Case Report. The patient K.S. is having her palpitations again. She has been evaluated in the past and not received a diagnosis. This time, she has an event recorder on and she pushes the button. After the palpitations have not gone away for an hour, she decides to go ahead and call in the event. Once on the phone, she downloads the rhythm and is told to immediately call 911, to be taken to the emergency room. K.S. decides to drive herself. Once in the local ER, things move along pretty fast. She is in a wide complex tachycardia at a rate of 188 bpm (Figure 1). Lidocaine IV is pushed without result; Procan is tried next with conversion to SR. Now stabilized, she is transferred to our facility at St. Mary s Hospital for further evaluation and treatment. K.S. is a 70-year-old female with a history of palpitations for the last 3 years. She has had EP studies done while taking both beta blockers and calcium channel blockers. No definitive diagnosis was reached, despite aggressive protocols with atropine and dobutamine. She does have coronary artery disease with diffuse disease of the right coronary artery, a 50% lesion of the left anterior descending artery and mild ischemia of the infralateral wall on myoview. Her baseline 12-lead electrocardiogram does not show any underlying bundle branch block (Figure 2). She was treated medically and had an ejection fraction of 50%. Current medications include Imdur, Demadex, Zyloprim and a multivitamin. Another EP study is recommended and K.S. agrees to proceed. During the EPS, three catheters are inserted via the right femoral vein. The routine protocol of atrial and ventricular testing is carried out with no arrhythmias noted. Isuprel is started at 1 mcg/minute. and the atrial protocol is repeated. At a drive of 400/280 a tachycardia is started with long PR and 2:1 block below the His (Figure 3). This progresses to one-to-one conduction with LBBB abbherrancy (Figure 4). The morphology of this wide complex tachycardia is identical to what was seen in the outlying facility. The diagnosis of typical atrioventricular nodal reentry tachycardia (AVNRT) is made, the slow pathway potentials are identified and radiofrequency ablation of the pathway is completed. A repeat evaluation is completed with no recurrence of the tachycardia. K.S. has now been discharged with a script for Toprol XL due to her CAD and angina. At this time, she has had no recurrence of her palpitations. Conclusions. AVNRT is the most common form of supraventricular tachycardia (approximately 50% of cases). The most common form of AVNRT has the long PR interval and is considered to be Typical AVNRT. Occasionally, bundle branch block can occur due to alterations in His Purkinje conduction. When this occurs, the arrhythmia can be mistaken for ventricular tachycardia. Evaluation in the EP lab can make the diagnosis, and ablation of the slow pathway will provide the cure for these patients.