The physicians at UCSF are considered pioneers in heart disease treatment and research for conditions such as arrhythmia, heart failure, and catheter ablation, which was developed here. UCSF is also home to 11 research institutes, 1,500 laboratories, and more than 2,000 ongoing research projects. This is a case report of a patient with focal atrial tachycardia arising from the coronary sinus. The RPM System (EP Technologies, Boston Scientific, Natick, Massachusetts), was used to make the diagnosis and precisely localize and ablate the site of origin of the atrial tachycardia. The patient was a 36-year-old male with past medical history significant for rheumatic heart disease. He had aortic valve replacement (AVR), (Carbo- metrix) and mitral valve commissurotomy in 1991. He had paroxysmal supraventricular tachycardia for one year prior to admission. An electrophysiology study done at an outside hospital suggested a left-sided atrial tachycardia. He was treated with verapamil and sotalol, but persistent symptoms remained. He was scheduled for EP study at our institution, and his medications were held in preparation for the procedure. He was presented to the ER with narrow complex tachycardia at 260 beats per minute. He was treated with intravenous beta-blockers and taken to the EP lab the next day. The RPM system is an integrated mapping system and electrophysiology recording system that utilizes two reference catheters and one mapping/ablation catheter, both equipped with ultrasound transducers. One reference catheter is placed in the right ventricular (RV) apex and the other reference catheter is positioned in the coronary sinus. All three catheters are 7 French (Fr) in size. The reference catheters have a fixed curve, whereas the ablation catheter is a bi-directionally steerable, closed-loop, cooled ablation catheter. The shaft of the coronary sinus reference catheter has nine 1-mm ring electrodes and one 2-mm tip electrode, whereas the RV reference and the ablation catheter have three 1-mm ring electrodes and one 2-mm tip electrode. The mapping system uses ultrasound-ranging techniques to determine the position of the ablation catheter relative to the 2 reference catheters, and to create a three-dimensional electroanatomical anatomy of the heart, including the display of ablation and reference catheters. It is also possible to display the curve of the catheter, since one of the transducers is placed proximal to the deflection point of the shaft of the catheter. This provides a more fluoro-like display, and makes it easier to reposition the ablating catheter to an endocardial site with the same curve as before, resulting in a more exact catheter position. The acquisition module and the ultrasound unit are connected to a dual processor PC. The system is capable of simultaneously processing seven catheters (including reference catheters) and 64 signals (including 12-lead electrocardiograms and two pressure signals). The original position of the reference catheters is displayed on the real-time window, so that the catheters can be repositioned in the event that they are displaced. In addition, the system is also capable of recording voltage maps that allow for identification of scar areas (low voltage). After obtaining informed consent, the patient was taken to the EP lab in post-absorptive state. Local anesthesia was achieved with lidocaine/ bupivacaine. Hemostatic sheaths were placed percutaneously, using the Seldinger technique, in the right femoral vein and right internal jugular vein. Fixed curve reference catheters (7 Fr) were placed in the coronary sinus (decapolar), and right ventricle (quadripolar). A non-RPM fixed curve quadripolar 5 Fr catheter was used to record signal at the bundle of His region. A bidirectional 7 Fr quadripolar cooled-tip mapping/ablation catheter (Chilli RPM, EP Technologies, Boston Scientific, Natick, Massachusetts) was placed in the right atrium. Pro- grammed electrical stimulation was performed with pacing at twice the diastolic threshold. Narrow complex tachycardia was easily inducible with atrial overdrive pacing, and sustained with isoproterenol. Tachycardia cycle length was 235-250 ms and earliest atrial activation was noted in the CS bipoles 7,8. Spontaneous 2:1 block was noted during tachycardia (Figure 1). Ventricular overdrive pacing entrained the tachycardia and upon cessation of pacing the tachycardia resumed with a VAA response consistent with a diagnosis of atrial tachycardia. Right-sided activation map was performed, including the coronary sinus (CS) to the distal pole of the CS catheter during tachycardia. It showed earliest activation in CS bipoles 7,8 that was adjacent to the left atrium (Figure 2). Given this finding, a transseptal catheterization was performed (under Ultra ICE, EP Technologies, Boston Scientific). The mapping catheter was placed in the left atrium through the Mullins sheath. Therapeutic anticoagulation was achieved with heparin. Activation map within the left atrium demonstrated a focal atrial tachycardia with earliest activation close to the CS bipoles 7,8 (Figure 3). There was no early activation specifically within the pulmonary veins or left atrial appendage. At this point the catheters were removed from the left side of the heart. Activation map in the coronary sinus showed earliest activation in the posterior floor of the sinus between bipoles 5,6 and 7,8. The area of interest was marked and ablation was performed in sinus rhythm. Ablation done at this site with low power was successful in making the tachycardia non-inducible. Continuous ST monitoring was performed during ablation and no changes were observed. After 30 minutes, no sustained atrial tachycardia was inducible, despite administration of isoproterenol. This case demonstrates the successful utilization of the RPM System to map and ablate focal atrial tachycardia arising from the coronary sinus in this highly symptomatic patient. In this case, the RPM System allowed the construction of a three-dimensional activation map in the left and right atria with minimal use of fluoroscopy during tachycardia. It also records the full catheter position (shaft and tip) at any endocardial site. This makes it possible to return to a point of interest with the exact catheter position as before. The RPM system can also be used in guiding catheter ablation in patients with atrioventricular nodal reentrant tachycardia and atrioventricular reentrant tachycardia where it is used to tag the His bundle region and calculate the distance from that region to the ablation site (Figure 4). The ability to record voltage maps and to ablate with a cooled-tip catheter are especially useful in catheter ablation of patients with atrial flutter and scar-related ventricular tachycardia.