Asymptomatic QTc prolongation associated with quetiapine fumarate overdosein a patient being treated with risperidone

We report a patient who ingested a 2000-mg overdose of quetiapine fumarate (Seroquel (TM)). Her maintenance medications also included risperidone, ven lafaxine, topiramate, and clonazepam. On presentation, she was drowsy, but had no other significant CNS signs and no cardiac symptoms or abnormal phys ical signs. Approximately 2 h after the quetiapine ingestion, an electrocar diogram (ECG) showed normal sinus rhythm at 95 beats/min with a corrected Q T (QTc) interval of 537 ms (upper limit of normal = 440 ms). Plasma quetiap ine concentration at that time was 1800 ng/ mi. Continuous ECG monitoring f or the subsequent 18 h did not reveal any episode of ventricular tachycardi a. A 12 - lead ECG 18 h post- overdose was normal with a QTc interval of 40 1 ms and the corresponding plasma quetiapine concentration was 160 ng/ml. S he made an uneventful medical recovery from the toxic ingestion. This case suggests that when patients overdose on quetiapine while taking therapeutic doses of risperidone, such overdoses, even if not massive, can cause consi derable QTc interval prolongation. We recommend that quetiapine overdose pa tients undergo continuous ECG monitoring for 12-18 h post-ingestion.