Diphenhydramine and QT prolongation - A rare cardiac side effect of a drug used in common practice.

There have been few cases in recent times where QT interval prolongation has been studied with regards to the use of diphenhydramine. We present a case of a patient who presented because of shortness of breath and needed emergent hemodialysis; during the course of which he developed prolonged QT interval on electrocardiography, which was correlated interestingly with the use of diphenhydramine. Pruritus is a common symptom experienced by dialysis patients. A less known, but rare side effect of diphenhydramine is prolongation of QT interval. The histamine H1 receptor antagonist diphenhydramine inhibits the fast sodium channels and at higher concentrations inhibits the repolarizing potassium channels which leads to prolongation of the action potential and the QT interval. Diphenhydramine toxicity is dose-dependent with a critical dose limit of 1.0 g. Although a lot is known about the potential side effects of antihistamines, only a few cases have cited the cardiac side effects. Thus, it is important for the clinician to be aware of this potentially serious consequence of a commonly used drug, especially in the end-stage renal disease population. It is important for clinicians to be aware of this rare yet dangerous side effect of diphenhydramine. <Learning objective: Although a lot is known about the potential side effects of antihistamine drugs little is described regarding cardiac side effects. Some antihistamines such as terfenadine have been discontinued from the US market due to prolonged QT interval; therefore, it is important for the clinician to be aware of potentially serious consequences of a commonly used drug such as diphenhydramine. We describe a case of a patient who developed QT prolongation with a temporal association with diphenhydramine usage.>.