Cryo-ablation vs. Radiofrequency for AVNRT

Abstract & Commentary

By John P. DiMarco, MD, PhD

Source: Deisenhofer I, et al. Cryoablation versus radiofrequency energy for the ablation of atrioventricular nodal reentrant tachycardia (the CYRANO Study): Results from a large multicenter prospective randomized trial. Circulation. 2010;122:2239-2245.

This paper describes the results of a study which compared two energy types, cryoenergy and radiofrequency (RF), for catheter ablation in patients with atrioventricular nodal reentrant tachycardia (AVNRT). This study was performed at six European and one Chinese center each of which have a large volume of patients undergoing electrophysiologic studies and catheter ablation procedures. Patients with inducible at electrophysiologic study were randomly assigned to ablation using either cryoablation or radiofrequency energy. In the cryoablation group, the investigators used a 6-mm tipped cryoablation catheter for both mapping and ablation. With this catheter, they performed cryomapping to localize likely ablation sites. During cryomapping, the catheter tip is cooled to -30 degrees Celsius. This produces transient and reversible electrophysiologic changes with no risk for permanent damage. By repeating stimulation during cryomapping, they were able to localize the position of the AV nodal slow pathway. Once they identified a site at which induction of AVNRT during cryomapping was prevented, they delivered two 4-minute lesions with the tip cooled to -80 degreees Celsius. Repeat attempts to induce AVNRT were made during lesion delivery. Fast pathway conduction was also monitored to avoid inadvertent fast pathway damage. In the RF ablation group, a 4-mm tipped RF catheter was used to map the slow pathway. Electrogram characteristics including the A:V ratio and the presence of a slow pathway potential-guided ablation. Ablations were performed during sinus rhythm. The occurrence of accelerated junctional beats during ablation was used as an indicator of a probably successful ablation site.

The endpoints for the cryoablation procedure was the elimination of inducible AVNRT and the absence of more than a single AV nodal echo beat during repeat atrial stimulation. The primary endpoints for the comparison included short-term ablation efficacy, efficacy after six months and ablation-induced AV block that required pacemaker implantation. The authors also measured procedure and fluoroscopy times, device-related complications, and pain during the ablation procedure.

A total of 509 patients with inducible AVNRT were entered into the trial. Of these, 258 were randomized to radiofrequency and 251 to cryoablation. Thirteen patients were lost to follow-up, 7 in the RF group and 6 in the cryoablation group. The patients were all over 18 years of age and two-thirds were female. The mean age was 51.5 + 15.5 years. Ninety-eight percent of the patients had the typical or slow-fast form of AVNRT.

Procedural success was noted in 243 of 251 patients in the cryoablation group (96.8%) and 254 of 258 patients (98.4%) in the radiofrequency catheter ablation group (p = 0.255). No patient in the cryoablation group developed catheter-induced AV block. In the radiofrequency ablation group, one patient developed complete AV block twenty hours after the ablation and required permanent pacing. Ablation-induced permanent first degree AV block was also noted in two patients. At the six month time point, recurrent AVNRT had been documented in 23 of 243 patients (9.4%) in the cryoenergy group and in 11 of 254 (4.3%) in the radiofrequency group. Almost all of these patients underwent repeat procedures using radiofrequency and were long-term successes. The difference in recurrence rates between the cryoablation group and the radiofrequency ablation group was significant with a p value of 0.029.

Procedure duration was significantly longer in the cryoablation group. Fluoroscopy times were similar, but the total procedure time was 18 minutes longer. It was also noted that the cryoablation generator and leads were more susceptible to malfunction during the procedure. Device malfunctions were noted in 13 cryoablation patients versus only two radiofrequency ablation patients. Pain was decreased using cryoablation. The subjective pain score in the radiofrequency group was 20.3 + 22 on a scale of 1 to 100. In the cryoablation group, it was 7.3 + 13.9.

Commentary

Cryoablation takes several minutes to produce a permanent lesion. If AV block is noted during a lesion application with cryoenergy, the application can be discontinued and the tissue rewarmed with complete recovery of conduction. In contrast, radiofrequency application can produce permanent damage within seconds. Therefore, most electrophysiologists recognize that they need to carefully monitor VA conduction during the periods of accelerated junctional rhythm and need to be prepared to immediately turn off RF application if any VA block is noted. With careful attention to technique, the inadvertent production of permanent AV block should be very uncommon — probably in the range of 1 in 200 cases as seen in this trial. It should remembered, however, that these were mostly middle-aged, adult patients. Cryoablation seems to be most popular among pediatric electrophysiologists. Here, the consequences are producing life-long AV block in a child or adolescent are greater and the size of the heart in which the ablation lesions must be delivered may be smaller. Therefore, I favor cryoablation in children under the age of 13 or 14. The small increment in safety seems to fully justify the increased need for a repeat procedure. I also use cryoablation in selected patients where the slow pathway-ablation site appears to be very close fluoroscopically, or on 3-D mapping, to the bundle recording catheter. This is not common, but I do switch from RF to cryoenergy in perhaps 5% of my AVNRT cases. Finally, cryoablation has unique advantages in patients with anteroseptal pathways and with septal atrial tachycardias close to the AV node and should often be used in those situations. The latter 2 conditions are times when cryoablation is clearly the better approach

It should also be noted that in this paper the authors accepted one AV nodal echo beat after cryoenergy delivery as an acceptable outcome. Some electrophysiologists who use cryoablation frequently say that all AV nodal echo beats should be eliminated with cryoenergy even though this does not seem to be required with radiofrequency energy. It is possible that further applications to eliminate the single echo beat seen in some of the patients here might have decreased the recurrence rate. This would likely have increased the procedure time but may have brought the recurrence rate down to more acceptable levels.