Prolonged QT interval with markedly abnormal ventricular repolarization in diphenhydramine overdose.

Diphenhydramine is a histamine-1 receptor antagonist of ethanolamine origin with anticholinergic, sedative, antivertigo, antiemetic, antidyskinetic, and local anesthetic properties. It inhibits the fast sodium channels and, at higher concentrations, also the potassium channels, inhibition of which may result in QT interval prolongation on electrocardiogram. The markedly abnormal ventricular repolarization is rare with diphenhydramine overdose. We report a case where a young woman who took moderate overdose of diphenhydramine (625 mg) along with acetaminophen developed prolonged QT interval with strikingly abnormal T waves. These changes reverted to normal with treatment. Patient did not experience torsade de pointes, possibly secondary to the protective effect of the diphenhydramine overdose-related tachycardia. As the tachycardia caused by diphenhydramine overdose seems to protect against torsade de pointes, it may be practical to avoid bradycardia in acute phase of diphenhydramine toxicity. Acetaminophen has not been shown to prolong QT interval or effect cardiac repolarization.