Case Report M.W. is a 23-year-old male with a history of recurrent palpitations for several years. They generally occurred only with exercise, but he is a college football player with potential for a career with the NFL. He could often get near-syncopal with these events, which were documented by event monitoring to be secondary to SVT. He was sidelined from the football team and avoided by the previously interested NFL scouts until his medical condition resolved. He was brought for EP studies to a hospital in Rockford, Illinois. Antegrade conduction was normal, and SVT was easily inducible. A retrograde concealed accessory pathway that appeared to be very close to the AV node region was identified. The hospital where this procedure was performed was not currently equipped to do cryoablation, so they aborted the ablation attempt and referred him to Evanston Hospital for that procedure. On November 16, 2005, we performed the ablation. Given the previously identified pathway site, we did it using an ESI balloon array for help with precise location. Diagnostic catheters were placed in the RA, His and RV. Programmed stimulation was used to induce tachycardia. Its atrial activation was mapped using the ESI balloon array (Figure 1). A 6 mm CryoCath Freezor Xtra catheter was advanced to the tricuspid annulus and used to map for the earliest atrial activation site during tachycardia. Catheter manipulation intermittently eliminated conduction through the accessory pathway repeatedly. This made it difficult to assess the outcome of ablation, but it allowed us to compare the atrial activation site purely through the AV node (Figure 2) with that through the accessory pathway (Figure 1). The distance between them was approximately 9 mm. Cryoablation at the site perceived as ideal by the 3D imaging was delivered at this site, despite mechanical elimination of conduction through the pathway prior to the start of the application. Two consolidation applications were delivered immediately superior and lateral to the putative successful site. Accessory pathway conduction never recurred, and no arrhythmias were inducible after ablation. AV nodal physiology was normal, and no different before and after the ablation. The patient has returned to playing football and remains asymptomatic almost exactly three months after the procedure. The NFL scouts no longer have to fear an arrhythmic condition as a limitation for this college athlete. Discussion Accessory pathways are located in the anteroseptal or midseptal region in approximately 6% of cases.1 In our own series of 359 patients with accessory pathway ablations treated between 1990 and 1999, nineteen (5%) had anteroseptal pathways. Ablation of these pathways in close proximity to the AV node has been performed successfully for many years. Surgical interruption of these pathways often resulted in a cure from tachycardia but the need for a pacemaker. The same was uniformly true with low-energy DC ablation. Radiofrequency (RF) ablation improved results significantly; unfortunately, it results in permanent AV nodal block much more frequently than in locations distant to this area with AV block rates ranging from 3% to upwards of 10%.2 In our series, of 19 patients with anteroseptal pathways, one patient developed complete AV nodal block (5%). However, these numbers underestimate the magnitude of the problem. Many operators abort the ablation procedure and relegate the patient to treatment with antiarrhythmics when pathways appear to be uncomfortably close to the AV node. Cryoablation is much more forgiving than RF in that if an undesired conduction effect is noted during a cryoapplication, prompt discontinuation almost always results in restoration of normal physiology. To date, there have been no reports of unintended permanent AV block with cryoablation. Anteroseptal pathways, and other pathways in which ablation near the AV node is required, are therefore ideally treated by cryoablation to maximize safety. Using the 6 and 8 mm CryoCath cryoablation catheters, long-term ablation efficacy results compare favorably to those with RF energy. Mechanical interruption of conduction through an accessory pathway has been reported in 9.7% of accessory pathway ablations in a prospective study. Its incidence is highest in anteroseptal pathways, in which it occurred in 38.5% of cases.3 It was therefore not unexpected that it occurred in our case, making it yet more challenging. Two technologies aided us in succeeding in treating this patient successfully. The ESI balloon array localized the precise location of the atrial insertion of this pathway in a single beat of AV reentrant tachycardia. It provided us with the location of the atrial connection to the AV node as well. In this way, when we advanced our catheter to this location and mechanically interrupted its conduction, we still knew we were in an adequate area. Furthermore, we did not have to rely on fluoroscopic imaging and its inherent inaccuracies in anatomical location. Using cryoablation provided the stability required of the catheter. Had it moved towards the AV node, it could have produced AV block. Movement of the catheter during RF application is inevitable and unpredictable, given cardiac and respiratory motions. With cryoablation, catheter stability is excellent generally within 15 - 25 seconds of the start of the application. We knew our catheter would remain where we placed it once we reached freezing temperature. Conclusion This case illustrates the usefulness of two technological advances in the treatment of SVT by ablation. The ESI balloon array provided us with precise anatomical localization of the site of the accessory pathway, even though its conduction was interrupted mechanically. Cryoablation provided us with the necessary safety to ablate in a sensitive area prone to disastrous complications. It also provided us with the necessary catheter stability to make the procedure easier. It is obvious that AV block is a catastrophic complication in a patient looking at a career as a professional athlete. It may not result in as much loss of income in a patient looking at a more sedate profession, but it is no less life altering. As this case illustrates, ablation close to the AV node can be undertaken with a very reasonable degree of safety and effectiveness, if we avail ourselves of the necessary existing tools.