Prashant Nair, Ariel Roguin, 200
“Magnetic Resonance Imaging in patients with ICDs and Pacemakers”
In-Vitro and Animal Studies:
The potential hazardous effects of MR imaging in patients with cardiac pacemakers have
been studied since 1983. Pavlicek and colleagues were the first to report the effects of MR
imaging on pacemaker function11. They showed that RF fields present in an MR unit could
possibly inhibit demand pacemakers and time-varying magnetic fields could generate pulse
amplitudes to mimic cardiac activity. The threshold for initiating the asynchronous mode of a
pacemaker was reported to be as low as 17 Gauss (1 Gauss = 10-4 Tesla). The possibility of
altering pacemaker parameters was presented as a serious limitation of MR imaging. Fetter et
al.12 showed that pacemakers reverted from the demand to the asynchronous mode within the
magnetic field of the scanner (0.15 Tesla), but microscopic testing showed no evidence of reed
switch sticking or magnetizing, or damage to other discrete pacemaker components. Other
investigators studied the feasibility of dual-chamber pacing systems in the MR environment.
Erlebacher et al. tested different DDD pacemakers in a saline phantom, and showed that during
scanning at 0.5 Tesla, all units malfunctioned due to RF interference which caused total
inhibition of atrial and ventricular output, or resulted in atrial pacing at very high rates.9 The
potential for rapid cardiac stimulation during MR was also reported in animal studies22. Lauck et
al. investigated the performance of different stimulation modes (VVI, VVIR, VOO, DDD,
DDDR and DOO) during MR scan at 0.5 Tesla23. Reversible activation of the reed switch with
consecutive asynchronous stimulation was observed in all pacemakers. Pacemakers in the
asynchronous mode were not affected with regard to stimulation rate and capture during
scanning. In contrast, pacemakers with automatic mode switching to demand pacing or
programmed inactivation of the reed switch were triggered in the dual chamber mode and were
inhibited in the single chamber mode. Thus, the investigators recommended programming into
the asynchronous mode prior to scanning, and in those without permanent pacemaker
dependency, complete inactivation of the system, if possible.
The effects of more powerful MR scanners (i.e., 1.5 Tesla) on cardiac pacemakers were
initially reported by Hayes et al24. In-vivo evaluation of different single and dual chamber
pacemakers showed reversion into asynchronous mode and transient reed switch inhibition.
Seven of the eight pulse generators paced rapidly when exposed to the RF signal associated with
a marked decrease in blood pressure. Stimulation cycle length was 200 ms (300 beats/min)
corresponding to the frequency of pulsing. It was proposed that rapid pacing was the result of an
"antenna" effect that couples the RF energy back into the pacemaker output circuits.
More recently, Achenbach et al.25 showed in a phantom study on 11 pacemakers and 25
leads that no pacemaker malfunction was observed in asynchronous pacing mode (VOO/DOO),
whereas inhibition and rapid pacing were observed during spin-echo imaging if the pacemakers
were set to VVI or DDD mode. The authors suggested that rapid pacing was caused by induction
of currents above sensing threshold in the atrial lead and consequent triggering of ventricular
stimulation. Direct interference with the pacemaker electronics seemed to be an unlikely
explanation, because the rapid pacing rate was always equal to the programmed frequency limit.
Importantly, most of the above were reports of earlier generation pacemakers. Recent
reports testing improved technology devices found no functional issues in most pacemakers
exposed to prolonged MR scan26,27.
Measuring lead heat in the MR environment is technically difficult and depends on the
methods used; explaining why several group report different results. Heating effect of pacemaker
leads was investigated by Achenbach et al.25 Continuous registration of the temperature at the
lead tip using an optical temperature sensor showed a maximal temperature increase of 63.1°C
during 90 seconds of scanning. In seven electrodes, the temperature increase exceeded 15°C.
Luechinger et al28 used pacemaker leads with additional thermocouple wires as temperature
sensors implanted in nine animals to measure heating. They recorded temperature increases of up
to 20°C were during MR imaging of the heart. However, they found only minor stimulation
Indian Pacing and Electrophysiology Journal (ISSN 0972-6292), 5(3): 197-209 (2005)