Indications for Digoxin Fab in Digoxin Toxicity
Digoxin-specific antibody fragments (digoxin-Fab) should be administered immediately to any patient with digoxin toxicity who presents with life-threatening arrhythmias, hemodynamic instability, or hyperkalemia >5.0 mEq/L. 1
Class I (Strongest) Indications for Digoxin-Fab
The American Heart Association provides the highest level recommendation (Class I, Level of Evidence B) for digoxin-Fab in severe life-threatening cardiac glycoside toxicity. 1 This represents the strongest guideline support for any intervention in digoxin toxicity. 2
Life-Threatening Arrhythmias Requiring Immediate Treatment:
- Ventricular arrhythmias: Ventricular tachycardia (especially fascicular or bidirectional), ventricular fibrillation, or multifocal ventricular ectopy 1, 2
- Severe bradyarrhythmias: High-grade AV block (second or third-degree), progressive bradycardia, or cardiac standstill/asystole 1, 2
- Symptomatic bradycardia with hemodynamic compromise: Heart rate typically <45 bpm with associated symptoms or instability 1, 3
Hyperkalemia as a Critical Marker:
- Potassium >5.0 mEq/L in acute digoxin poisoning is a marker of severity and associated with poor prognosis, warranting empiric digoxin-Fab administration 1
- Potassium >6.0 mEq/L represents an even stronger indication for immediate treatment 4, 5
Serum Digoxin Concentration Thresholds:
- Digoxin level >4 ng/mL with any clinical manifestations of toxicity 2
- Digoxin level >2 ng/mL with life-threatening arrhythmias or hemodynamic instability 1, 5
- However, do not delay treatment waiting for digoxin levels if clinical presentation suggests life-threatening toxicity 6
Class IIa (Reasonable) Indication
- Symptomatic bradycardia in the setting of digoxin toxicity without immediate life-threatening features is reasonable to treat with digoxin-Fab, with clinical response rates of 80-90% 1
Expected Clinical Response
- Dysrhythmia resolution typically occurs within 30-45 minutes of digoxin-Fab administration 4, 2, 5
- Free digoxin concentration falls to nearly zero within minutes of administration 5
- In cardiac arrest from digoxin poisoning, survival was 54% (30 of 56 patients) with digoxin-Fab treatment 2
Dosing Strategy Based on Clinical Scenario
Acute Overdose with Known Ingested Dose:
- Administer 2 vials of Fab for every milligram of digoxin ingested 1
- Note: This will overestimate required dose due to incomplete bioavailability (60-80%), vomiting, and activated charcoal effect 5
Chronic Toxicity or Unknown Dose:
- Calculate vials using formula: serum digoxin concentration (ng/mL) × weight (kg) / 100 1
- Alternatively, give 40 mg (1 vial) initially and repeat after 60 minutes if patient remains symptomatic 5
- Most chronic toxicity cases require only 40-120 mg (1-3 vials) total 5
Critical/Emergent Situations:
- Administer empirically 10-20 vials when therapy is required before serum digoxin level can be obtained or in life-threatening toxicity 1
- In imminent cardiac arrest, a full neutralizing dose may be justified 5
Titrated Approach (Cost-Effective):
- For acute poisoning: 80 mg bolus, repeated as required according to clinical parameters, as most patients require less than half the calculated neutralizing dose 5
Critical Pitfalls to Avoid
- Never use hemodialysis, hemofiltration, hemoperfusion, or plasmapheresis for digoxin removal—these are ineffective due to digoxin's large volume of distribution (5-10 L/kg) 1, 6
- Do not administer calcium IV rapidly to digitalized patients, as this can precipitate serious arrhythmias 6
- Monitor potassium levels closely both before and after digoxin-Fab administration, as levels can shift dramatically (hypokalemia may develop post-treatment) 4, 2, 6
- Do not wait for confirmatory digoxin levels before treating life-threatening toxicity—treat immediately based on clinical presentation 6
Temporizing Measures While Awaiting Digoxin-Fab
- Atropine may be reasonable for symptomatic bradyarrhythmias, though evidence is limited to case reports with variable effects 2, 7
- Temporary cardiac pacing may be considered for refractory bradydysrhythmias, though complications are common (36% in one series) 2
- Lidocaine or phenytoin may be reasonable for ventricular dysrhythmias until digoxin-Fab can be administered 1, 2, 7
Post-Treatment Monitoring
- Monitor for rebound toxicity, exacerbation of heart failure, and increased ventricular rate in atrial fibrillation 4, 6
- Serum digoxin levels become unreliable after antidigoxin antibody administration due to measurement of bound digoxin 2
- Watch for rare allergic reactions (infrequent occurrence) 4, 8
- Response is typically rapid, occurring within 30 minutes to 4 hours 2