The differential for the widening of the QRS waveform with sinus rhythm includes aberrant conduction, hyperkalemia, accessory pathway conduction, and ischemia. Widening of QRS may also be seen in the antiarrhythmic use of class 1 sodium blockers. Aberrant conduction may occur during atrial tachycardias without adequate time for repolarization of part of the conduction system, which is followed by broader QRS complexes. In this patient with normal baseline ECG results and no known prior conduction disease during a stable sinus rate, it is unlikely aberrant conduction. The transient nature of this rhythm without characteristic peaked T waves and other features, such as PR prolongation, P-wave flattening, or bradycardia, makes hyperkalemia unlikely as the mechanism, and potassium levels were confirmed to be normal. Enhanced accessory pathway conduction may manifest at different heart rates, especially when there is variable conduction velocity within the atrioventricular node, which can result in variability of the QRS duration. However, there was less concern for preexcitation without evidence of a δ wave in this patient. Additionally, this patient was not receiving antiarrhythmic therapy for any effects by sodium channel blockade resulting in QRS widening.