The cryo balloon comprises of a balloon on the end of a catheter which is inflated in the left atrium and then jammed in each of the pulmonary veins in turn. When the balloon is in the vein it is then frozen by passing gas through the catheter. Where the balloon is in contact with the vein it causes freezing of the tissue and cell death. This renders the tissue electrically inert. The damage caused by cryoablation is less traumatic than RF and therefore it is less likely to cause damage to the veins. Unsubstantiated claims that cryo is less likely to cause stroke have never been proven.
In addition cryoablation can be associated with damage to the nerve that stimulates the diaphragm (the phrenic nerve) but this does not usually cause symptoms and in all published reports has recovered within 12 months. The potential problem with cryoablation is that it only works if the balloon is in contact with the tissue and while the balloon is perfectly circular, the pulmonary veins are not. Therefore it is not uncommon for operators need to use an additional catheter at the end of the procedure (either cryo or RF) to close the gaps left by the balloon. Cryoablation is used by many centres because it is felt to be faster and safer, and good results have been published in non-randomised studies.
RADIOFREQUENCY CATHETER ABLATION
The most common method for AF ablation used today is to perform point-by-point RF ablation to create a wide encirclement of the veins.
Multiple randomised trials have shown that this technique is superior to drug therapy and therefore it is reasonable to use this as the gold standard to judge other therapies and techniques by. Alternative approaches have focused on using alternative energy sources, or technologies to make delivery of the RF easier.
CIRCULAR ABLATION CATHETERS
This is a system where the catheter is not bent by a puller wire inside the catheter and is floppy. The tip of the catheter has metal in it and strong magnetic fields are used to bend the catheter.
The catheter can be advanced using a motorised drive unit placed on the patient’s groin which allows the operator to control catheter movement from outside the catheter lab and away from x-rays. The advantage to this system is that perforation by the catheter seems very unlikely because the catheter is so floppy that it cannot be pushed through the heart wall.
Whether this is really an issue is not clear because although blood leaking out of the heart is a common complication of AF ablation this is rarely results from catheter perforation and may be related to the transseptal puncture required to access the left atrium with any technology, or as the result of an RF lesion causing perforation. Nevertheless the magnetic system allows precise manipulation of the catheter around the heart and therefore the potential for very accurate RF lesion placement.
One of the technical challenges has been to produce a cool-flow catheter that is sufficiently floppy to allow the magnets to manipulate it.
Cool flow describes a catheter where saline is passed down the central lumen of the catheter and sprayed out of the end over the catheter tip. This is generally used in AF ablation because we believe it may reduce the chances of blood clot formation on the catheter tip and therefore stroke (unproven in randomised trials).
The difficulty of maintaining the integrity of a central lumen used for the saline and making the catheter floppy has now been overcome and this catheter is now in use. This may however explain why magnetic navigation has been in clinical use for almost 10 years and no data demonstrating that it is effective for AF ablation have been published.
There are concerns that the magnets may not be able to bend the catheter with sufficient force to produce adequate RF lesions but this again is not substantiated.