To the Editor:—
In their article about pacemakers and extracardiac radiofrequency ablation, Tong et al. describe some appropriate precautions; i.e. , have temporary equipment available for backup pacing and check the pacemaker with a programmer both before and after the radiofrequency ablation event.1However, the authors should have included a statement about the placement of the current return pads, which should be as close as possible to the radiofrequency ablation delivery electrodes to prevent or minimize radiofrequency ablation current travel across the generator and lead system (they did not specify the location of these pads). In addition, this article has a number of inaccuracies.
First, the authors have misinterpreted electrical artifacts displayed by their digital monitor (and marked with upward arrows in strips D and E [fig. 1]) as pacemaker output pulses, making an incorrect diagnosis of “intermittent temporary runaway pacing.” None of these pulses appears to “capture” the ventricle and pace the heart, and other pacing pulses exist in an appropriate fashion that do depolarize the ventricle. In fact, these are pacemaker “pseudospikes,” which result from inappropriate digital processing of the electrical noise from the radiofrequency ablation by the electrocardiographic monitor. This phenomenon has been well described.2,3Additional psuedospikes can be seen in strip D after complexes 5 and 8 and in strip E after complexes 2, 6, and 7. Also, the term “runaway pacing” refers to continuous, high-rate pacing resulting from internal component failure within a generator. Correction is always by pacemaker replacement.4,5Further, the pseudospikes appear at rate of 375 bpm (strip E , 160 ms), arguing against runaway pacing, as the Guidant Meridian pacemaker (Guidant Corporation, St. Paul, MN) has a runaway limit of 205 bpm.6
Next, the authors report that their patient had “complete atrioventricular block.” Complete atrioventricular block is not demonstrated by any of the five strips that accompany the text. Strip C and strip E (complexes 4–7) show narrow complex QRS complexes, suggesting that the patient had intact atrioventricular nodal conduction. The rhythm might be sinus at approximately 70 bpm with a P-R interval of 300 ms. It is not clear if the atrial events are paced, as operating room monitors do a poor job of detecting and showing atrial pacing events. Because the authors did not describe the programmed P-R interval, we have no way to know whether atrial events are paced or native. The atrioventricular delay in strip D and possible paced atrial events in strip A , complexes 8–10 (open downward arrows ), do appear to be less than 200 ms. However, without knowing the atrioventricular delay programming and without real-time data from the pacemaker, limited conclusions can be made about the longer atrioventricular delays seen throughout the five strips.
The authors state that the patient's heart rate immediately changed from 63 to 96 bpm with the onset of radiofrequency ablation delivery, seen in strip A complexes 4–7. Yet, there is the suggestion of P waves (open downward arrows , added) at these complexes. The possible atrial pacing artifacts at complexes 8–10 (strip A ) might have resulted from rate smoothing, which will limit any decreasing rate in this pacemaker, if programmed Thus, without knowledge of the pacemaker settings as well as real-time data from the pacemaker (telemetry), no conclusion can be reached about the etiology of this increased heart rate.
At most, these strips might demonstrate atrial and ventricular oversensing, which should be expected during radiofrequency ablation, according to the Meridian Physician's Manual.6In a DDD-programmed pacemaker, the occurrence of atrial oversensing, without ventricular oversensing, will result in higher rates of ventricular pacing than expected; usually, these events take place at the upper limit for tracking. With the presumed upper limit for tracking in this case of 120 bpm, atrial oversensing could have taken place at A4–6 and B5–6. Ventricular oversensing is difficult to prove with these strips, as the longest R-R interval occurs at A9–10 and represents a heart rate of 70 bpm. In the setting of ventricular oversensing, the pacemaker will fail to pace the ventricle and an inappropriately long R-R interval will occur. No such intervals are demonstrated.
In summary, Tong et al. remind us that extracardiac radiofrequency ablation in a patient with a cardiac generator should be approached with caution owing to possible electromagnetic interference with the generator and potential misinterpretation of electrocardiographic behavior.
University of Texas M.D. Anderson Cancer Center, Houston, Texas. firstname.lastname@example.org