Emergency Medicine · Toxicology
The facts most likely to be tested
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First-line antidote for life-threatening arrhythmias or severe hyperkalemia is digoxin-specific Fab fragments.
Classic pathognomonic visual change is xanthopsia, seeing yellow-green halos around lights.
Most commonly precipitated by hypokalemia or acute renal failure raising digoxin levels.
ECG hallmark of true toxicity is bidirectional ventricular tachycardia or frequent PVCs.
Confirmed by a serum digoxin level >2.0 ng/mL, though toxicity can occur at lower levels with hypokalemia.
Correct underlying hypokalemia and hypomagnesemia, which increase digoxin binding and worsen toxicity.
Classically avoid calcium gluconate for hyperkalemia because of the board-tested risk of a fatal stone heart.
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A 74-year-old woman with atrial fibrillation and chronic kidney disease presents with nausea, anorexia, and confusion. She reports seeing yellow-green halos around lights. Her potassium is 6.2 mEq/L and her serum digoxin level is 4.1 ng/mL. ECG reveals a bidirectional ventricular tachycardia.
Which of the following is the most appropriate treatment?
Digoxin-specific antibody (Fab) fragments.
Life-threatening arrhythmia, severe hyperkalemia, and a markedly elevated digoxin level all mandate digoxin-specific Fab fragments. Calcium is classically avoided in digoxin-induced hyperkalemia because of the theoretical risk of inducing a fatal stone heart.
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Etiology / Epidemiology
Caused by supratherapeutic levels of digoxin, most commonly triggered by concurrent hypokalemia or acute renal failure.
Clinical Manifestations
Classically presents with GI distress, generalized weakness, and pathognomonic xanthopsia (yellow-green halos around objects).
Diagnosis
Confirmed by serum digoxin level >2.0 ng/mL, with ECG demonstrating PVCs or the highly specific bidirectional ventricular tachycardia.
Treatment
Administer digoxin-specific Fab fragments as the first-line antidote for life-threatening arrhythmias or severe hyperkalemia.
Prognosis
Highly lethal if unrecognized due to refractory ventricular arrhythmias, but rapidly reversible with targeted antidote therapy.
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Epidemiology & Etiology
Digoxin possesses a notoriously narrow therapeutic index, making toxicity common. Acute toxicity often results from intentional overdose, whereas chronic toxicity is typically precipitated by renal insufficiency, volume depletion, or interacting drugs like amiodarone, verapamil, or macrolides. Hypokalemia is the most common precipitating factor because potassium and digoxin compete for the exact same cellular binding site.
Pertinent Anatomy
Digoxin acts directly on the myocardial cell membrane and the vagus nerve. It increases vagal tone specifically at the AV node, leading to slowed conduction and prolonged AV nodal refractory periods.
Pathophysiology
Digoxin toxicity occurs via excessive inhibition of the myocardial Na+/K+-ATPase pump. This causes massive intracellular sodium accumulation, which subsequently reverses the Na+/Ca2+ exchanger, leading to severe intracellular calcium overload. Excess calcium increases resting membrane potential and lowers the threshold for delayed afterdepolarizations, directly triggering ventricular arrhythmias. Concurrently, hypokalemia drastically exacerbates toxicity by increasing digoxin binding affinity to the uninhibited Na+/K+-ATPase pumps.
Clinical Manifestations
Early symptoms are predominantly gastrointestinal (anorexia, nausea, vomiting) and neurologic (lethargy, confusion, weakness). The classic pathognomonic visual change is xanthopsia, described by patients as seeing yellow-green halos around lights. Cardiac manifestations range from asymptomatic bradycardia and AV blocks to life-threatening arrhythmias.
Diagnosis
The gold standard for confirmation is a serum digoxin level >2.0 ng/mL, though clinical toxicity can occur at lower levels if hypokalemia is present. An immediate serum potassium level is critical to assess for hyperkalemia (a marker of severe acute toxicity) or hypokalemia (a trigger for chronic toxicity). The ECG classically shows the Salvador Dali mustache sign (scooped ST segments) indicating digoxin effect, but true toxicity most commonly presents with PVCs or the highly specific bidirectional ventricular tachycardia.
Treatment
Immediately discontinue digoxin and any potassium-depleting diuretics. Administer digoxin-specific Fab fragments as the definitive first-line treatment for life-threatening arrhythmias, end-organ dysfunction, or potassium >5.0 mEq/L in the setting of acute overdose. Correct underlying hypokalemia or hypomagnesemia to reduce digoxin binding. Classically, avoid calcium gluconate for hyperkalemia in this specific setting due to the board-tested theoretical risk of inducing a fatal stone heart.
Prognosis
Rapid administration of digoxin-specific Fab fragments effectively reverses fatal arrhythmias and normalizes potassium levels. Rebound toxicity can occur as tissue stores equilibrate back into the serum, requiring continuous cardiac monitoring and serial electrolyte checks.
Differential Diagnosis
1. Beta-blocker toxicity: Presents with bradycardia and hypotension but is uniquely accompanied by hypoglycemia.
2. Calcium channel blocker toxicity: Presents with bradycardia and hypotension but is uniquely accompanied by hyperglycemia.
3. Amiodarone toxicity: Causes bradycardia and QT prolongation but lacks the GI symptoms and xanthopsia.
4. Hypokalemia (isolated): Causes arrhythmias and U waves on ECG but lacks visual changes or profound GI distress.