Initial Experience with CryoCath Cryoablation at the Heart Hospital of New Mexico

Luis Constantin, MD
Luis Constantin, MD
In the third installment of the Cryotherapy section, Dr. Constantin describes his experience using the CryoCath Freezor ablation system. In this article, he outlines the benefits of using cryoablation and compares its uses with radiofrequency. One of the more compelling aspects of the field of clinical cardiac electrophysiology is the accelerated evolution of applied technologies impacting favorably on the lives of patients. A very recent addition hails from our neighbors in Canada. On April 22, CryoCath Technologies received FDA approval to market the Freezor ® Cryoablation System in the United States for the treatment of atrioventricular nodal reentry tachycardia (AVNRT). The first approved procedure was performed less than two weeks later at the Texas Heart Institute. Freezor ® Cryoablation System in the United States for the treatment of atrioventricular nodal reentry tachycardia (AVNRT). The first approved procedure was performed less than two weeks later at the Texas Heart Institute. At the Heart Hospital of New Mexico, we recently acquired a cryoablation system and have been busy exploring its use in treating a variety of arrhythmias. Although our experience is neither extensive nor controlled, we are acquiring a sense of the remarkable potential for its application and identifying some of the limitations of the current menu of tools. Although FDA clearance was given to marketing the technology for the treatment of AVNRT, CryoCath is working diligently on several additional catheter designs and applications. The company recently reported promising initial results in the treatment of isthmus dependent atrial flutter, and in mid November initiated a trial of its Arctic Circler catheter for pulmonary vein isolation. Although CryoCath is an approved supplier of catheter-based cryoablation devices, competing systems will eventually emerge. For the experienced catheter ablator, acquisition of the necessary skill set will require negligible effort. Although the Freezor console displaces more space than any standard radiofrequency (RF) generator, and sports a different set of virtual buttons, connections and LCD images, the look and feel of the available catheters is similar to that of RF ablation designs. The Freezor ® is a 7 French catheter with a 4 mm tip, not unlike most commonly used RF catheters. My own experience suggests that their handling is most like that of the Chilli ® (Boston Scientific, Natick, Massachusetts) irrigated tip catheter. Like the Chilli, the Freezor catheter is somewhat stiffer and slightly less yielding than standard RF units. I suspect this reflects the common use of a double lumen. Ablation with radiofrequency energy is achieved by the application of high-frequency alternating current and resultant resistive heating of tissue in the vicinity of the catheter tip. In cryoablation, pressurized liquid nitrous oxide is delivered to the tip of the catheter from the cryoconsole through an ultra-fine injection tube. As the refrigerant enters the tip expansion chamber (maintained under vacuum), the cold liquid refrigerant evaporates as it absorbs heat from the tip that is in contact with the tissue. The warmed vapor is returned to the console through a lumen maintained under vacuum. The local tissue temperature typically plummets to less than -75 ºC, achieving the desired effect of cell necrosis. Although lesion dimensions as assessed by the volume of injured tissue are similar to those achieved with RF, the appearance of the cryolesion differs considerably, owing to the preservation of architectural elements. Thus, a lower theoretical risk of perforation exists, although this is not a frequent complication of RF procedures. The maximum temperature drop at the lesion periphery is achieved in 2.5-3.0 minutes, though 4-minute applications are recommended. A second major benefit is the reversible effect observed when cooling is initially limited; it is called cryomapping. Theoretically, this should reduce or altogether prevent the inadvertent creation of complete heart block when working in the vicinity of the atrioventricular node (AVN). In support of this are the results of CryoCath s clinical trials and a recent report from Europe regarding the excellent outcome in patients with midseptal and para-Hisian accessory pathways. Cryoablation may also prove safer than RF if data suggesting less thrombus formation, less perforation and nonoccurrence of pulmonaryvein stenosis prove correct. We have applied cryoablation successfully in treating AVNRT, but have also used it primarily or adjunctively in the treatment of Wolff-Parkinson-White syndrome, inappropriate sinus tachycardia, idiopathic ventricular tachycardia originating from the right ventricular outflow tract, atrial flutter and atrial fibrillation. The cryoablation procedure for AVNRT employs the same array of diagnostic catheters used for the RF approach. However, the technique differs slightly because accelerated junctional rhythm, the sine qua non of successful RF ablation, does not emerge during cryoablation of the AVN slow pathway. Thus, the commonly recognized indication that ablation is in progress at the appropriate target site is absent. Performance of extrastimulus testing while actively cryo-ablating, provides assessment of immediate outcome since slow pathway block is observed directly as it occurs (Figure 1). This, coupled with the longer duration of energy application, however, may lead to frustration on the part of practitioners accustomed to junctional rhythm as the form of near immediate feedback suggestive of slow pathway damage. Whether or not complete AVN slow pathway ablation is required to insure long-term success or whether evidence of slow pathway attenuation or the induction of only single AVN echoes will suffice remains uncertain. Similarly, the endpoints for cryoablation of other arrhythmia substrates have yet to be defined. One of the purported advantages of cryoablation is the phenomenon of cryoadhesion. The tissue adjacent to the catheter tip adheres to it as the temperature passes through -20 to -30 ?C. Once this occurs, the catheter can be released from the operator s hands with equanimity, as the catheter will not migrate to another site. This has obvious advantages over RF ablation, where constant application of torque may be required for ablation success and to avoid complications. On the other hand, unintentional tugging of the adherent catheter could disrupt the tissue, an unlikely theoretical hazard. A related drawback results from catheter movement during cardiac systole or in association with the respiratory cycle. The catheter tip may alternately move on and off the intended ablation site, and the critical temperature effecting adhesion may be reached just as the catheter moves off the target, necessitating discontinuation of the application and repositioning of the catheter at the appropriate site. Thus, as in RF ablation, catheter contact and stability prior to reaching cryoadhesion is crucial and the use of long and/or preformed sheaths can be essential. (Contact may be more important with cryo than with RF because more contact with tissue means more ability to remove heat and kill cells.) We recently observed this during an attempted cryoablation of inappropriate sinus tachycardia. The procedure was performed with intracardiac ultrasound guidance. The ultrasound image revealed the catheter would occasionally slide off target sites on and along the crista terminalis. In several instances, cryoadhesion would occur just after the catheter had moved away from a desired ablation site. In spite of this, I am of the opinion that cryoadhesion is mostly advantageous. A potential limitation is a byproduct of the otherwise elegant catheter design. As previously stated, application of torque is often needed to maintain ablation catheter position. Because the double lumen is subject to kinking, which will stem the flow of refrigerant, cryoablation catheters may not survive the application of forces well tolerated by those used for radiofrequency ablation. For this reason, I am reluctant to consider cryoablation when multiple catheter deflections and a major application of torque may be required, as can be necessary in some left-sided accessory pathway procedures when using a retrograde, transaortic approach. In Europe, Canada and the US, however, the experience with Freezor in ablation of left-sided pathways using the transaortic approach suggests that this may not prove to be a problem. Nevertheless, for such procedures, I prefer the transeptal technique. Catheter kinking was problematic during our initial pulmonary vein isolation procedure using cryoablation. For this we used a 6 mm tip Freezor ® Xtra catheter. After the first catheter failed, a second catheter was used. Unfortunately, we ultimately had to draw on our more familiar RF arsenal to complete the task. Since then, and with appropriate sheath support, we have been able to achieve partial or complete isolation of pulmonary veins with this catheter in several patients (Figure 2). In summary, our experience with the CryoCath Freezor ablation system has been instructional and interesting, but also gratifying and even entertaining. If cryoablation proves to be as effective as, but safer than RF, it may very well become the preferred catheter ablation technology. I am excited by its potential for an expanded role in ablation therapy and look forward to using the next generation of cryoablation tools. For more information on cryoablation and CryoCath, please visit: www.cryocath.com/en/