title: Determinants of gradient field-induced current in a pacemaker lead system in a magnetic resonance imaging environment.
abstract: BACKGROUND
  The determinants of low-frequency-induced current by magnetic resonance imaging (MRI) gradient fields in a pacemaker lead system are largely unknown.
  
  
  OBJECTIVE
  The 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.
  
  
  METHODS
  Six 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.
  
  
  RESULTS
  With a conventional implant, low-frequency-induced current was < or =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.
  
  
  CONCLUSIONS
  Under 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.
authors.affiliation: Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA. htandri1@jhmi.edu <htandri1@jhmi.edu>
authors.forename: Harikrishna
authors.initials: H
authors.lastname: Tandri
authors.forename: Menekhem M
authors.initials: MM
authors.lastname: Zviman
authors.forename: Steven R
authors.initials: SR
authors.lastname: Wedan
authors.forename: Thomas
authors.initials: T
authors.lastname: Lloyd
authors.forename: Ronald D
authors.initials: RD
authors.lastname: Berger
authors.forename: Henry
authors.initials: H
authors.lastname: Halperin
mesh_terms: D000818:Animals; D004285:Dogs; D004562:Electrocardiography; D004574:Electromagnetic Fields; D004869:Equipment Safety; D008279:Magnetic Resonance Imaging; D010138:Pacemaker, Artificial / Q000009:adverse effects*