Determinants of gradient field-induced current in a pacemaker lead system in a magnetic resonance imaging environment

BackgroundThe determinants of low-frequency–induced current by magnetic resonance imaging (MRI) gradient fields in a pacemaker lead system are largely unknown.ObjectiveThe purpose of this study was to determine the magnitude of MRI low-frequency–induced current in an implanted pacemaker lead system and to investigate in vivo determinants of low-frequency–induced current in an animal model.MethodsSix mongrel dogs underwent conventional single-chamber pacemaker implantation with a current recorder connected in series. Pulse generator (PG) was programmed to VOO 120 bpm with subthreshold output. MRI was performed in a 1.5-T scanner. Low-frequency–induced current was recorded during unipolar pacing, bipolar pacing, and bipolar pacing with the PG case electrically isolated from the pocket. In each mode, low-frequency–induced current was recorded with and without a large loop of additional lead connected in series.ResultsWith a conventional implant, low-frequency–induced current was ≤0.5 mA in all three pacing modes. With five external loops, the magnitude of low-frequency–induced current increased to >30 mA, with consistent myocardial capture in unipolar and bipolar pacing. However, in bipolar pacing with the PG electrically isolated from the pocket, low-frequency–induced current decreased to <0.5 mA with no myocardial capture even with additional looped leads.ConclusionUnder conventional implant conditions, the magnitude of low-frequency–induced current is <0.5 mA and is unlikely to cause myocardial capture; however, arrhythmia induction cannot be excluded. With sufficient increase in effective loop area (additional looped leads), direct myocardial capture by the low-frequency–induced current is possible. In this study, breaking the return pathway by electrically isolating the PG case from the circuit abolished low-frequency–induced current.