This patient should be given adenosine. She has hemodynamically stable narrow-complex tachycardia consistent with supraventricular tachycardia. The rhythm is regular and no obvious P waves are visible; therefore, atrioventricular nodal reciprocating tachycardia (AVNRT) is the most likely cause. AVNRT accounts for up to two thirds of cases of supraventricular tachycardia. Patients often report neck pulsations, which are caused by simultaneous contraction of the atria and ventricles. Because the patient failed to terminate her tachycardia with vagal maneuvers, adenosine should be administered. Adenosine is highly effective at termination of nodal-dependent rhythms and can help identify the underlying etiology. For example, continued atrial activity (P waves) during atrioventricular block can help identify atrial flutter and atrial tachycardia. Patients given adenosine should be on a cardiac monitor with a running rhythm strip on paper to document the results. Prior to giving adenosine, patients should be warned that they may experience nausea, flushing, chest pain, or a sense of dread. Patients with bronchospastic lung disease should not receive adenosine.

Although amiodarone would be effective for terminating this patient's arrhythmia, it has many long-term risks, including thyroid, liver, pulmonary, and neurologic toxicity. In this young patient, amiodarone would not be an appropriate option.

Cardioversion is not indicated because the patient is hemodynamically stable, and pharmacologic attempts at cardioversion, such as adenosine, have not been attempted.

Ibutilide is an intravenous Vaughan-Williams class III antiarrhythmic drug FDA approved for pharmacologic cardioversion of atrial fibrillation. The patient has regular supraventricular tachycardia, not atrial fibrillation.