Dose Calculation Principles

• One 40 mg vial neutralises approximately 0.5 mg digoxin.
• If the amount ingested is known, dose can be estimated from the ingested digoxin dose.
• If a reliable serum concentration is available, dose can be estimated from serum digoxin concentration and body weight.
• In emergency practice, dose is often guided by clinical severity and TOXBASE/NPIS advice.
• Repeat dosing may be required if toxicity persists or recurs.

💉 Administration

• Route: intravenous infusion.
• DigiFab® is reconstituted with water for injections, then diluted in sodium chloride 0.9% according to product information/local policy.
• Manufacturer information commonly advises infusion over about 30 minutes.
• In cardiac arrest or immediately life-threatening toxicity, more rapid administration may be considered under expert/resuscitation guidance.
• Use aseptic preparation and monitor continuously during administration.

🔎 Monitoring

• Continuous ECG monitoring.
• Heart rate, blood pressure, oxygen saturations and conscious level.
• Serum potassium frequently - hypokalaemia can occur after Fab treatment.
• U&E and creatinine because Fab–digoxin complexes are renally cleared.
• Magnesium and calcium.
• Clinical response: rhythm, perfusion, symptoms and haemodynamics.
• Do not rely on total serum digoxin concentration after Fab treatment, as assays may measure bound digoxin and appear falsely high.

⚠️ Cautions

• Previous allergy to sheep protein, papaya/papain or Fab products may increase hypersensitivity risk.
• Renal impairment may delay clearance of Fab–digoxin complexes and increase rebound risk.
• Hypokalaemia may develop as digoxin effect is reversed, so potassium must be monitored closely.
• Reversal of digoxin may worsen heart failure or allow recurrence of rapid atrial fibrillation in patients dependent on digoxin for rate control.
• Pregnancy, children and complex poisoning require specialist toxicology advice.

⛔ Contraindications

• There are no absolute contraindications in immediately life-threatening digoxin toxicity.
• Known severe hypersensitivity to digoxin-specific antibody fragments is a major caution, but life-saving treatment may still be required with senior specialist input.

⚕️ Side Effects

• Hypokalaemia.
• Hypotension during or after infusion.
• Allergic reactions, including urticaria, angioedema, bronchospasm or anaphylaxis.
• Febrile reactions.
• Serum sickness, usually delayed.
• Worsening heart failure after reversal of digoxin effect.
• Rapid ventricular response in atrial fibrillation if digoxin effect is reversed.
• Rebound digoxin toxicity, especially in renal impairment.

🚫 Important Pitfalls

• Do not delay Fab in unstable life-threatening toxicity while waiting for a serum digoxin concentration.
• Do not interpret early post-ingestion digoxin concentrations without considering timing.
• Do not use serum digoxin levels after Fab treatment to guide immediate clinical response.
• Do not forget potassium: hyperkalaemia suggests severe acute toxicity; hypokalaemia may follow treatment.
• Do not give routine calcium for hyperkalaemia in digoxin toxicity without senior/toxicology advice because of historical concern about arrhythmia risk.
• Do not perform electrical cardioversion unless immediately necessary, as digoxin-toxic myocardium may be very irritable.

📝 Exam Pearls

• 🚨 Digoxin toxicity can cause both bradyarrhythmias and ventricular arrhythmias.
• 🧪 Hyperkalaemia in acute digoxin poisoning is a marker of severe toxicity.
• 💊 Digoxin-specific Fab is the antidote for life-threatening toxicity.
• 🧮 One 40 mg vial binds about 0.5 mg digoxin.
• 📈 Digoxin level after Fab can be misleading because bound digoxin may still be measured.
• 🫘 Chronic toxicity is common in older patients with renal impairment or interacting drugs.
• 📞 In the UK, check TOXBASE/NPIS and local policy early.

🧠 Teaching Note

Digoxin toxicity is dangerous because it creates a combination of increased automaticity and impaired AV conduction. That is why the ECG can show apparently contradictory rhythms: ventricular ectopics or VT alongside bradycardia or AV block. In acute poisoning, hyperkalaemia reflects severe sodium-potassium ATPase inhibition and is a major warning sign. Fab fragments work by pulling free digoxin out of the circulation, shifting the equilibrium away from tissue binding and rapidly reducing toxicity.

📚 References

• BNF/NICE – Digoxin-specific antibody fragments
• DigiFab® Summary of Product Characteristics
• TOXBASE – UK National Poisons Information Service
• RCEM Learning – Digoxin toxicity
• Management of digoxin toxicity – review article