Background The patient is a 17-year-old male diagnosed with ventricular preexcitation since the age of 9. Initially he complained of only intermittent palpitations, but more recently demonstrated an adenosine-sensitive sustained supraventricular tachycardia (SVT). Although he was well-controlled on beta-blocker therapy, he was a competitive swimmer and preferred nonpharmacologic curative therapy. He underwent diagnostic electrophysiology testing eight months prior to the curative ablation. This evaluation diagnosed a manifest posteroseptal pathway (Figure 1) and a concealed right free wall pathway. Unfortunately, ablation attempts were unsuccessful. Repeat procedure six months later was also unsuccessful, despite the use of a unidirectional cooled tipped catheter. Eight months after the initial procedure, he presented for repeat electrophysiology testing and ablation at our institution. Electrophysiology Study Written informed consent was obtained. Conscious sedation was used throughout the procedure. Diagnostic catheters were placed in the coronary sinus, His bundle position, tricuspid annulus (Halo), and right ventricle; then the electrophysiologic properties of the pathway were assessed. Earliest antegrade activation appeared to be in the distal Halo (Figure 2). Pacing the right ventricle demonstrated earliest retrograde activation in the low lateral right atrium (Figure 3). Ventricular pacing reproducibly initiated orthodromic atrioventricular (AV) tachycardia (tachycardia cycle length 320 ms). In addition to being observed spontaneously, SVT was also induced with ventricular premature depolarizations, atrial pacing, and atrial premature depolarizations. Retrograde effective refractory period (ERP) was 260 ms. Antegrade ERP was 280 ms at paced cycle length of 400 ms. Attempts to assess antegrade 2:1 block in the pathway resulted in atrial fibrillation (AF). In addition, AF (requiring electrical cardioversion) was observed at different times during the procedure; the shortest RR was 320 ms (Figure 1). Electroanatomic mapping with a 4-mm NaviStar catheter (Biosense Webster, Inc., a Johnson and Johnson company, Diamond Bar, CA) was performed. Detailed antegrade and retrograde mapping was completed. Earliest antegrade activation (19 ms pre delta) was in the posteroseptal right atrioventricular junction, whereas earliest retrograde activation was in the low lateral right atrium. The coronary sinus was mapped. Subsequently, the mapping catheter was placed retrograde across the aorta into the left ventricle, and the left posteroseptal region was mapped in detail. The earliest antegrade location remained in the right posterior septum. Repeat mapping of retrograde accessory pathway conduction from the right ventricular aspect of the tricuspid was also performed, with earliest atrial activation confirmed to be in the low lateral tricuspid annulus. Radiofrequency application to the right posterior septum at the site of earliest preexcitation failed to eliminate accessory pathway conduction. Catheter stability targeting the low lateral tricuspid annulus was difficult to maintain. A Mullin s sheath modified to remove the distal curve was inserted into the right heart to provide support and improve stability of the catheter. RF energy delivered with a standard 4 mm tip ablation catheter in this region was limited either by continued difficulty with maintaining catheter tip stability at the annulus or maximum power delivery of 10 Watts with target tip temperature of 55 degrees Celsius when ablating the atrial aspect of the annulus. Despite radiofrequency energy application, antegrade and retrograde conduction persisted. A bi-directional cooled tip catheter, the Chilli II (Boston Scientific, Maple Grove, MN), was inserted through the modified sheath and a stable tricuspid annular catheter position was obtained. Power-controlled RF energy was delivered along the right inferolateral tricuspid annulus, starting at 15 W with incremental titration of power, with maximum target temperature of 38 degrees Celsius and maximum impedance drop of 8 ohms. Antegrade and retrograde conduction were eliminated over the accessory pathway without additional lesions to the posterior septal region. Only poor AV nodal conduction was evident retrograde, and supraventricular tachycardia was no longer inducible. Discussion Many features of this case highlight the difficulty that may be encountered with accessory pathway ablation. Manifest preexcitation for this patient is consistent with a posteroseptal pathway,5 but the retrograde activation mapped to the posterolateral tricuspid annulus. The wide disparity between antegrade and retrograde activation lead to the suspicion of two separate pathways. However, a single lesion was able to terminate conduction in both directions, implying a single slanted pathway of significant length.1 The inability of prior ablation to the posterior tricuspid annulus suggested that the pathway was indeed protected from easy elimination with endocardial ablation. The procedure also highlights the technical difficulties in ablating on the lateral tricuspid annulus, both in terms of catheter stability and/or power delivery. Elevated tip temperatures limited adequate power delivery with a standard 4-mm catheter and may have impeded successful ablation. Although the use of an irrigated tip ablation catheter is rarely necessary for accessory pathway elimination, prior studies have demonstrated safety and efficacy of using irrigated tipped catheters for pathways refractory to standard techniques. In fact, an irrigated tip catheter was used earlier in this patient, albeit ineffectively. This was possibly related to the location of the pathway and the difficulty of keeping the catheter in stable position with good contact using a unidirectional catheter. The recently released bi-directional cooled tip Chilli II catheter allowed for a wider range of catheter manipulation, better contact, and effective power delivery to eliminate accessory pathway conduction. Conclusion Right-sided accessory pathways may be difficult to successfully ablate. An unusual case of a long right posterior slanted pathway illustrates many of these difficulties. This case highlights the utility of a bi-directional cooled tip catheter to facilitate catheter stability and effective power delivery in successfully eliminating accessory pathway conduction refractory to standard ablation tools and techniques.