Long RP tachycardia is an uncommon form of supraventricular tachycardia (SVT) that exhibits a longer RP interval than PR interval on 12-lead electrocardiograms (ECG)1. The differential diagnosis of a long RP tachycardia includes the uncommon form of AV nodal reentrant tachycardia (AVNRT), the permanent form of reciprocating tachycardia (PJRT), and atrial tachycardia (AT). Drug responses vary markedly from patient to patient in uncommon SVT. Few small patient cohorts have been studied involving uncommon SVT whose attacks could not be adequately controlled by antiarrhythmic agents2. We present the case of a 47-year-old woman with stable long RP tachycardia refractory to first and second-line treatment in the Emergency Department, who rapidly destabilized to transient heart failure and hemodynamic compromise, ultimately requiring synchronized electrical cardioversion. In our report, we highlight the presentation of long RP tachycardia in the Emergency Department; its management in the acute setting and dealing with its ECG interpretational challenges. Our reasons for highlighting this case are: 1. This is a relatively uncommon ECG presentation to the Emergency Department, which poses its own analytical dif culties. 2. The dynamic nature of drug-resistant tachyarrhythmia in the acute setting makes this case clinically relevant to the Emergency Physician.