A 69-year-old man is brought to the emergency department by his daughter due to 3 days of … — USMLE Step 3

A $69$ -year-old man is brought to the emergency department by his daughter due to $3$ days of progressive nausea, vomiting, profound weakness, and visual disturbances. He reports that room lights appear to have a "yellowish halo." His medical history includes heart failure with reduced ejection fraction, persistent atrial fibrillation, and chronic kidney disease stage 3b. His daily medications are carvedilol, sacubitril-valsartan, dapagliflozin, and digoxin. Eight days ago, he was prescribed a $7$ -day course of clarithromycin at an urgent care clinic for an exacerbation of chronic bronchitis.

Vital signs are temperature $37.1^\circ$ C ( $98.8^\circ$ F), blood pressure $105/60$ mmHg, heart rate $46$ /min, and respiratory rate $16$ /min. Physical examination reveals an irregularly irregular, bradycardic rhythm.

Laboratory findings:
Serum sodium: $136$ mEq/L
Serum potassium: $6.3$ mEq/L
Blood urea nitrogen: $44$ mg/dL
Serum creatinine: $2.2$ mg/dL (baseline $1.8$ mg/dL)
Serum digoxin level: $4.8$ ng/mL (therapeutic range: $0.5-0.9$ ng/mL)

An electrocardiogram (ECG) demonstrates atrial fibrillation with a slow ventricular response and frequent, multiform premature ventricular contractions.

Which of the following is the most appropriate initial pharmacologic intervention for this patient?

Intravenous digoxin-specific antibody fragmentsAnswer
B
Intravenous calcium gluconate
C
Intravenous regular insulin and dextrose
D
Oral sodium polystyrene sulfonate
E
Intravenous amiodarone

Answer

The most appropriate initial intervention is intravenous digoxin-specific antibody fragments.

This patient presents with severe digoxin toxicity, likely precipitated by the recent addition of clarithromycin. Clarithromycin is a potent inhibitor of P-glycoprotein, a transport protein responsible for the renal efflux of digoxin; inhibition leads to rapid accumulation of digoxin to toxic levels. Digoxin toxicity manifests with gastrointestinal distress, visual disturbances (yellow halos), and life-threatening arrhythmias. At a cellular level, digoxin directly inhibits the Na+/K+-ATPase pump, preventing potassium from entering the cell and leading to extracellular hyperkalemia. In the setting of digoxin toxicity, a serum potassium level greater than

5.0

mEq/L is an indicator of severe poisoning and a strong predictor of mortality. It is an absolute indication for the immediate administration of intravenous digoxin-specific antibody fragments (DigiFab). These fragments bind free digoxin in the serum, pulling the drug off its receptors and rapidly reversing the Na+/K+-ATPase blockade, which corrects both the arrhythmogenic state and the hyperkalemia.

Step-by-Step Solution

Analyze the patient's clinical presentation, medication history, and recent antibiotic use.

The patient has classic signs of digoxin toxicity (nausea, visual halos, bradycardia, premature ventricular contractions) precipitated by clarithromycin, a macrolide antibiotic that inhibits P-glycoprotein and significantly increases serum digoxin levels.

Identifying the precipitating drug interaction and the toxidrome is required to correctly interpret the laboratory findings.

Evaluate the laboratory results and ECG findings to determine illness severity.

The patient has severe hyperkalemia (

6.3

mEq/L), acute-on-chronic kidney injury, a highly elevated digoxin level (

4.8

ng/mL), and an ECG showing significant rhythm disturbances (bradycardia with multiform PVCs).

Severity markers dictate whether supportive care is sufficient or if an active antidote is immediately required.

Select the most appropriate evidence-based pharmacologic intervention.

Severe hyperkalemia (K+ >

5.0

mEq/L) in the setting of digoxin toxicity is a primary indication for intravenous digoxin-specific antibody fragments.

Digoxin-specific antibody fragments directly bind free digoxin, reversing the Na+/K+-ATPase blockade, which simultaneously corrects the hyperkalemia and the arrhythmogenic state.

Key Concept

Digoxin-specific antibody fragments are the first-line pharmacologic intervention for digoxin toxicity complicated by severe hyperkalemia (potassium > 5.0 mEq/L) or life-threatening arrhythmias. Intravenous calcium is contraindicated.

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