Determinants of gradient field-induced current in a pacemaker lead system in a magnetic resonance imaging environment
Johns Hopkins University School of Medicine, Baltimore, MD 21287, USA. <> Heart Rhythm
(Impact Factor: 5.08).
04/2008; 5(3):462-8. DOI: 10.1016/j.hrthm.2007.12.022
The determinants of low-frequency-induced current by magnetic resonance imaging (MRI) gradient fields in a pacemaker lead system are largely unknown.
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.
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.
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.
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.
Available from: Olaf Muehling
- "This includes not only SAR level and body landmark, but also MRI manufacturer, generator and lead system manufacturer and model
. The proximity of the generator to the end of the bore, proximity of the generator to the surface of the bore, body habitus, lead geometry and loops as well as lead system integrity
[38,39] may also play a role. There may be an unpredictable change in pacemaker function such as inhibition, rapid ventricular pacing occur
 or no change at all
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Conventional cardiac pacemakers are still often regarded as a contraindication to magnetic resonance imaging (MRI). We conducted this study to support the hypothesis that it is safe to scan patients with cardiac pacemakers in a 1.5 Tesla MRI, if close supervision and monitoring as well as adequate pre- and postscan programming is provided.
We followed up 356 patients (age 61.3 ± 9.1 yrs., 229 men) with single (n = 132) or dual chamber (n = 224) cardiac pacemakers and urgent indication for a cranial MRI for 12 months. The scans were performed at 1.5T. During the scan patients were monitored with a 3-lead ECG and pulse oximetry. Prior to the scan pacemakers were programmed according to our own protocol.
All 356 scans were completed without complications. No arrhythmias were induced, programmed parameters remained unchanged. No pacemaker dysfunction was identified. Follow-up examinations were performed immediately, 2 weeks, 2, 6, and 12 months after the scan. There was no significant change of pacing capture threshold (ventricular 0.9 ± 0.4 V@0.4 ms, atrial 0.9 ± 0.3 V@0.4 ms) immediately (ventricular 1.0 ± 0.3 V@0.4 ms, atrial 0.9 ± 0.4 V@0.4 ms) or at 12 months follow-up examinations (ventricular 0.9 ± 0.2 V@0.4 ms, atrial 0.9 ± 0.3 V@0.4 ms). There was no significant change in sensing threshold (8.0 ± 4.0 mV vs. 8.1 ± 4.2 mV ventricular lead, 2.0 ± 0.9 mV vs. 2.1 ± 1.0 mV atrial lead) or lead impedance (ventricular 584 ± 179 Ω vs. 578 ± 188 Ω, atrial 534 ± 176 Ω vs. 532 ± 169 Ω) after 12 months.
This supports the evidence that patients with conventional pacemakers can safely undergo cranial MRI in a 1.5T system with suitable preparation, supervision and precautions. Long term follow-up did not reveal significant changes in pacing capture nor sensing threshold.
Available from: António Ferreira
- "These gradients are repeatedly and rapidly turned on and off (the slew rate of a gradient system is a measure of how quickly this can happen). These rapidly changing magnetic fields can induce electrical currents in pacemaker leads, causing oversensing, undersensing, or even life-threatening arrhythmias.8,13,18,19 "
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ABSTRACT: Use of both magnetic resonance imaging (MRI) and pacing devices has undergone remarkable growth in recent years, and it is estimated that the majority of patients with pacemakers will need an MRI during their lifetime. These investigations will generally be denied due to the potentially dangerous interactions between cardiac devices and the magnetic fields and radio frequency energy used in MRI. Despite the increasing reports of uneventful scanning in selected patients with conventional pacemakers under close surveillance, MRI is still contraindicated in those circumstances and cannot be considered a routine procedure. These limitations prompted a series of modifications in generator and lead engineering, designed to minimize interactions that could compromise device function and patient safety. The resulting MRI-conditional pacemakers were first introduced in 2008 and the clinical experience gathered so far supports their safety in the MRI environment if certain conditions are fulfilled. With this technology, new questions and controversies arise regarding patient selection, clinical impact, and cost-effectiveness. In this review, we discuss the potential risks of MRI in patients with electronic cardiac devices and present updated information regarding the features of MRI-conditional pacemakers and the clinical experience with currently available models. Finally, we provide some guidance on how to scan patients who have these devices and discuss future directions in the field.
Available from: Edmond M Cronin
- "3. Electrical current induction: this has been demonstrated in vitro and in animal studies, due to both the RF field and pulsed gradients.[8,15] Rapid capture of the myocardium could result in hemodynamic compromise or ventricular fibrillation. "
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ABSTRACT: Pacemakers and other cardiac implantable electronic devices (CIEDs) have long been considered an absolute contraindication to magnetic resonance imaging (MRI), a crucial and growing imaging modality. In the last 20 years, protocols have been developed to allow MR scanning of CIED patients with a low complication rate. However, this practice has remained limited to a relatively small number of centers, and many pacemaker patients continue to be denied access to clinically indicated imaging. The introduction of MRI conditional pacemakers has provided a widely applicable and satisfactory solution to this problem. Here, the interactions of pacemakers with the MR environment, the results of MR scanning in patients with conventional CIEDs, the development and clinical experience with MRI conditional devices, and future directions are reviewed.
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