HAMBURG, GERMANY. High intensity focused ultrasound (HIFU) is used successfully in the treatment of prostate cancer. Early trials also found it effective in the ablation of atrial fibrillation. The procedure is similar to that used in a standard pulmonary vein isolation (PVI) using radiofrequency energy except that the lesion-creating catheter is in the form of an inflatable balloon equipped with a 9 MHz ultrasound crystal which, when turned on, generates the heat necessary to create a complete lesion ring in less than a minute. Thus, the HIFU technique has the advantage of short procedure time and minimal radiation exposure (mean fluoroscopy time of about 30 minutes). Unfortunately, the HIFU procedure is also associated with the potential for serious adverse events such as phrenic nerve injury and the creation of esophageal ulcers or an, mostly fatal, atrioesophageal fistula (a hole between the heart and the esophagus).
Electrophysiologists at the Asklepios Klink St. Georg recently completed a trial to see if implementing a stringent safety protocol when performing HIFU ablations would reduce the number and severity of adverse events. Their trial involved 28 patients with paroxysmal (19) or persistent (9) atrial fibrillation of 6 years standing. The average age of the patients was 63 years and 18 (64%) were male. A temperature probe was inserted in the esophagus prior to the start of ablation which used a 20, 25 or 30 mm diameter sonication ring at an acoustic power setting of 45 watt. The effectiveness of the HIFU application in achieving electrical isolation of the pulmonary veins was measured with a Lasso catheter inserted in the ablated vein. A safety algorithm was used to ensure that tissue temperatures, especially esophageal temperature, did not exceed certain specified limits.
Complete acute isolation of all pulmonary veins was achieved in 9 patients (32%) using HIFU only with the addition of radiofrequency ablation bringing the total number of patients having all veins isolated to 16 or 57%. Eleven of 13 HIFU ablations (85%) were aborted prematurely because of dislodgement of the catheter or because of excessive esophageal temperatures. Adverse effects were significant with 21% experiencing transient or persistent phrenic nerve palsy (corresponding incidence for radiofrequency ablation is less than 0.1%). Esophageal ulcers were observed in two (8%) of patients undergoing post-procedure endoscopy, one patient died due to an atrioesophageal fistula, and one died of unexplained causes 49 days following the procedure. The authors conclude that, "application of HIFU energy for PVI in humans cannot be advised at this point in time."
Neven, K, et al. Fatal end of a safety algorithm for pulmonary vein isolation with use of high-intensity focused ultrasound. Circulation: Arrhythmia and Electrophysiology, Vol. 3, June 2010, pp. 260-65
Editor's comment: If you are ever offered a PVI procedure involving HIFU, please say NO THANKS.