Test Description

Mechanism of Action

Digoxin inhibits Na+/K+-ATPase, leading to increased intracellular sodium, which via the Na+/Ca2+ exchanger increases intracellular calcium. This produces a positive inotropic effect (increased contractility). It also enhances vagal tone, slowing AV conduction and reducing heart rate.

Why the Level Matters

Digoxin has one of the narrowest therapeutic indices of any commonly used medication. The difference between therapeutic and toxic levels is small, and toxicity can be fatal. Factors that alter digoxin sensitivity include electrolyte abnormalities, renal function, drug interactions, and thyroid status.

Timing is critical: Digoxin levels drawn before 6 hours post-dose will be falsely elevated due to the distribution phase and DO NOT reflect steady-state tissue levels. Early post-ingestion levels in acute overdose do NOT correlate with toxicity — use clinical findings instead.
Quick Reference
  • Therapeutic Range: 0.8 – 2.0 ng/mL (some guidelines recommend 0.5 – 0.9 ng/mL for heart failure)
  • Potentially Toxic: >2.0 ng/mL (though toxicity can occur within "therapeutic" range)
  • Sample Timing: Draw at least 6 hours post-dose (distribution phase takes 6-8 hours)
  • Antidote: Digoxin-specific antibody fragments (DigiFab / Digibind)
  • Critical Lab: Always check potassium — hypokalemia potentiates digoxin toxicity
  • Classic ECG: "Scooped" ST depression (Salvador Dalí mustache), increased PR interval, decreased QT interval
  • Elimination: Primarily renal (60-80%); half-life 36-48 hours (prolonged in renal failure)
Reference Ranges
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Clinical Context Target Range Notes
Heart Failure (HFrEF) 0.5 – 0.9 ng/mL DIG trial post-hoc analysis showed lower mortality; higher levels may increase mortality
Atrial Fibrillation Rate Control 0.8 – 2.0 ng/mL Traditional range; higher levels often needed for rate control
Potentially Toxic >2.0 ng/mL Risk increases significantly above this level
Trough Level Draw ≥6 hours post-dose Distribution phase lasts 6-8 hours; earlier draws are unreliable
Heart failure target has shifted: Modern evidence (post-hoc DIG trial analyses) suggests target levels of 0.5–0.9 ng/mL for heart failure — lower than the traditional 0.8–2.0 range. Levels >1.2 ng/mL were associated with increased mortality in HF patients.
Digoxin Toxicity

Acute vs. Chronic Toxicity

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Feature Acute Ingestion Chronic Toxicity
Patient Often young, intentional OD or accidental pediatric ingestion Often elderly on chronic therapy
Digoxin Level Very high (may be >10 ng/mL); early level unreliable May be only mildly elevated or even "therapeutic"
Potassium HYPERkalemia (Na+/K+-ATPase inhibition → K+ shifts extracellularly) HYPOkalemia (often the precipitant — diuretic use, renal losses)
GI Symptoms Prominent: nausea, vomiting, abdominal pain Often more insidious: anorexia, nausea
Cardiac Bradycardia, high-grade AV block, ventricular tachycardia Any dysrhythmia possible; atrial tach with block is "classic"
Visual Symptoms Less common acutely Classic "yellow vision" (xanthopsia), halos, blurred vision

ECG Findings in Digoxin Use vs. Toxicity

  • Therapeutic "digoxin effect" (NOT toxicity): "Scooped" ST depression, shortened QTc, flattened/inverted T waves — this is a drug effect and does NOT indicate toxicity
  • Toxicity dysrhythmias: Nearly any arrhythmia can occur; classic patterns include:
    • Atrial tachycardia with AV block — most characteristic
    • Bidirectional ventricular tachycardia — nearly pathognomonic
    • Accelerated junctional rhythm
    • Regularized atrial fibrillation (junctional escape in AF patient)
    • High-grade AV block, sinus bradycardia
    • Ventricular tachycardia or fibrillation
The hallmark of digoxin toxicity is "increased automaticity with decreased conduction." If a patient in atrial fibrillation suddenly has a regular rhythm (junctional escape) or develops ventricular arrhythmias, suspect digoxin toxicity.
Factors Potentiating Toxicity

Electrolyte Abnormalities

  • Hypokalemia — most important; K+ and digoxin compete for the same binding site on Na+/K+-ATPase. Low K+ = more digoxin binding = more toxicity at any given level
  • Hypomagnesemia — potentiates digoxin toxicity; often coexists with hypokalemia
  • Hypercalcemia — increases myocardial sensitivity to digoxin

Renal Insufficiency

Digoxin is 60-80% renally eliminated. Decreased GFR leads to drug accumulation. Dose reduction is required in renal failure. Elderly patients are at particularly high risk due to age-related decline in renal function.

Drug Interactions

  • Amiodarone — increases digoxin level by ~50% (reduce digoxin dose by half when starting amiodarone)
  • Verapamil, diltiazem — increase digoxin levels and have additive AV nodal blocking
  • Quinidine — doubles digoxin level
  • Spironolactone — may interfere with digoxin assay and reduce renal clearance
  • Erythromycin, tetracycline — increase digoxin absorption by eliminating gut flora that metabolize digoxin
Hypothyroidism increases sensitivity to digoxin; hyperthyroidism decreases it. Thyroid function should be assessed in cases of unexpected digoxin toxicity.
Treatment — DigiFab

Indications for Digoxin-Specific Antibodies (DigiFab)

  • Life-threatening dysrhythmias (VT, VF, high-grade AV block with hemodynamic instability)
  • Hyperkalemia >5.0 mEq/L in acute ingestion (indicates significant Na+/K+-ATPase inhibition)
  • Acute ingestion of >10 mg in adults or >4 mg in children
  • Digoxin level >15 ng/mL at any time or >10 ng/mL at 6 hours post-ingestion
  • Hemodynamically significant bradycardia unresponsive to atropine

DigiFab Dosing

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Scenario Dosing Method
Known level (chronic) Vials = [serum digoxin (ng/mL) × weight (kg)] / 100
Known ingestion amount Vials = [amount ingested (mg) × 0.8] / 0.5
Empiric — acute, life-threatening 10-20 vials (often give 10 initially)
Empiric — chronic toxicity 3-6 vials (start with 1-2 in less severe cases)

Important Management Notes

  • Atropine — first-line for symptomatic bradycardia; may not work in severe toxicity
  • Avoid calcium — traditionally contraindicated in hyperkalemia from digoxin toxicity ("stone heart" theory); recent evidence is debated, but DigiFab is preferred
  • Avoid cardioversion if possible — may precipitate refractory VF; use lowest energy if absolutely necessary
  • Treat hyperkalemia with insulin/glucose, sodium bicarbonate, DigiFab; avoid calcium if possible
  • Activated charcoal — consider if acute ingestion within 1-2 hours
  • After DigiFab: Digoxin levels are meaningless (measures bound + unbound; total level will be very high but drug is inactivated)
After DigiFab administration, standard digoxin assays will show a dramatically ELEVATED level because they measure both free and antibody-bound digoxin. This does not indicate worsening toxicity. Free digoxin levels (if available) should be used instead.
Clinical Pearls
Digoxin toxicity can occur at "therapeutic" levels: Patients with hypokalemia, hypomagnesemia, hypercalcemia, hypothyroidism, or renal insufficiency may develop toxicity with levels in the "normal" range. Always correlate the level with clinical findings and electrolytes.
Hyperkalemia in acute digoxin overdose is a marker of severity: In acute overdose, K+ >5.0 mEq/L indicates massive Na+/K+-ATPase inhibition and is an independent predictor of mortality. This is an indication for DigiFab even before cardiac arrhythmias develop.
Bidirectional VT is nearly pathognomonic: If you see alternating QRS axis on the ECG (beat-to-beat axis alternation), think digoxin toxicity first. The only other common cause is aconitine poisoning.
Oleander and foxglove poisoning: Cardiac glycosides are found in several plants (oleander, foxglove, lily of the valley) and in bufo toad venom. These cause identical toxicity to digoxin. DigiFab has variable cross-reactivity with plant glycosides — higher doses may be needed.
References
  1. Goldberger, Z. D., & Goldberger, A. L. (2012). Therapeutic ranges of serum digoxin concentrations in patients with heart failure. American Journal of Cardiology, 109(12), 1818-1821.
  2. Hauptman, P. J., & Kelly, R. A. (1999). Digitalis. Circulation, 99(9), 1265-1270.
  3. Chan, B. S., & Buckley, N. A. (2014). Digoxin-specific antibody fragments in the treatment of digoxin toxicity. Clinical Toxicology, 52(8), 824-836.
  4. Levine, M., et al. (2011). Assessment of hyperkalemia in the setting of acute digoxin toxicity and empiric Fab fragment use. American Journal of Emergency Medicine, 29(9), 1167-1171.
  5. Nelson, L. S., Howland, M. A., Lewin, N. A., et al. (Eds.). (2019). Goldfrank's Toxicologic Emergencies (11th ed.). McGraw-Hill. Chapter: Cardiac Glycosides.
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The clinical content and references are curated and reviewed by myself; however, AI was used to assist in organizing, paraphrasing, and formatting the information presented.