Digoxin Toxicity After Clarithromycin in Heart Failure
A 68-year-old man with heart failure with reduced ejection fraction and atrial fibrillation has been stable on digoxin 0.25 mg daily, furosemide 40 mg twice daily, spironolactone 25 mg daily, and carvedilol. Baseline creatinine is 1.2 mg/dL and potassium 4.6 mmol/L. He develops community-acquired pneumonia and is prescribed clarithromycin 500 mg twice daily for seven days. On day five he reports nausea, vomiting, anorexia, confusion, and seeing yellow-green halos around lights. Vital signs show HR 46/min, BP 92/58 mmHg, and RR 20/min. Laboratories reveal potassium 5.8 mmol/L, creatinine 1.8 mg/dL, and serum digoxin 3.2 ng/mL. The pharmacy calls to ask whether the digoxin dose should be adjusted, stopped, or whether an antidote is indicated. You must explain the pharmacokinetic interaction and prioritize immediate management.
1. Explain the pharmacokinetic interaction that produced digoxin toxicity.
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Clarithromycin inhibits P-glycoprotein in the intestinal wall and renal tubular cells. Digoxin is a P-glycoprotein substrate, so inhibition increases intestinal absorption and reduces renal tubular secretion, raising serum digoxin. The patient also has worsening renal function, which independently reduces digoxin clearance, and hyperkalemia, which increases myocardial susceptibility to digoxin toxicity. Symptoms of nausea, vomiting, confusion, and yellow-green visual halos fit digoxin toxicity. Bradycardia and hypotension reflect increased vagal tone and impaired conduction. The serum level of 3.2 ng/mL is above the usual therapeutic range, though toxicity is a clinical diagnosis and can occur at lower levels in susceptible patients. Clarithromycin should be stopped and an alternative antibiotic chosen.
2. What is your immediate management, and when would you use digoxin-specific antibody fragments?
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Stop digoxin and clarithromycin immediately. Obtain continuous ECG monitoring, check serial potassium, magnesium, and renal function, and treat hyperkalemia. Avoid intravenous calcium because it can potentiate digoxin toxicity. For bradycardia with hypotension, atropine may be used cautiously; pacing may be needed. Digoxin-specific antibody fragments are indicated for life-threatening arrhythmias, hemodynamic instability, or hyperkalemia greater than 5.5 mmol/L in the setting of digoxin toxicity. This patient has bradycardia, hypotension, renal failure, and potassium 5.8 mmol/L, so Fab should be arranged urgently. Treat the pneumonia with a non-interacting regimen after cultures. Monitor until rhythm and potassium normalize.