Digoxin Toxicity

Meet the patient — two vignettes, one antidote

A 78-year-old woman on long-term digoxin for atrial fibrillation plus furosemide is brought in confused, vomiting for three days, complaining that the lamp looks green. Her pulse is 38, her ECG shows atrial tachycardia with 2:1 block, and her potassium is 3.1 mmol/L.[1]

A 25-year-old man presents two hours after swallowing 30 digoxin tablets in a deliberate overdose. He is vomiting profusely, bradycardic, and his potassium is already 6.8 mmol/L.[1]

Two patients, one mechanism, one antidote, two potassium pictures. The chronic case is hypokalaemic and the precipitant; the acute case is hyperkalaemic and the severity marker. Hold that fork and the whole page slots into place. The classic exam trap is the first ECG — an atrial tachyarrhythmia with a slow ventricular response on digoxin is toxicity until proven otherwise.[1][2]

What digoxin toxicity is — and why a normal level never excludes it

Digoxin toxicity is the clinical and biochemical syndrome of excessive cardiac glycoside effect, arising from inhibition of the sodium-potassium ATPase. It may occur with a serum digoxin concentration within or above the conventional therapeutic range, so toxicity is fundamentally a clinical diagnosis — a normal number never excludes it in the symptomatic patient.[1]

Digoxin remains in clinical use for two indications: rate control in atrial fibrillation (especially with coexisting heart failure or in sedentary patients) and as an adjunct in heart failure with reduced ejection fraction (HFrEF) to reduce hospitalisations. Its narrow therapeutic index — the toxic dose is only slightly above the therapeutic dose — makes toxicity common and frequently examined.[1]

The DIG trial reframed the drug. It established that digoxin reduces heart-failure hospitalisations but does not reduce all-cause mortality in HFrEF; the contemporary target serum concentration is therefore 0.5 to 0.9 ng/mL (lower than the older 0.8 to 2.0 ng/mL), which reduces toxicity risk without losing benefit. The RATE-AF trial later confirmed digoxin monotherapy is non-inferior to bisoprolol for symptom control in permanent AF, supporting its continuing role in selected patients while reinforcing the need for active toxicity monitoring.[3][4]

The topic is high-yield for five reasons: the characteristic symptom combination (GI plus visual plus cardiac); the potentiating effect of hypokalaemia; the pathognomonic arrhythmia (bidirectional VT); the classic teaching arrhythmia (atrial tachycardia WITH AV block); and the specific antidote (digoxin Fab fragments).[1][2]

Pharmacology — the numbers that explain every clinical feature

A working knowledge of digoxin pharmacology explains nearly every clinical and investigative feature, and is a frequent short-answer question in its own right.[1]

[1]

The clinical consequences of these pharmacokinetic facts are exactly what examiners probe. Because digoxin is renally cleared with a long half-life, any fall in glomerular filtration — acute kidney injury, dehydration, a heart-failure episode, or simply advancing age — drives accumulation and chronic toxicity over days to weeks. Because the volume of distribution is large (it distributes into skeletal muscle, not fat), the serum level is a small fraction of total body load — which is why haemodialysis does NOT remove digoxin and why the post-Fab total serum level becomes uninterpretable. Because the therapeutic window is narrow, the dose that controls atrial fibrillation is dangerously close to the dose that causes vomiting and arrhythmia.[1]

Classification — chronic versus acute, and the potassium fork

Cardiac glycoside toxicity is classified two ways that matter clinically — by temporal pattern and by source. The temporal pattern is the one that decides your potassium interpretation and your Fab dose.[1]

[1]

By source, cardiac glycosides include pharmaceutical digoxin and digitoxin, plants (foxglove or Digitalis purpurea, yellow oleander or Cascabela thevetia, white oleander or Nerium oleander, lily-of-the-valley or Convallaria majalis, red squill or Urginea), and toad venom cardioactive steroids (Chan Su, Kyushin, 'love stone'). All share the Na+/K+ ATPase mechanism and cross-react with digoxin Fab, which is why the antidote works across this whole toxin family.[2]

The single most important distinction at the bedside is digoxin EFFECT versus digoxin TOXICITY. Sagging ST depression with a shortened QT and a normal heart rate is digoxin effect (benign, do not treat); any arrhythmia (atrial tach with AV block, bidirectional VT, frequent PVCs, AV block) plus symptoms is digoxin toxicity (treat). ST changes alone are not toxicity.[1]

Why hypokalaemia is the engine of chronic toxicity

Digoxin toxicity is common because the drug's therapeutic index is narrow and its use is concentrated in the elderly — the very population most vulnerable to accumulation. Most cases are chronic accumulative toxicity in older patients with renal impairment and polypharmacy, not acute overdose.[1]

Patient-related precipitants, in order of how often they bite:[1]

• Renal impairment — digoxin is predominantly renally cleared (60 to 80 per cent unchanged in urine); falling creatinine clearance is the single biggest driver of chronic accumulation. Dose by renal function.
• Advanced age and low lean body mass — digoxin distributes into muscle (large Vd); sarcopenia lowers the Vd and raises serum concentration for a given dose.
• Hypothyroidism — reduces digoxin clearance.
• Electrolyte depletion — hypokalaemia, hypomagnesaemia, hypercalcaemia — all potentiate toxicity at the pump.
• Hypoxia and acidaemia — increase myocardial sensitivity to digoxin.
• Heart failure exacerbation and dehydration — reduce renal perfusion and clearance.[1]

The central role of HYPOKALAEMIA — state the mechanism in every viva. Potassium and digoxin compete for the same extracellular K+-binding site on the Na+/K+ ATPase alpha-subunit. When serum K+ falls, digoxin binds more avidly and its action is potentiated — so a patient on digoxin plus a loop or thiazide diuretic who becomes hypokalaemic can become toxic at a previously tolerated dose. Hypokalaemia also independently promotes DADs and arrhythmia. Hypomagnesaemia acts by the same logic (magnesium is the physiological blocker of the digoxin-binding site; low Mg2+ leaves the site more available) and must be corrected alongside potassium or hypokalaemia will be refractory.[1]

Drug interactions that raise digoxin levels — almost all act via P-glycoprotein inhibition:[1]

• Amiodarone — inhibits P-glycoprotein (digoxin renal or efflux transporter); halve the digoxin dose.
• Verapamil or diltiazem — P-gp inhibition.
• Quinidine — displaces digoxin from tissue binding plus P-gp inhibition (the classic interaction; halve the dose).
• Propafenone, flecainide — P-gp inhibition.
• Macrolides (clarithromycin, erythromycin) and itraconazole — P-gp or CYP inhibition.
• Ciclosporin, dronedarone, ritonavir — P-gp inhibition.
• Spironolactone or eplerenone — P-gp inhibition and K+-sparing (may raise the level but also protect K+).
• Loop and thiazide diuretics — via hypokalaemia and hypomagnesaemia (the commonest mechanism in practice).[1]

Regional epidemiology — cardiac glycoside plant self-poisoning. In South Asia (especially Sri Lanka and India) and parts of Africa, yellow oleander (Cascabela thevetia) and white oleander (Nerium oleander) self-poisoning causes thousands of admissions and substantial mortality each year, and historically drove the empirical use of digoxin Fab in plant poisoning. Cerbera odollam ('suicide tree') and Calotropis (madar) are other regional sources. In the West, foxglove herbal or tea ingestion and toad venom remedies are rarer sources.[2]

Pathophysiology — seven steps to the pathognomonic rhythm

The molecular mechanism is the single most frequently examined concept, reproduced in full.[1]

Step 1 — pump inhibition. Digoxin binds with high affinity to the alpha-subunit of the Na+/K+ ATPase (the sodium-potassium pump) and inhibits it. The pump normally extrudes 3 Na+ and imports 2 K+ per ATP hydrolysed, so its inhibition raises intracellular Na+ and lowers intracellular K+.[1]

Step 2 — intracellular calcium rises. The raised intracellular Na+ reduces the transmembrane Na+ gradient that normally drives the Na+/Ca2+ exchanger (NCX) to extrude Ca2+. With a weaker Na+ gradient, NCX activity falls (and may reverse), so intracellular Ca2+ accumulates. The extra Ca2+ is taken up into the sarcoplasmic reticulum via SERCA, so each subsequent action potential releases a larger bolus of Ca2+ onto the contractile apparatus — positive inotropy (the therapeutic effect in heart failure).[1]

Step 3 — why toxicity causes automaticity and arrhythmia. Excess intracellular Ca2+ overloads the SR. During phase 3 to 4 of the action potential, spontaneous SR Ca2+ release generates an inward Na+/Ca2+ exchange transient inward current (and a Ca2+-activated non-selective cation current), producing delayed afterdepolarisations (DADs). When a DAD reaches threshold, it triggers an extrasystole — this triggered activity underlies the ectopic and automatic atrial and ventricular arrhythmias of digoxin toxicity (PVCs, atrial tachycardia, bidirectional VT). Early afterdepolarisations (EADs) are bradycardia- or QT-related and are NOT the mechanism here — digoxin causes DADs, the high-calcium mechanism.[1]

Step 4 — the vagal (parasympathetic) effect. Digoxin enhances vagal efferent activity and increases the sensitivity of the SA and AV nodes to acetylcholine. The result is slowing of the sinus node and, importantly, slowing of AV nodal conduction — the basis of digoxin's use for ventricular rate control in atrial fibrillation. In toxicity this produces sinus bradycardia, AV block (1st, 2nd, 3rd degree), and junctional escape rhythms.[1]

Step 5 — the hallmark: automaticity PLUS conduction block together. Toxic digoxin simultaneously drives ectopic or automatic foci (via Ca2+ overload) AND blocks the AV node (via the vagal effect). The combination is essentially diagnostic: an atrial tachycardia (automatic focus firing fast) WITH AV block (the ventricular response is slow because the AV node is blocked). This is why atrial tachycardia WITH AV block is the classic teaching arrhythmia of digoxin toxicity.[1]

Step 6 — why bidirectional VT is pathognomonic. Bidirectional ventricular tachycardia arises from triggered automaticity from two foci — typically alternating sites in the left and right bundle-Purkinje system (or fascicles) — each producing a beat of opposite QRS axis, so the QRS alternates in direction beat-to-beat in the same lead. This alternating-axis VT is essentially diagnostic of digoxin (or a cardiac glycoside) toxicity in the right context, although rare catecholaminergic polymorphic VT and Andersen-Tawil syndrome are mimics.[1]

Step 7 — the potassium paradox. In acute overdose, massive pump inhibition prevents cellular K+ uptake and K+ leaks from cells, so serum K+ rises (hyperkalaemia); the degree of hyperkalaemia parallels severity, and a serum K+ over 5.0 to 5.5 mmol/L in acute poisoning is itself an indication for Fab. In chronic toxicity, the patient is usually on a diuretic and is hypokalaemic — and the hypokalaemia is itself the precipitant that potentiated the toxicity. This dual behaviour of potassium is the single most testable 'trap' in digoxin toxicity: the same electrolyte is a precipitant in chronic disease and a severity marker in acute overdose.[1]

Clinical presentation — three systems, and the ECG is the danger

The clinical picture spans three systems — gastrointestinal, neurological or visual, and cardiac — and the cardiac effects are the danger. Symptoms in chronic toxicity creep in over days to weeks; in acute overdose they appear within hours.[1]

Gastrointestinal (often the earliest features): anorexia (frequently the very first symptom), nausea and vomiting (very common), abdominal pain, diarrhoea, and weight loss (chronic).[1]

Neurological and visual: fatigue, malaise, and generalised muscular weakness (common and non-specific); headache, dizziness, confusion, and delirium — in the elderly this can masquerade as dementia or 'failure to thrive'; severe cases: seizures, coma (rare). The visual disturbance is the signature — blurred vision, photophobia, scotomata, photopsias (flashing lights), and xanthopsia (objects appear yellow-green; halos around lights) — the single most famous visual sign, though not universally present. Gynaecomastia occurs in chronic toxicity (a weak oestrogen-like effect of digitalis).[1]

Cardiac — the danger. Virtually any arrhythmia can occur, but the teaching rests on the principle that toxicity produces automaticity AND AV block together:[1]

[1]

The cardinal principle to state in a viva: in digoxin toxicity the heart shows simultaneous enhanced automaticity (fast ectopic foci) and impaired conduction (slow or blocked AV node). An atrial tachyarrhythmia with a slow ventricular response on digoxin is toxicity until proven otherwise.[1]

[1]

The elderly trap. Chronic toxicity in an older patient on digoxin plus a diuretic may present with vague anorexia, weight loss, confusion, or falls, with only minor ectopy on ECG. The diagnosis is missed if digoxin is not actively considered. Any elderly patient on digoxin with new GI, visual, or CNS symptoms OR new ectopy has digoxin toxicity until proven otherwise.[1]

Acute overdose presentation: deliberate self-harm (or paediatric or accidental) ingestion of a large dose produces prominent early nausea and vomiting, then bradycardia and AV block, and often severe hyperkalaemia. The risk of a life-threatening ventricular arrhythmia is high in the first 6 to 12 hours.[1]

[1]

Differential — driven by the presenting feature

The differential is driven by the presenting feature: a patient on digoxin with GI plus visual plus cardiac features, an isolated arrhythmia, or delirium and weakness.[1]

[1]

The decisive distinction within the topic itself is digoxin EFFECT versus digoxin TOXICITY: sagging ST depression with a shortened QT and a normal heart rate is digoxin effect (benign, do not treat), whereas any arrhythmia (atrial tach with AV block, bidirectional VT, frequent PVCs, AV block) plus symptoms is toxicity (treat).[1]

Bedside assessment — the ECG and the Fab triggers

Focused history: indication and dosing (AF rate control or HFrEF; the current dose and any recent change); adherence (extra tablets in chronic, or a large single ingestion in acute or self-harm); renal function and recent illness (any dehydration or AKI that reduced clearance); concurrent drugs (diuretics for K+ loss, amiodarone, verapamil or diltiazem, quinidine, macrolides, spironolactone); symptoms (GI, visual — colour change and halos, weakness, confusion); and the possibility of deliberate self-harm or plant or herbal ingestion (oleander, foxglove, toad-venom remedies).[1]

Focused examination: vital signs (heart rate — bradycardia common; blood pressure; respiratory rate; oxygen saturation; temperature; GCS); hydration status (dehydration worsens renal function and accumulation); visual fields and colour vision (xanthopsia); cardiovascular examination; and abdominal examination (nausea or vomiting).[1]

ECG — obtain a 12-lead and a continuous rhythm strip; recognise both faces:[1]

• Digoxin effect (benign): symmetrical 'sagging' or 'scooped' ST depression (the 'reverse tick' or sagging sign, best in V5 to V6 and lead II), shortened QT, flattened or biphasic T waves, and occasionally U waves. These changes correlate with digoxin concentration and are not toxicity by themselves.
• Digoxin toxicity (dangerous): frequent PVCs or bigeminy, atrial tachycardia WITH AV block, bidirectional VT, sinus bradycardia, AV block (1st to 3rd degree), junctional escape rhythm, VT or VF.[1]

Establish continuous cardiac monitoring and IV access immediately for any suspected case, and assess the severity markers that trigger Fab: haemodynamic instability or shock, life-threatening ventricular arrhythmia, symptomatic bradycardia or AV block unresponsive to atropine, serum K+ over 5.0 mmol/L in acute poisoning, and massive known ingestion.[1][2]

Investigations — draw the level BEFORE Fab

First-line investigations: serum digoxin level (draw BEFORE giving Fab), serum potassium, magnesium, calcium, urea or creatinine or eGFR, venous blood gas, 12-lead ECG with continuous cardiac monitoring, FBC, LFTs, glucose; troponin if ischaemia suspected; beta-hCG in women of childbearing potential; paracetamol and salicylate levels in any deliberate overdose. TSH to exclude hypothyroidism (which reduces clearance) is reasonable in chronic toxicity. An abdominal X-ray may reveal radiopaque digoxin tablets ('pill burden') in acute overdose — digoxin tablets contain a radiopaque filler, so a visible pill mass is a useful clue, not a requirement.[1]

Serum digoxin concentration — thresholds and caveats:[1]

• Therapeutic range 0.5 to 2.0 ng/mL (modern HFrEF target 0.5 to 0.9; traditional 0.8 to 2.0).
• Chronic toxicity usually appears above 2.0 ng/mL, but toxicity can occur within range (especially elderly or renal) — correlate clinically.
• Acute overdose may show levels over 10 to 15 ng/mL.
• Levels are most informative drawn more than 6 hours after ingestion — earlier samples reflect distribution, not total load.
• Endogenous digoxin-like immunoreactive substances (DLIS) can falsely elevate the assay in renal failure, neonates, and pregnancy — a high level in these settings needs clinical correlation, not reflex Fab.
• After Fab is given, the total serum digoxin level RISES (the assay measures bound plus free digoxin, and bound drug is removed from tissue into the circulation) and is uninterpretable — use the free digoxin level or equimolar dose calculations; never chase the post-Fab total level.[2]

The indications for digoxin Fab fragments — reproduce verbatim (life-threatening means give Fab):[1][2]

1. Ventricular arrhythmia — VT or VF.
2. Potassium over 5.0 mmol/L in acute poisoning (many references use over 5.5 mmol/L) — reflects severe pump inhibition.
3. Severe bradycardia or 2nd- or 3rd-degree AV block unresponsive to atropine.
4. Cardiac arrest attributable to digoxin.
5. Ingested dose over 10 mg in an adult, or over 4 mg (or 0.1 mg/kg) in a child.
6. Serum digoxin concentration over 10 to 15 ng/mL measured more than 6 hours post-ingestion.
7. End-organ dysfunction — cardiogenic shock, severe hyperkalaemia, life-threatening instability.[1][2]

The Fab (DigiFab) dose calculations — reproduce any one:[1]

• If the serum concentration and weight are known: number of vials equals (serum digoxin in ng/mL multiplied by weight in kg) divided by 100. Each DigiFab 40 mg vial binds 0.5 mg of digoxin.
• If the ingested dose is known (acute): vials equals total mg ingested multiplied by 1.6 (DigiFab); or, for Digibind 38 mg per vial, vials equals total mg ingested divided by 0.5.
• If the dose or level is unknown: give 1 to 2 vials empirically for chronic toxicity, or 5 to 10 vials for acute life-threatening overdose, and titrate to clinical response. In cardiac arrest, give 10 to 20 vials as a single IV bolus.[2]

Resuscitation — ABCDE, then stop the drug and correct the electrolytes

[1]

Begin with ABCDE: secure the airway, give high-flow oxygen if hypoxic, establish IV access, attach continuous cardiac monitoring, and identify and treat the immediately life-threatening arrhythmia first.[1]

Immediate non-drug measures:[1]

• STOP digoxin and review or stop interacting drugs (amiodarone, verapamil, macrolides, quinidine).
• Correct the precipitant (dehydration, AKI, interacting drug).
• Activated charcoal 50 g (1 g/kg in children) within 1 hour of acute ingestion (and multiple-dose activated charcoal for plant glycoside poisoning, which undergoes enterohepatic recirculation).[1]

Potassium management — different in chronic versus acute (the paradox in action):[1]

• Chronic toxicity with hypokalaemia: correct cautiously to keep serum K+ at 4.0 to 5.0 mmol/L (IV potassium chloride, typically 10 to 20 mmol/h via a central line with monitoring). Over-correction into hyperkalaemia worsens toxicity because the failing pump cannot tolerate it. Replace magnesium (1 to 2 g IV magnesium sulphate) alongside — hypomagnesaemia makes hypokalaemia refractory.
• Acute overdose with hyperkalaemia: do NOT treat with IV calcium (see below). Use insulin or dextrose (10 U soluble insulin plus 25 g dextrose IV), salbutamol nebuliser or IV, and sodium bicarbonate if acidaemic; sodium polystyrene sulfonate is slower. Fab is the definitive potassium-lowering treatment in acute digoxin toxicity because it restores the pump, allowing K+ to re-enter cells — the potassium will fall, sometimes sharply, so monitor and avoid over-shooting into hypokalaemia after Fab.[1]

Bradycardia or AV block resuscitation:[1]

• Atropine 0.5 mg IV (repeated every 3 to 5 min to a maximum of 3 mg).
• If unresponsive: transcutaneous pacing, then transvenous pacing. Avoid isoprenaline (provokes ectopy or arrhythmia in the sensitised myocardium). Avoid beta-blockers and non-dihydropyridine calcium channel blockers (verapamil or diltiazem) — they worsen AV block and raise digoxin levels via P-gp.[1]

Ventricular arrhythmia resuscitation:[1]

• Lidocaine 1 to 1.5 mg/kg IV (class Ib — shortens the action potential, suppresses DADs — the preferred agent) or phenytoin 10 to 15 mg/kg IV (also class Ib; uniquely, phenytoin counteracts the AV-nodal digoxin effect while suppressing ventricular automaticity — the classic viva drug for digoxin VT).
• Magnesium sulphate 1 to 2 g IV (blocks DADs; useful for torsades-like and ventricular ectopy).
• Avoid class Ia (quinidine, procainamide, disopyramide) — they prolong conduction and can worsen toxicity — and class Ic (flecainide, propafenone).
• DC cardioversion is a LAST RESORT: if unavoidable for unstable VT or VF, use LOW energy starting at 10 to 20 J (synchronised), escalating cautiously — high-energy shocks risk precipitating refractory VF in the digoxin-sensitised myocardium.
• In digoxin-induced cardiac arrest, follow standard ALS but give Fab early (10 to 20 vials IV bolus) and continue resuscitation; reversal can be dramatic.[1][2]

The key principle: give Fab EARLY for life-threatening features rather than escalating antiarrhythmics — it is the definitive antidote.[2]

Definitive management — the Fab ladder

[1]

Fab fragment (DigiFab or Digibind) details — the definitive antidote:[2]

• Purified Fab (fragment antigen-binding) fragments of anti-digoxin IgG raised in sheep; they bind free digoxin in the plasma, creating a concentration gradient that draws digoxin out of the tissues; the Fab-digoxin complex is renal-cleared.
• Onset 15 to 45 min, peak effect within 1 to 2 h; the effect can be dramatic — arrhythmias terminate, hyperkalaemia resolves, conduction restores.
• Dosing options (reproduce any one): by level and weight, vials equals (serum digoxin ng/mL multiplied by weight kg) divided by 100; by ingested dose, vials equals mg ingested divided by 0.5 (Digibind) or mg ingested multiplied by 1.6 (DigiFab); empirically, chronic toxicity 1 to 2 vials, acute life-threatening 5 to 10 vials, cardiac arrest 10 to 20 vials IV bolus.
• Avoid underdosing — give a full neutralising dose; a partial dose leaves free digoxin on board and causes rebound. For chronic toxicity in renal failure, smaller empirical doses (1 to 2 vials) are used because the total body load is smaller and recrudescence is harder to detect.
• Monitor potassium closely after Fab — as the pump is unblocked, K+ shifts back into cells and the serum K+ falls, sometimes rapidly; have IV potassium ready and recheck within 30 to 60 min.
• The post-Fab total digoxin level is uninterpretable (it rises) — use the free level.[2]

The explicit contraindication to IV calcium — the 'stone heart' controversy. The classic teaching (animal data from 1969 to 1970) holds that calcium worsens the already calcium-overloaded, digoxin-toxic myocardium and may produce irreversible systolic arrest ('stone heart'). Modern retrospective human series have challenged this and found no clear harm, and the dogma is increasingly regarded as unproven. However, the exam-standard and conservative practice remains to AVOID IV calcium in digoxin toxicity; manage hyperkalaemia with Fab, insulin or dextrose, salbutamol, and bicarbonate instead.[1]

Extracorporeal removal is ineffective for digoxin because of its large volume of distribution and extensive tissue binding; Fab is the modality. Haemodialysis may still be required for the underlying renal failure or severe refractory hyperkalaemia, but not for digoxin removal itself.[2]

Escalation triggers to ICU: any life-threatening arrhythmia; need for Fab; need for pacing; haemodynamic instability; acute massive ingestion; serum K+ over 6 mmol/L; end-organ failure.[1]

The scenarios examiners set

• Chronic toxicity in the elderly or renal patient (the commonest scenario): vague GI or CNS symptoms, low-to-normal digoxin level, hypokalaemia from a diuretic. Management is stop the drug, correct K+ and Mg2+, hold interacting drugs, and a low threshold for Fab if unstable. Re-evaluate the dose and dosing interval against renal function before any restart.[1]
• Acute deliberate overdose (suicidal): massive ingestion, severe hyperkalaemia (K+ over 5.5 is a Fab indication), life-threatening arrhythmia, very high digoxin level. Activated charcoal if early; high-dose Fab by formula or 5 to 10 vials; ICU.[2]
• Cardiac glycoside plant poisoning (yellow or white oleander, foxglove, lily-of-the-valley, Cerbera odollam, Calotropis or madar): identical toxidrome; common in South Asia. Fab fragments cross-react (dose empirically — 5 to 10 vials, may need repeat); multiple-dose activated charcoal for enterohepatic recirculation of plant glycosides. Mortality falls sharply with early Fab.[2]
• Toad venom (Chan Su, Kyushin, 'love stone'): cardioactive steroid (bufalin or cinobufagin) poisoning with the same Na+/K+ ATPase toxidrome; cross-reacts with the digoxin assay and with Fab.[2]
• Paediatric ingestion: even a few adult tablets are dangerous (threshold over 0.1 mg/kg or over 4 mg); low threshold for Fab, weight-based dosing, activated charcoal if early. Children may present with vomiting and bradycardia after finding tablets.[1]
• Patient already on digoxin who is started on amiodarone, verapamil, or a macrolide: the classic iatrogenic interaction — halve the digoxin dose at the time the interacting drug is introduced and recheck the level in 5 to 7 days.[1]

Complications and pitfalls

Cardiac complications: refractory ventricular arrhythmias (VT or VF), complete heart block or asystole, cardiogenic shock, sudden cardiac death.[1]

Non-cardiac complications: severe hyperkalaemia (acute overdose), hypokalaemia-induced worsening of toxicity (chronic), mesenteric ischaemia (rare, from splanchnic vasoconstriction), thrombocytopenia and gynaecomastia (chronic).[1]

Treatment-related complications: hypokalaemia after Fab (K+ shifts intracellularly as the pump is unblocked — anticipate and replace); rebound toxicity if the Fab dose is insufficient or as Fab is cleared and tissue digoxin redistributes; heart-failure exacerbation from withdrawal of the inotropic effect once digoxin is neutralised; rare hypersensitivity or serum-sickness to sheep Fab (low rate even on re-exposure).[1]

Classic pitfalls — examiners will name these back at you:[1]

• Treating digoxin EFFECT (ST changes) as toxicity — ST depression alone is benign.
• Giving IV calcium for hyperkalaemia in acute digoxin overdose.
• Using quinidine, procainamide, or flecainide (class Ia or Ic) for the arrhythmia.
• Over-correcting hypokalaemia into hyperkalaemia in chronic toxicity.
• Misinterpreting the post-Fab total digoxin level (it rises spuriously — use the free level).
• Relying on dialysis to remove digoxin (it does not).
• Missing non-pharmaceutical glycoside exposure (oleander, foxglove, toad venom) in a patient with no digoxin prescription.
• Falsely reassuring yourself with a 'therapeutic' level in a symptomatic elderly or renal patient.[1]

Rebound phenomenon: after Fab, tissue digoxin redistributes into plasma and, as Fab is cleared (over 24 to 72 h), free digoxin can rise again — monitor for recrudescence, especially after massive overdose; check the free digoxin, not total.[2]

The potassium paradox restated: chronic toxicity is driven or potentiated by hypokalaemia (diuretics), whereas the drug itself (Na+/K+ pump failure) causes hyperkalaemia in acute overdose. The potassium value guides Fab need in acute poisoning (K+ over 5.5 is an indication), and Fab itself lowers potassium — so anticipate hypokalaemia after reversal.[1]

Prognosis and disposition

Overall mortality of treated digoxin toxicity is low in modern practice with early Fab — mortality fell from roughly 30 per cent in the pre-Fab era to under 5 per cent in contemporary series. Untreated acute massive overdose and delayed presentation still carry high mortality.[2]

Predictors of severity or poor outcome: acute overdose, very high digoxin level, hyperkalaemia over 5.5 mmol/L, advanced age, renal failure, delay to Fab, and life-threatening arrhythmia at presentation.[1]

Disposition: asymptomatic or minimal chronic toxicity with corrected precipitant and normal K+ — ward observation, hold the drug, recheck the level and renal function, review interacting drugs, restart at a reduced dose. Life-threatening features or acute overdose — ICU; after Fab, observe 24 to 72 h for rebound.[1]

Prevention at discharge: dose reduction and recheck against renal function, avoid interacting drugs, patient education on symptoms, periodic monitoring of serum digoxin, K+, Mg2+, and renal function; assess deliberate-self-harm risk and refer to psychiatry where relevant.[1]

Special populations

• Elderly or renal impairment: the commonest and most vulnerable group. Reduce the dose by renal function (CrCl-based), target the lower serum level (0.5 to 0.9 ng/mL), monitor regularly, and maintain a low threshold to suspect. Sarcopenia lowers the Vd and raises the serum concentration.[1]
• Pregnancy: digoxin crosses the placenta and is in fact used therapeutically for fetal SVT; toxicity management is unchanged and Fab is safe in pregnancy. Endogenous DLIS may elevate the assay. The fetus shares the maternal exposure and risk.[1]
• Paediatrics: paediatric formulations exist; the toxicity threshold per kg is lower. Weight-based Fab dosing; a small number of adult tablets is dangerous; activated charcoal if early.[1]
• End-stage renal disease or dialysis: very high baseline risk (digoxin is renally cleared). Do NOT rely on dialysis for removal (Fab only); DLIS may falsely elevate the assay, so correlate clinically.[2]
• Deliberate self-harm or psychiatric patient: assess intent and risk, ensure safeguarding, and arrange psychiatric referral after medical stabilisation; secure the drug supply.[1]

Evidence, guidelines, and regional differences

• Narrative review and consensus (Andrews et al., European Journal of Emergency Medicine 2023): consolidates the contemporary diagnosis, indications for Fab, and dosing — the practical reference for ED management.[1]
• Fab evidence (Chan and Buckley, Clinical Toxicology 2014): the efficacy and safety case for digoxin-specific antibody fragments in both pharmaceutical and plant glycoside poisoning.[2]
• IV calcium controversy: the classic 'stone heart' teaching rests on 1969 to 1970 animal data; modern retrospective human data found no clear harm, but the exam-standard and conservative practice remains to AVOID IV calcium in digoxin toxicity and to use Fab or insulin-dextrose for hyperkalaemia.[1]

[1]

Exam pearls — the mnemonic and the pearls

The mantra: automaticity plus AV block; hypokalaemia potentiates; Fab for life-threatening; avoid calcium, Ia and Ic, and dialysis.[1]

[1]

One-liners examiners reward:[1]

• Digoxin inhibits Na+/K+ ATPase, raising intracellular Ca2+ (inotropy) and vagal tone (AV block); hypokalaemia potentiates (K+ competes for the pump site).
• Clinical triad: GI (nausea or vomiting, earliest) plus CNS (confusion, weakness) plus visual (yellow-green halos or xanthopsia) plus cardiac.
• Cardiac hallmark: arrhythmia from automaticity plus AV block together; atrial tachycardia WITH AV block classic; bidirectional VT pathognomonic; PVCs the commonest.
• Precipitants (chronic): hypokalaemia (key), renal failure, hypomagnesaemia, hypercalcaemia, amiodarone, verapamil, quinidine, macrolides.
• Fab fragments (DigiFab) for life-threatening: VT or VF, severe brady or AV block, K+ over 5.5 (acute), shock, massive overdose; binds digoxin; effect 15 to 45 min; dose equals (level multiplied by kg) divided by 100, or 5 to 10 vials empirically, 10 to 20 in arrest.
• AVOID: class Ia (quinidine, procainamide, disopyramide), class Ic (flecainide), IV CALCIUM ('stone heart'), beta-blockers and CCB, and dialysis (digoxin not dialysable).
• Hyperkalaemia in ACUTE overdose predicts severity and is a Fab indication; post-Fab total digoxin level RISES spuriously — use the free level; K+ falls after Fab (anticipate hypokalaemia).
• Digoxin is RENALLY cleared, large Vd, NOT dialysable, half-life 36 to 48 h; tablets are radiopaque (abdominal X-ray clue).
• DC cardioversion is a last resort — LOW energy (10 to 20 J) if unavoidable.
• Digoxin EFFECT on ECG (sagging ST, short QT) is benign — treat the patient (clinical), not the number.[1]

Ward-round test — five stems with answers in the Reveal.[1]

A patient on digoxin develops an atrial tachyarrhythmia at 170 per minute with a slow ventricular response of 50. Diagnosis and first three steps?

Digoxin toxicity until proven otherwise — atrial tachycardia WITH AV block is the classic teaching arrhythmia (automatic focus firing fast, AV node blocked, slow ventricular response). 1. Stop digoxin and any interacting drug; draw serum digoxin and K+ BEFORE any Fab. 2. Correct K+ and Mg2+ (this patient is usually hypokalaemic). 3. Continuous cardiac monitoring; give Fab if there is haemodynamic instability, life-threatening ventricular arrhythmia, or symptomatic bradycardia unresponsive to atropine.[1]

What dose of Fab if serum digoxin is 12 ng/mL and weight is 70 kg?

Vials equals (12 multiplied by 70) divided by 100 equals 8.4 — round up to 9 vials of DigiFab (each 40 mg vial binds 0.5 mg digoxin). In cardiac arrest or if the level is uncertain, give 10 to 20 vials IV empirically and titrate to clinical response. Remember to monitor potassium after Fab — it will fall as the pump is restored.[1]

Acute digoxin overdose, K+ 7.0 mmol/L. Why is IV calcium the wrong move, and what do you give instead?

The classic 'stone heart' teaching (animal data) holds that calcium worsens the already calcium-overloaded, digoxin-toxic myocardium; the exam-standard and conservative practice is to AVOID IV calcium. Instead use Fab (the definitive potassium-lowering treatment, as it restores the pump), plus insulin-dextrose (10 U plus 25 g), salbutamol, and sodium bicarbonate if acidotic. Anticipate that K+ will fall sharply after Fab.[1][2]

A patient with no digoxin prescription arrives with vomiting, confusion, bidirectional VT, and a 'digoxin level' of 5 ng/mL. What is going on?

Non-pharmaceutical cardiac glycoside exposure — yellow or white oleander, foxglove, lily-of-the-valley, red squill, or toad venom (Chan Su). All share the Na+/K+ ATPase mechanism, cross-react with the digoxin assay, and are reversed by digoxin Fab fragments. In South Asia, oleander self-poisoning is the classic cause. Give Fab empirically (5 to 10 vials) and multiple-dose activated charcoal for enterohepatic recirculation of plant glycosides.[2]

After Fab, the total serum digoxin level is reported as 25 ng/mL and rising. Do you give more Fab?

No — the post-Fab total digoxin level is uninterpretable. The assay measures bound plus free digoxin, and bound drug redistributes from tissue into the circulation, so the total rises spuriously. Monitor the free digoxin level and the clinical response (rhythm, K+, haemodynamics); give more Fab only for clinical recrudescence, not for the number. Also recheck K+ — it falls as the pump is restored.[2]

[1] [2] [3] [4] [1]

References

1. [1]Andrews P, Anseeuw K, Kotecha D, et al. Diagnosis and practical management of digoxin toxicity: a narrative review and consensus Eur J Emerg Med, 2023.PMID 37650725
2. [2]Chan BS, Buckley NA. Digoxin-specific antibody fragments in the treatment of digoxin toxicity Clin Toxicol (Phila), 2014.PMID 25089630
3. [3]Digitalis Investigation Group. The effect of digoxin on mortality and morbidity in patients with heart failure N Engl J Med, 1997.PMID 9036306
4. [4]Kotecha D, Bunting KV, Gill SK, et al. Effect of Digoxin vs Bisoprolol for Heart Rate Control in Atrial Fibrillation on Patient-Reported Quality of Life: The RATE-AF Randomized Clinical Trial JAMA, 2020.PMID 33351042