Poisoning from - digoxin

Description

Narrow therapeutic index - toxicity from acute overdose OR chronic accumulation (renal impairment, dehydration, drug interactions - more insidious, harder to spot)
GI (nausea, vomiting, abdominal pain) + neuro (confusion, weakness) + visual (xanthopsia - yellow-green vision, halos) + cardiac arrhythmia - any arrhythmia is possible

Chronic toxicity - elderly, renal impairment, on diuretics (hypokalaemia potentiates toxicity)
Acute overdose - deliberate self-poisoning; more predictable dose-response but still dangerous

Inhibits Na+/K+-ATPase -> inc intracellular Na+ -> dec Na/Ca exchange -> inc intracellular Ca2+ -> inc contractility (therapeutic effect) but also inc automaticity/arrhythmia risk
inc vagal tone -> AV nodal block, bradycardia
Hyperkalaemia in acute overdose reflects the degree of Na/K-ATPase inhibition and correlates with severity/mortality
Hypokalaemia in chronic use (diuretics) potentiates toxicity by inc digoxin binding to the (fewer functioning) Na/K-ATPase sites

Digoxin level - but level correlates poorly with toxicity in chronic use (tissue-bound, slow equilibration); more reliable after acute overdose (>6h post-ingestion for a true level)
ECG: any arrhythmia - classically bidirectional VT, atrial tachycardia with block; also bradyarrhythmia, ectopy; "reverse tick" ST depression = therapeutic effect, not toxicity
Potassium level - key severity marker in acute overdose (K+ >5-5.5 mmol/L predicts high mortality without antidote)
Renal function - chronic toxicity usually driven by accumulation from AKI/CKD

Digoxin-specific antibody fragments (DigiFab) - definitive antidote
- Indications: life-threatening arrhythmia, K+ >5 mmol/L (acute), haemodynamic instability, or a large acute ingestion
- Dose calculated from ingested amount (acute) or empirically (chronic toxicity, unknown load)
Do NOT treat hyperkalaemia with calcium in digoxin toxicity - historically taught to worsen cardiac toxicity ("stone heart"); use insulin/dextrose, correct with DigiFab which resolves the hyperkalaemia by treating the cause
Correct hypomagnesaemia; avoid overcorrecting potassium once DigiFab given (rebound hypokalaemia as it enters cells)
Atropine for symptomatic bradycardia; avoid other antiarrhythmics that impair conduction further
Activated charcoal if early

Hypokalaemia, hypomagnesaemia, hypercalcaemia, renal impairment, hypothyroidism - all potentiate toxicity at "therapeutic" levels
Drug interactions: amiodarone, verapamil, clarithromycin - inc digoxin levels

Chronic toxicity often insidious - non-specific GI/confusion preceding arrhythmia by days
With DigiFab, arrhythmia and hyperkalaemia typically resolve within hours; without it, mortality is significant in severe toxicity

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