Toxicology – Antidote: Digoxin Immune Fab

Indications

Digoxin immune Fab is the specific antidote for severe or life-threatening cardiac glycoside toxicity. It may be considered when poisoning produces findings such as:

• Hemodynamic instability or significant hypotension
• Severe symptomatic bradycardia or advanced conduction abnormalities
• Dangerous ventricular dysrhythmias
• Significant hyperkalemia in acute poisoning
• Rapidly worsening toxicity after a major exposure

It can also bind several naturally occurring cardiac glycosides found in plants such as foxglove, oleander, dogbane, milkweed, and lily of the valley.

Mechanism of Action

Digoxin immune Fab consists of antibody fragments that bind circulating digoxin and related cardiac glycosides.

Once bound:

• Free digoxin concentrations fall
• Digoxin dissociates from its tissue receptors
• Inhibition of the Na⁺/K⁺-ATPase decreases
• The digoxin–Fab complexes are subsequently cleared from the body

This can rapidly reverse serious cardiac and systemic toxicity.

Digoxin Toxicity Mechanism

Normally, digoxin inhibits the Na⁺/K⁺-ATPase pump, increasing intracellular sodium.

This reduces the activity of the Na⁺/Ca²⁺ exchanger, causing intracellular calcium to rise. More calcium becomes available for release from the sarcoplasmic reticulum, producing digoxin’s positive inotropic effect.

At toxic concentrations, however, excessive intracellular calcium promotes abnormal automaticity and dysrhythmias. Digoxin also increases vagal activity, which slows the sinus rate and conduction through the AV node.

Administration

The amount of digoxin immune Fab required depends on factors such as:

• Whether toxicity is acute or chronic
• The estimated amount of digoxin involved
• Serum digoxin concentration when appropriately timed
• Severity of the patient’s clinical findings

In severe emergencies, empiric treatment may be given without waiting for laboratory confirmation. Dosing should follow current toxicology or poison-center guidance.

Monitoring After Fab

Monitor:

• ECG and cardiac rhythm
• Potassium
• Renal function
• Blood pressure and clinical response

Potassium may decrease rapidly as toxicity reverses, so hypokalemia can develop after treatment.

Important Laboratory Point

After an acute ingestion, a digoxin concentration obtained too early—before tissue distribution is complete—can be difficult to interpret; levels are generally most useful several hours after ingestion.

After digoxin immune Fab is administered, routine digoxin assays can measure both bound and unbound drug and may therefore show a markedly elevated total digoxin concentration that does not represent ongoing toxicity.

Key Points

• Digoxin immune Fab directly binds and neutralizes circulating cardiac glycosides.
• Severe dysrhythmias, cardiovascular instability, and significant hyperkalemia are important warning signs of serious toxicity.
• Digoxin’s increased contractility comes from increased intracellular Ca²⁺, while its slowing of heart rate and AV conduction is largely related to enhanced vagal effects.
• Do not interpret routine serum digoxin levels normally after Fab has been administered.