Reversible Second-Degree Heart Block Attributed to Ondansetron: A Rare Side Effect.

A primigravida at 36 weeks with gestational diabetes mellitus and hypothyroidism and no prior chronic medical illness was admitted for safe confinement. A cesarean section was required to deliver the baby with breech presentation complicated by a slow progression of labor. Asymptomatic sinus bradycardia with a heart rate of 40 per minute was observed during the induction of anesthesia. Before bupivacaine administration for spinal anesthesia, she was administered pantoprazole 40 mg and ondansetron 4 mg intravenously. ECG recording showed a type 1 Mobitz second-degree heart block. Follow-up ECG showed progression of heart block to type 2 Mobitz second-degree heart block. The second-degree heart block persisted for 16 hours, during which the patient was asymptomatic, and the ventricular rate was maintained at a range of 60-80 per minute. After normalization of rhythm, the patient was observed in the ICU. She received another dose of ondansetron 4 mg intravenously for vomiting, and the heart block recurred. The rhythm disturbance was attributed to ondansetron. Her rhythm normalized after 36 hours, and she was subsequently discharged home three days later.