Ablation involves killing a very small amount of abnormal heart muscle responsible for the heart racing. This produces a pea-sized area of dead heart muscle called a lesion. In treating heart rhythm problems with ablation techniques, doctors can choose from a number of energy sources with which to produce lesions in the heart tissue. The use of radiofrequency (RF) waves to burn tissue is a well-established treatment for arrhythmias, but concerns about safety and the continuing evolution of cryothermal (cold-energy) catheters have helped convince many doctors to add cryoablation to their practice.
RF: advantages and drawbacks
Radiofrequency ablation has an established record of successful use in treating a variety of arrhythmias. Many different RF catheters are available, and make it possible for the doctor to move them to certain sites inside the heart. Once the RF catheter is in position in the heart, it takes only a minute to make a lesion – an important consideration in terms of keeping the length of the procedure to a minimum. Currently, RF ablation is the mainstay of treatment of heart-racing problems.
However, RF ablation has some limitations and drawbacks. A large study of RF ablation conducted in 18 very good hospitals in the 1990s found an overall success rate of 95%, but about 5% of patients required a second try. Major complications occurred in 3% of patients including death, stroke, accidental damage to the normal pathway (the AV node) requiring a permanent pacemaker, perforation of the heart, heart attack, collapsed lung, and blood-clot formation. Minor complications occurred in 8% of patients including fluid around the heart or lung, breathing problems, nerve injury, pneumonia, and low blood pressure. These outcomes may be slightly better today, but not remarkably so. Therefore, the search for better energy sources for ablation has continued and now includes microwave, laser, high-frequency ultrasound and cryo.
Cryo: advantages and drawbacks
Cryoablation catheters (also known as cryocatheters) have been developed that possess similar ease of handling to RF catheters. When they were tested in animals and compared with RF ablation catheters, cryocatheters produced less irritation of the inner lining of the heart and blood clotting. Cryocatheters can be used in areas of reduced blood flow more effectively than RF catheters.
Cryocatheters stick tightly to the heart muscle and do not move around as RF catheters do.A large study was performed with the first generation of cryocatheters with the same size metal tip as RF catheters. This trial, the “Frosty Trial,” involved almost 200 patients with various forms of heart racing. All patients were treated with a cryocatheter. The immediate success was 93% in patients with the most common type of heart racing involving the top of the heart – sufficiently high to receive FDA approval of this catheter for this arrhythmia.
The success for ablating other short circuits as well as the normal heart pathway prior to pacemaker insertion was less than that typically achieved with RF ablation. Nevertheless, the catheter performed well. Complications occurred in 4% of patients and were thought not to be caused by the cryocatheter itself.
There is evidence supporting the claim that cryoablation is safer than RF ablation. In a study of people with atrial fibrillation, for example, cryoablation was found to be less likely to generate blood clots than RF ablation. Another advantage of cryo energy is that it allows the doctor to freeze tissue to test whether it is responsible for the heart racing – a procedure known as cryomapping. Heat-based therapies such as RF ablation don’t allow the doctor to do that – once the tissue is burned, it stays burned. In contrast, cryoablation allows re-warming of the frozen tissue to restore its function. Also, with cryoablation there is less risk of perforating the heart since cryoablation does not weaken the heart tissue like RF ablation does. Also, accidental damage to tissues adjacent to the heart, such as the esophagus or a heart artery is less likely than with RF ablation.
Traditionally, the drawbacks of cryoablation have focused on three issues: too small lesion size; catheter maneuverability, and the time required to produce a lesion.
In the Frosty Trial, the smallest tip cryocatheter was used – and lesions were too small to have a lasting effect in some patients. In Europe, two trials were performed comparing cryoablation with RF ablation for the most common cause of heart racing from the top of the heart. Both the RF and the cryocatheters had 4-mm tip electrodes. These studies showed the same success rates and safety. Fewer lesions were required to cure patients treated with cryoablation than those receiving RF ablation. Success at the end of the procedure was the same in the groups, but in one of the studies the heart racing came back again more in patients treated with cryoablation than in patients treated with RF ablation. The reason for this is that the 4mm cryocatheter used in these studies made too small of a lesion to cure some patients. A more recent study of 75 patients who all underwent cryoablation using a catheter with a larger metal tip was successful at the end of the procedure in 99% of patients with recurrence of the heart racing in only 5%. These results are comparable to those of RF ablation. Therefore most doctors are now using the cryocatheters with the larger metal tip sizes.
The second disadvantage of cryoablation is that the cryocatheters do not handle as well as RF ablation catheters. It is more difficult for the doctor to make tight turns, loops and other advanced maneuvers with the catheter inside the heart. However, newer cryocatheters on the market offer better maneuverability.
Compared with RF ablation’s ability to make a lesion in 60 seconds, a comparable lesion with cryoablation takes several minutes if done correctly. This difference is magnified if adjacent lesions are to be placed to minimize the chance that the heart racing problem will recur.