To the Editor:—
Radiofrequency ablation is becoming an important therapy but can cause a problem if applied very close to pacer leads. Hayes et al. 1reported no problems with distant extracardiac ablation. We report an extracardiac case with a different finding.
The patient was admitted for radiofrequency ablation of a right adrenal metastasis. He also had heart block that was shown on electrophysiologic testing to be a high-grade suprahisian block. A permanent dual-chamber pacemaker (Meridian 1276; Cardiac Pacemakers Inc., St. Paul, MN) was implanted and programmed to Dual Chambers paced, Dual Chambers sensed, Dual response to sensing with Rate modulation (DDDR) mode (rate, 60–120 beats/min; output, 2.5 V/ 0.5 ms; sensitivity: atrium, 0.75 mV, ventricle, 2.5 mV). The radiofrequency equipment used was Cool-tip model 1025 (Radio Frequency Ablation System; Radionics, Tyco, Burlington, MA). Energy was delivered at a maximum of 200 W, 480 kHz at the lowest possible impedance, and achieved a treatment volume of 4.2–7.0 cm. Under intravenous general anesthesia, four 12-min radiofrequency ablations were performed, two in the inferior and two in the superior portions of the tumor mass. At the start of radiofrequency ablation, the pacemaker rate immediately changed from 63 to 96 beats/min (fig. 1A) and then showed an irregular pacing rhythm with intermittent runaway pacing during application of the radiofrequency current (fig. 1B). The underlying problem of complete atrioventricular block was also identified. However, pacing rhythm resumed in the postoperative care unit, without any device malfunctions.
The literature on intracardiac radiofrequency ablation is not applicable to this patient. Differences relate to time of current use, surface area, and target organ. The devices used for extracardiac radiofrequency ablation may have a continuous higher energy supply and a longer duration of ablation to provide a maximized ablation zone. Hayes et al. reported no electromagnetic interference with the pacemaker during intrahepatic radiofrequency ablation. In our case, electrocardiographic monitoring clearly demonstrated an influence on the pacemaker that inhibited the implanted pulse generator but did not permanently damage or reprogram it. This effect may be a manifestation of the extremely high field strengths induced by ablation in close proximity to the permanent pacing lead. 2When we estimated and compared the distance between the delivery system and the pacing lead with that reported by Hayes et al. , no cases were found to be closer than 5 cm. However, the radiofrequency equipment used in our case has a higher energy supply and a larger treatment volume when compared with what Hayes et al. used in their patients. The other reasons for the differences may be a different pacemaker manufacturer and a variable programmed mode during radiofrequency ablation. 3Based on these observations and the limited information obtained from the reported cases, we propose the following as precautions for extracardiac radiofrequency ablation in patients with a permanent pacing system: (1) The distance between the extracardiac radiofrequency delivery system and the ventricular pacing lead must be 5 cm or more, depending on the power and treatment volume of the radiofrequency equipment used. 1,2(2) Temporary external sources of pacing should be available as a standby, should the permanent system be inhibited. 2(3) The generator should be examined before and after the procedure in case any changes occur that necessitate reprogramming. 2