Left bundle branch block morphology on electrocardiogram after massive diphenhydramine overdose

Diphenhydramine is a first-generation antihistamine with various pharmacologic effects including sodium channel blockade. Electrocardiographic findings often presents with QRS prolongation with a rightward axis. Left bundle branch block (LBBB) is a rare manifestation in this context. We present a case of a 35-year-old woman who presented to the emergency department after ingesting 100 tablets of diphenhydramine 25 mg (46 mg/kg) with alcohol to self-harm. She experienced two generalized tonic-clonic seizures en route to the hospital. Initial vital signs were: pulse of 105 bpm, blood pressure of 141/101 mmHg, oxygen saturation of 100 % by mask ventilation, and temperature of 95.9 °F. She was intubated for airway protection. Laboratory results revealed hypokalemia (2.3 mmol/L), elevated lactate (10.4 mmol/L), and serum ethanol concentration of 187 mg/dL. A whole blood diphenhydramine concentration, taken six hours post-presentation, was elevated at 7324.9 ng/mL (therapeutic levels: 25-112 ng/mL). An electrocardiogram showed sinus tachycardia, heart rate 116 beats/minute, PR interval of 54 ms, QRS duration of 152 ms, and new LBBB. The patient received 150 mEq of sodium bicarbonate and a repeat electrocardiogram demonstrated sinus tachycardia and resolution of LBBB. During her hospital course, she maintained normal sinus rhythm without conduction abnormalities. She was extubated the following day and transferred to a psychiatric unit two days later. Reversible LBBB can be observed due to various etiologies including sodium channel blockade. Our case demonstrates that patients who develop a new LBBB after diphenhydramine overdose can be treated with standard therapy, including sodium bicarbonate.