Drs. Filipovic, Michaux, and Seeberger remark that our two-part review article does not contain a clear statement as to how to manage patients who are not pacemaker-dependent. 1,2They suggest that reprogramming the device to an asynchronous mode could produce harmful interactions with the patient's intrinsic rhythm, including the R-on-T phenomenon causing ventricular fibrillation (VF). While they illustrate the R-on-T asynchronous pacing in intraoperative Holter recordings from two patients, there is no VF in either recording.
A search of the Ovid Medline database (1966–present) reveals only a single case report describing VF induced by R-on-T ventricular pacing (i.e. , stimulation during ventricular repolarization, particularly during the upstroke of the T wave). 3This occurred after coronary angiography in an elderly man who had suffered a recent myocardial infarction (MI) and presented with syncope. A temporary ventricular-inhibited pacemaker had been placed after sinus pause was documented. The R-on-T phenomenon was attributed to sensing failure. Ventricular tachyarrhythmias initiated by R-on-T ventricular extrasystoles have been described both in the setting of thrombolysed acute MI 4and in patients with acute MI prior to the thrombolytic era. 5At least one report suggests that in the setting of acute MI, R-on-T extrasystoles appear more likely to trigger VF than ventricular tachycardia. 6To our knowledge, there are no reports of R-on-T extrasystoles or asynchronous pacing triggering ventricular tachyarrhythmias in anesthetized patients.
Magnet-induced asynchronous ventricular pacing is used routinely in pacemaker follow-up, both as part of examinations in pacemaker clinics and physicians’ offices and during transtelephonic screening, and asynchronous (“noise mode”) pacing is induced automatically when pacemakers identify persistent electromagnetic interference (EMI), as described below. The induction of VF by ventricular stimuli occurring during ventricular repolarization appears to be so rare that most pacemaker practitioners, even those with many years of experience, have never seen it.
As discussed, 2EMI between 5 and 100 Hz overlaps the frequency range of intracardiac signals, and is not filtered out by the pacemaker's sensing circuitry. Possible responses to EMI are: (1) inhibition or triggering of stimulation; (2) asynchronous pacing; (3) mode resetting; or (4) damage to the pulse-generator circuitry. 7To protect against inappropriate inhibition of pacing stimuli, some devices may revert to asynchronous pacing at the basic-rate interval when exposed to continuous EMI above a certain frequency. 7In others, repetitive noise detection in the noise-sampling period causes temporary reversion to an asynchronous “noise” mode, typically VOO or DOO. 7Whether EMI causes inhibition or asynchronous pacing depends on interference-signal duration and field strength. 8At the lowest field strength, there is no effect. But, as the field strength increases, there is a greater tendency to inhibition because the noise may be sensed intermittently. With still higher field strengths, noise is sensed continuously and asynchronous pacing occurs. There can be considerable variation between different pacemakers and their susceptibility to EMI. 7,8
We agree with Filipovic et al. that if a patient is pacemaker-dependent and EMI is likely to cause significant interference, a triggered or asynchronous mode should be programmed. 2However, for patients with adequate intrinsic rhythm, we suggest several measures to reduce the likelihood of adverse effects due to surgical EMI. 2Also, if electrocautery must be used in the vicinity of the pulse generator or leads (closer than 15 cm), the pulse generator should be identified and its response to strong, continuous EMI ascertained. 2If electrocautery usage might compromise the patient in any way, a compatible programming device must be available in the operating room, with the pulse generator accessible to the programming head and someone experienced in programming present. 7
Therefore, based on available evidence and our own experience, we believe that asynchronous pacemaker operation, whether preprogrammed or consequent to sensed strong, continuous EMI (i.e. , noise mode), is highly unlikely to trigger ventricular tachyarrhythmias. We suppose this might be possible in patients with acute MI or in patients with severe underlying heart disease and dangerous imbalance (e.g. , severe hypokalemia, digitalis, or catecholamine excess).
Finally, Filipovic et al. suggest that to protect the “battery” (we are sure they mean the pulse generator), bipolar atrial and ventricular sensing should be used. If the implanted pacing system has this capability, the bipolar configuration is desirable because it offers superior immunity to far-field sensing and because the very short distance between electrodes minimizes the effective antenna length and thus the susceptibility to radiated electromagnetic interference of the sort commonly encountered. 2However, we disagree with the implication that any pacemaker can always be switched on by placing a magnet over the pulse generator. This will not occur if the magnet response has been programmed off, an option in some pulse generators. 2