Palpitations and a QTc of 520 ms in a 60-year-old woman

A 60-year-old woman was admitted to hospital with a diagnosis of community-acquired pneumonia. Her medical history included hypertension and depression. On admission, she was prescribed clarithromycin 500 mg twice daily and citalopram 20 mg once daily (her long-term antidepressant). She also received intravenous ondansetron 4 mg once daily for nausea, starting on day 2. On day 3, she complained of palpitations. A 12-lead ECG showed a QTc interval of 520 ms. Her serum potassium was 3.8 mmol/L, serum magnesium was 0.9 mmol/L (within normal limits), and serum calcium was normal. Her other medications included amlodipine 5 mg daily and aspirin 75 mg daily. Ondansetron was discontinued immediately. In view of the prolonged QTc, she was transferred to a monitored bed for continuous telemetry. After 24 hours, the QTc was 440 ms. She had no further palpitations. Her discharge was delayed by one day to allow for observation after the QTc had normalised. She remained on citalopram and clarithromycin. She had not been rechallenged with ondansetron. Alternative causes considered include the QT-prolonging effects of citalopram and clarithromycin, but the QTc normalised while these drugs were continued, making them less likely. Electrolyte imbalance was considered, but serum electrolytes were within normal limits.