Distinct electrocardiographic findings of a drug-drug interaction

A man in his 80s presented to the emergency department with a one day history of light headedness, dyspnoea, poor appetite, and fatigue. He had a history of hypertension, chronic atrial fibrillation, and atrial flutter, and his drug history included warfarin, metoprolol, digoxin, and furosemide. He reported a fever after experiencing symptoms of an upper respiratory tract infection and had taken a four day course of azithromycin empirically before presenting.

In the emergency department, the patient’s blood pressure was 100/68 mm Hg and his heart rate was 49 beats/min. The results of laboratory tests were: brain natriuretic peptide 1560 ng/L (range 0-100 ng/L), serum creatinine 1.5 mg/dL (range 0.7 -1.3 mg/dL for man), serum potassium 3.8 mmol/L (range 3.5-5.5 mmol/L), and serum digoxin 4.23 nmol/L (range 1.02-2.56 nmol/L). A 12 lead electrocardiogram (ECG) was recorded at admission (fig 1). An ECG six months previously had shown atrial fibrillation, prominent U waves, and a controlled ventricular rate. The ECG was repeated two days after the …