Abstract & Commentary
Synopsis: Cryoablation offers an alternative approach to radiofrequency ablation.
Source: Friedman PL,et.al. Catheter Cryoablation of Supraventricular Tachycardia: Results of the Multicenter Prospective "Frosty" Trial. Heart Rhythm. 2004;1: 129-138.
For many years, radiofrequency energy delivery has been the standard tool for catheter ablation. Recently, a cryoablation catheter was introduced. This report details the clinical results using this catheter in a large clinical trial. Patients were eligible for inclusion in the trial if they were older than age 18 and had a clinical history of supraventricular tachycardia (SVT), suspected to be due to atrioventricular nodal reentry (AVNRT) or AV reentry using an accessory pathway (AVRT). Patients who were undergoing AV junctional ablation for atrial fibrillation, with excess ventricular rates, were also eligible. The study was conducted in 11 hospitals in the United States and 3 in Canada. The ablation catheter used is a 7 French quadripolar steerable catheter with a 4 mm tip. The catheter has an outer shaft that is maintained under constant vacuum, and an inner injection tube through which liquid N2O is injected. When the liquid N2O escapes at the end of the injection tube into the outer shaft, it evaporates, resulting in cooling at the tip. A thermocouple at the catheter tip allows temperature to be monitored. In operation, the operator can use either a cryomapping mode, in which temperature is maintained at or above -30° Centigrade, or a cryoablation mode, in which the coldest possible temperature tip is achieved generally less than or equal to -68° Centigrade. The standard period of application for cryoablation is 4 minutes. In this study, cryoablation was performed at sites chosen on the basis of electrical mapping, anatomic landmarks, or the results of cryomapping. No limit was placed on the number of cryomapping or cryoablation attempts. If cryoablation proved unsuccessful, patients could be crossed over to radiofrequency energy for their ablation.
A total of 166 patients were enrolled in the study. The group included 103 patients with AVNRT, 51 patients with AVRT, and 12 patients with atrial fibrillation. In 2 patients, the cryoablation catheter was not activated because of technical difficulties, and 1 patient had discovery of a left atrial tachycardia after the catheter was introduced. An additional 7 patients did not have a qualifying cryoablation due to inability to achieve target temperature. By intention to treat analysis, acute procedural success was achieved in 91% of the patients with AVNRT, 69% of the patient with AVRT, and 67% of the patients who underwent attempted AV junctional ablation.
The increased success rate in patients with AVNRT was statistically significant compared to the other 2 groups. Among the patients with accessory pathways, the success rate was lower (65%) for those with left lateral accessory pathways, compared to those with accessory pathways in other positions. There were 27 patients in whom acute procedural success was not achieved with cryoablation. Twenty-five of these underwent an attempted ablation with radiofrequency energy. Radiofrequency ablation was successful in 23 of these 25 subjects. Cryomapping was attempted before cryoablation in 135 of the patients, and successfully identified a suitable site of ablation in 87 of the 135 subjects. In selective patients, rewarming was performed to see if changes, seen during cryomapping, were reversible.
In 58 of the 62 cases, electrophysiologic changes, noted during cryomapping, completely resolved within 6 minutes. All patients showed at least partial recovery of conduction. There were 8 acute major complications related to catheter placement in the trial. None of the complications were specifically related to the cryoablation delivery. There were 10 subjects who had transient abnormalities of AV conduction noted during the procedure. All these conduction abnormalities resolved within 1 minute. No patient had persistent AV block, or required permanent pacemaker insertion. Among the patients in whom acute procedural success rate was achieved, the long-term success was 91% for all subjects, and 94% for those with AVNRT.
Friedman and colleagues conclude that cryoablation offers an alternative approach to radiofrequency ablation. They anticipate that improvements in catheter design in the future will increase the success rate without compromising efficacy.
Comment by John P. DiMarco, MD, PhD
Since its introduction more than 15 years ago, radiofrequency ablation has become the standard approach for the catheter ablation of many types of cardiac arrhythmias. For patients with the common forms of supraventricular tachycardia, success rates should exceed 95%, with permanent complications seen in less than 0.5%, if the procedure is performed by experienced operators. Many of the complications seen in patients with radiofrequency ablation are due to catheter manipulation and positioning, and would be expected to occur with almost any type of ablation, as was seen in this report. However, radiofrequency energy ablation and cryoablation are quite different in the time required for an irreversible effect to occur. Irreversible damage may be produced within seconds of radiofrequency energy delivery, and if the catheter is not in the correct position, or if it dislodges, AV block may be inadvertently produced. In patients with AV node reentrant tachycardia, the standard approach is to start as far away from the His bundle catheter as possible, but rare patients have posteriorly positioned fast pathways, and in other patients, the slow pathway and the fast pathway are immediately adjacent. In patients with midseptal and anteroseptal accessory pathways, it may be quite difficult to separate the normal conduction system from the accessory pathway. If one ablates an accessory pathway in these positions with radiofrequency energy, one often has a His bundle potential recorded on the ablation catheter that can be seen only after preexcitation has been eliminated. In these patients, having the option to produce a reversible lesion during cryomapping has potential advantages. Freezing takes several minutes to produce irreversible conduction block. If AV block is observed during a lesion, the cryomapping or cryoablation can be stopped, and the tissue should recover. These potential advantages with cryoablation are also relevant in young children, where the area for safe ablation might be quite small and for ablation of septal atrial tachycardias. The disadvantages of cryoablation are several. The lesions do take 4 minutes to produce, and if multiple lesions are required, this can prolong procedure duration. In addition, the marker frequently used to guide slow pathway ablation, is an accelerated junctional rhythm, and this is not seen during cryoablation. One has to rewarm and then retest after every lesion. Although in theory, the catheter should adhere to the tissue at the ablation site during cryomapping and cryoablation, stabilizing the catheter for 4 minutes may be difficult. The long-term results, reported after an acute procedural success, seem to show about the same risk of recurrence of 3% to 5%, as has been reported to occur with standard radiofrequency procedures.
Cryoablation does have a place in the armamentarium of electrophysiologists. Right now, it is the preferred approach if a high risk of producing AV block is anticipated because of the position of the structure targeted for ablation. In most patients with AVRT and AVNRT, this risk with radiofrequency ablation will remain the modality of choice. Hopefully, in the future, improved catheter design and handling will allow higher success rates to be achieved with cryoablation for all the structures we commonly target.
Dr. DiMarco, Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville, is on the Editorial Board of Clinical Cardiology Alert.