However, the search for other forms of energy applications has remained. Apart from laser, microwave and ultrasound techniques, freezing was considered an important option. From 1977 on, Surgeons continued to create lesions with cryoprobes, and the first animal studies conducted with catheters were very promising. As it became clear that reversible and irreversible blocks could be created in dogs with a transvenous approach and Halocarbon (freon), an important new tool was emerging. The process is simple: an acute ice ball is formed, and the ice crystals cause subcellular damage. The combined freeze-thawing process causes haemorrhage and inflammation that may eventually lead to fibrosis. In more recent devices, nitrous oxide replaces Freon.
Clinical applications and advantages
The introduction of this technology lead to sites where normal AV conduction had to be saved, and where radiofrequency could be too dangerous; e.g. attempts to ablate parahissian or paraseptal bypass tracts might touch the AV-node. The catheter tip can be cooled to -30C, but this cooling causes a biological effect which is still transient (2). When the pathway is blocked, and the normal conduction remains present, further freezing to – 70, or -85C can effectively destroy the targeted tissue. This is an advantage over radiofrequency energy. Efficacy depends on precise mapping and subsequent firm adhesion to the ablation site, occurring during the freezing. This means that application is also possible during a fast rhythm. This is not advised with radiofrequency, as a catheter can jump after termination of tachycardia and the mapping site would then be lost. The rate of cooling, and consecutive cycles can influence efficacy as well.
AV nodal reentry and accessory pathways can be treated efficiently. For AVNRT, therapy can be focused exclusively on the slow pathway, and with radiofrequency this is only indirectly possible. As our preliminary data show a similar efficacy of both approaches, cryoenergy has a chance of becoming the most important technique in this area. For other sites, catheter manufacturing will dictate whether the promise of less collateral damage will become a fact as well. E.g. for flutter, larger catheter tips are necessary, but it seems that patients will experience less pain with cryoenergy. Other investigators showed that cryoenergy is very effective for pulmonary vein ablation, without the complication of stenosis, which is the disadvantage of this treatment with radiofrequency (3). Whether lesions will be transmural and deep enough to be used for linear approaches remains to be investigated.
The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.
Cryoenergy ablation could become the technique of choice for AV nodal reentrant tachycardia, with fast pathway and accessory pathways running in the proximity of the normal conduction tissue ablation being completely avoided.
Cryoenergy holds promise for atrial fibrillation. More widespread use of it will depend on the evolution of catheter technology.