e were satisfied with our results in this trial, which, at the time of this writing, was in the last stages of review by the FDA, in preparation for commercial release of the Freezor ® catheter. Not unexpectedly, this catheter has been in common use in many European countries for some time. What follows is a review of this technology and its evolution to help assess its potential role in our current armamentarium. In the 1980s, DC ablation became the first non-surgical treatment of atrioventricular (AV) reentrant arrhythmias. The energy source had poor control and directionality, and the associated barotrauma limited the sites to which it could be applied safely. Nonetheless, it opened up the world of catheter ablation. The advent of radiofrequency energy as a source for ablation constituted a paradigm shift; RF offered much more precise control over the location of tissue destroyed by ablation without remote damage. The last 10 years have witnessed a revolution in the approach to the treatment of SVT, thanks to RF ablation. Because radiofrequency ablation has excellent success rates and its complication rate is acceptably small, it has changed from a novel approach to the therapy of choice for patients with symptomatic SVT. Nevertheless, as we treat a greater number of patients with SVT, we are encountering limitations. For example, the most commonly treated supraventricular tachycardia, AV nodal reentrant tachycardia, has an incidence of unexpected atrioventricular block of 0.8-2.0% (in highly experienced centers) when treated with RF ablation. While it occurs at a low rate, it is a devastating complication that is often much worse than the problem being treated. The same is also true in anteroseptal pathways, which pose an even higher risk of AV block. This is why we call the difficult-to-treat area around the atrioventricular node tiger country. As well, pathways exist in several other locations inherently difficult to treat by radiofrequency ablation (such as posteroseptal pathways that can be reached only from the middle cardiac vein). In a limited role, cryoenergy has been used in the treatment of arrhythmias for several decades. It was utilized, for example, with endocardial resection of ventricular tachycardia (VT) and with surgical atrial maze procedures. The use of cryoenergy for catheter ablation is just now becoming a reality as the technology has evolved to facilitate percutaneous treatment with 7 French catheters. It is likely that the use of cryoenergy will constitute yet another paradigm shift in the catheter treatment of arrhythmias. Cryo is different from radiofrequency because it results in lesions that are more circumscribed, better delimited, and virtually free from clot. Also distinct from RF ablation, cryotherapy is able to produce transient electrical block on the candidate ablation site by creating a zone of hypothermia in the target tissue. This facilitates cryomapping, the ability to test a site for safety and efficacy prior to creating a permanent lesion, which makes ablation in the vicinity of the AV node a much more comfortable procedure. It is easy to see how this characteristic makes anteroseptal pathways more amenable to safe treatment with cryotherapy. More importantly (given how common AV nodal reentry is), the risk of inducing permanent inadvertent atrioventricular block from AV nodal modification is decreased thanks to the novel cryomapping feature of cryotherapy. Atrioventricular nodal modification by cryotherapy is as easy or easier to perform than by RF, and we did not notice that the procedures took any longer. Having done it both ways, I would rather pull a cryo catheter than an radiofrequency catheter when faced with the need for an AV nodal modification, because of the reduced risk of AV block. In addition, a unique feature of cryotherapy is the adhesion of the subzero ablating catheter tip to the endocardial surface. This adhesion allows for a very stable catheter position, facilitating the use of pacing to test for effect, and preventing accidental dislodgement of the catheter with abrupt changes in rhythm during a procedure. In centers where it is impractical to have several highly trained operators in the room to share the responsibility of assessing AV nodal conduction and the position of the ablating catheter, cryoadhesion is ideal. Once the catheter has adhered, fluoroscopic imaging is no longer necessary as the catheter remains in place regardless of pacing maneuvers, termination of arrhythmia or finger fatigue. This greatly facilitates AV nodal modifications. These considerations also apply to cryoablation of anteroseptal pathways, which is easier and safer than RF ablation. These pathways are therefore more likely to be tackled by a wide range of electrophysiologists when performed by cryotherapy, especially since no cases of inadvertent permanent atrioventricular block have been reported with cryotherapy from the FROSTY and Euro trials. Cryoablation within vessels (e.g. the CS and its branches) is currently being studied. Preliminary evidence suggests strongly that it is safe, does not result in venous or arterial occlusion or narrowing, and can reach pathways that are in close proximity to coronary veins and arteries. Case reports from Europe support these conclusions. These findings immediately raise the possibility of applying cryotherapy to the pulmonary veins, as possibly the most dreaded complication we encounter in the electrophysiology lab is pulmonary veins stenosis. We currently withhold or postpone RF ablation for treatment of atrial fibrillation out of fear of complications, particularly pulmonary vein stenosis. If ablation within the pulmonary veins proves to be as effective by cryoenergy as by radiofrequency energy without a risk of pulmonary vein stenosis, we will likely be quicker to utilize ablation as a means of therapy for atrial fibrillation in appropriate patients. In fact, trials of a different cryotherapy device called Arctic Circler are currently underway in Canada and Europe for performing pulmonary vein isolation. We hope that CryoCath will bring this product to the United States under an IDE trail to help make the treatment of atrial fibrillation a reality. Our experience with cryotherapy in the research trial was highly positive. The learning curve was not at all steep and the physician and nursing team got comfortable with the use of this technology quickly. The future for cryotherapy looks bright it will not displace radiofrequency ablation, but rather supplement it for particular uses, including being used routinely for AV nodal reentry and some common atrioventricular reentrant SVTs. Thanks to this latest paradigm shift, and the promise of future advancements in cryotherapy, tiger country is becoming a cool place to ablate.