Wolff-Parkinson-White Syndrome (WPW)
Wolff-Parkinson-White Syndrome (WPW) is caused by abnormal connections or short circuits in the heart called accessory pathways. These abnormal pathways are present since birth and result from incomplete separation of the top from the bottom part of the heart. Initially, heart muscle from the top part of the heart is in direct contact with heart muscle from the bottom part of the heart. As the fetus ages, connective tissue grows, separating the top from the bottom part of the heart completely except for the one normal pathway, the atrioventricular node.
When this separation process is incomplete, an extra connection is present. This accessory pathway can conduct the heartbeat from the bottom to the top part of the heart only, from the top to the bottom only, or in both directions. In the first case (bottom to top only), the accessory pathway is said to be concealed and the heart rhythm abnormality resulting is called supraventricular tachycardia (SVT).
When the accessory pathway can conduct from top to bottom only, SVT is impossible, but fast heart rhythms from the top part of the heart such as atrial fibrillation and atrial flutter may conduct so rapidly to the bottom part of the heart that the person experiences a cardiac arrest. If the accessory pathway is not that fast, then a cardiac arrest will not occur and the person may feel just like the average person with atrial fibrillation or flutter. If the pathway can conduct in both directions, then both types of arrhythmias are possible. Although these pathways are present from birth, only about 1% of them runs in families.
Most people with WPW have “spells” consisting symptoms of abnormal heart beating (palpitations) or outright heart racing. Associated symptoms may include chest pain, pulsations in the neck, shortness of breath, light-headedness, fatigue, sweating, passing out or even (though rarely) cardiac arrest. After a spell, the person may have frequent urination (due to release of a protein from the top heart chambers that stimulates urine formation) or feel fatigued for hours to days. Some people with WPW, however, have not had any symptoms at all.
How serious is WPW?
In rare cases, WPW can lead to cardiac arrest; however, most people with WPW are at low risk of this catastrophe. Most doctors expert in WPW feel that the risk of death in WPW patients who are having heart racing is greater if nothing is done than if WPW is treated with ablation – hence, ablation is usually recommended.
Treatment options for patients with WPW include either antiarrhythmic drugs or ablation. Antiarrhythmic drugs, if a good option, would ideally work best in WPW patients who were at the highest risk for a cardiac arrest. Unfortunately, in these high risk patients, drugs do not work well enough to eliminate the risk of a cardiac arrest. Therefore the mainstay of treatment for WPW has been ablation, not medications.
RF ablation for WPW
In the 1990s, radiofrequency (RF) ablation replaced open-heart surgery for the treatment of WPW. In general, RF ablation is successful in curing WPW in over 95% of patients with a risk of about 3%. No large studies comparing RF ablation with cryoablation have been performed, and definitive comparisons are speculative. However, based on other research, there are several risks that are potentially greater with RF ablation when compared with cryoablation, including the following:
- Higher risk of stroke with left-sided pathways
- Higher risk of perforation
- Higher risk of accidental damage to important structures near the pathway
- Higher risk and lower chance of success for pathways located within heart veins
Cryoablation for WPW
A larger study of cryoablation called the FROSTY trial that enrolled 166 patients scheduled for ablation included 51 patients with WPW. Forty-four of this group underwent cryoablation using a catheter with a 4-mm tip electrode. Although cryoablation in these patients was found to be safe, in only 77% was the procedure successful. Many doctors have since given up on the 4-mm catheter believing that it makes too small a lesion to be effective. Preliminary use of catheters with larger tips (6 or 8 mm) in our institution has indicated that these produce better outcomes.
There are three pathway locations (comprising nearly half of all pathways) that may be more suitable for cryoablation than RF ablation. First, the accessory pathways close to the normal connection between the top and bottom of the heart (the AV node) are ideal for cryoablation. These pathways are labeled right anteroseptal, parahisian, intermediate septal or posterior septal. When the doctor has placed the cryoablation catheter where she/he believes the pathway to be, gentle cooling is delivered through the catheter. If the AV node starts to be affected, the cooling is immediately turned off, and the AV node can recover fully. The doctor can then reposition the catheter to a different location and repeat the process. Finally, a site will be found where cooling causes the pathway to lose function but the AV node is fine. At this site, freezing the tissue will cause the pathway to be destroyed without affecting the AV node.
On the other hand, if such a pathway is subjected to RF ablation, the only approach is to make the burn hoping to turn off power if an untoward effect is seen. Unfortunately, “you can’t take back a burn” applies here just as it does when a child touches a hot grill. Research has shown that lethal temperatures can persist for up to 10 seconds after RF power is turned off. The chance of having a pacemaker put in after RF ablation of these pathways in a large series was 2.5-3%. There was also an 11% chance of recurrence of heart racing in this population in the same study. The initial use of this cryoablation technique was in patients with such pathways because the risk of a pacemaker was such a concern with RF ablation. The safety and efficacy of cryoablation in case reports and small series was excellent, making this the ablation of choice for these patients. A larger series of 35 adolescent patients showed acute success rates comparable to what would have been expected with RF ablation, but with slightly higher recurrence rates.
The second pathway location where cryoablation has an advantage is the rare pathway located inside the coronary sinus, a vein on the back side of the heart. Animal studies have suggested that if these pathways are within 5 mm of a heart artery, there is a high chance that RF ablation will damage the heart artery, which could cause a heart attack. Cryoablation delivered close to such arteries is much less likely to damage the artery. Also, if such pathways are in very small veins, not enough power can be given with RF ablation to kill the pathway. There are also cases of getting the RF catheter stuck in the small vein when RF energy “welds” the catheter to the vein. Cryoablation in small vein locations with low blood flow allows the catheter to make a bigger lesion than normal, and welding does not occur.
Thirdly, pathways on the right side of the heart cross the tricuspid valve, which tends to move a lot during the normal heart beat. Not infrequently it is difficult for the doctor to get a RF ablation catheter to stay in contact with the pathway long enough to kill it. With cryoablation, the catheter sticks to the heart tissue within 10 seconds or so and will not move after freezing starts to occur. In a large series using RF ablation on such pathways, the success at the end of the first procedure was only 84% with 7% of patients requiring a second procedure. 14% of patients had recurrence of their heart racing. These results with RF ablation are not encouraging. One study of 3 patients having cryoablation for these pathways has been published showing success in all patients.