Risks to pacemaker patients undergoing magnetic resonance imaging examinations

Abstract

(i) movement of pacemaker device or leads, (ii) radiofrequency (RF)-induced heating of lead tip, (iii) pacemaker dysfunction (induction of ventricular fibrillation and risk of asynchronous pacing), (iv) damage, (v) rapid pacing, (vi) reed switch malfunction with or without asynchronous pacing, (vii) inhibition of pacing output, (viii) change or loss of programmed data caused by electrical reset, (ix) depletion of the pacemaker battery voltage, (x) change in pacing capture threshold, (xi) high abandoned lead risk, and (xii) back-up mode creating cardiac insufficiency in children due to reduction in rate or output. Before critically commenting on these real or alleged risks, an observation made in the midst of the 1980s with Medtronic pulse generators (PGs) of the Spectrax family is reported. There were case reports that these PGs developed several malfunctions during RF electrosurgery: reduction in output pulse, switching to back-up mode, inhibition, or total loss of function. Applying 500 kHz voltages to the input of a Spectrax PG, there was obviously rectification of the RF voltage inside the PG diminishing the battery voltage and thereby the output pulse, and causing the PG to enter the back-up mode. With still higher voltages, complete inhibition took place due to activation of the ‘run-away protection’. Reduction of the battery voltage to 1 V or less forced the circuitry into instability where the logic elements were neither ‘1’ nor ‘0’, resulting in high current consumption. If the battery resistance was 10 kV or higher, this state was no longer reversible when the interference [electromagnetic interference (EMI)] was ceased, thus yielding complete exhaustion within a short time. These phenomena no longer exist in modern Medtronic PGs. Another phenomenon, but not connected to Medtronic PGs, arose when the PG was protected against external defibrillation by a simple Zener diode. The often used Z 7 limited the EMI at the input of the PG to 7 V (Zener voltage) in one polarity and to 0.7 V (diode voltage) in the other polarity. All voltages higher than the diode or Zener voltages were short-circuited by the Zener diode, so that stronger currents flowed through the electrode/tissue interface. Fibrillation could result from these rectified currents. This effect of pacemaker EMI was well known from investigations of RF electrosurgery and pacemakers. 2 Orland and Jones 3 even proved that fibrillation was possible if the PG was exhausted. Modern PGs are now protected by back-to-back Zener diodes or similar circuits that avoid rectification. There is no doubt that the asymmetric Zener diode protection will also short-circuit RF voltages during MRI, yielding scan synchronous stimulation pulses as described by several authors. 4‐7 Against this background of RF experience, pacemaker experts engaged in the problem of MRI and pacemaker compatibility were at that time focused on what could possibly happen. Their experimental experience of whether these assumptions were realistic was limited in the 1980s. This should give rise to the question as to which of the adverse effects are realistic and which are without any experimental or clinical proof. To mention the latter is only of historical interest and should not be described as a threat that still exists. One phenomenon that was not discussed in the paper by Strach et al. 1 or in others either 8 is the EMI behaviour due to gradient fields. Assuming that the asynchronous magnet mode is switched off or that the reed switch remains open inside the MRI machine, the PG will switch to its ‘noise mode’ if it is exposed to EMI from the strong voltages pulses—we measured in vitro