Magnetic resonance imaging of pacemakers and implantable cardioverter-defibrillators without specific absorption rate restrictions

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1 Europace (2010) 12, doi: /europace/euq092 CLINICAL RESEARCH Pacing and CRT Magnetic resonance imaging of pacemakers and implantable cardioverter-defibrillators without specific absorption rate restrictions Michael Mollerus*, Glenn Albin, Margaret Lipinski, and Jill Lucca St. Mary s/duluth Clinic, 407 E Third Street, Duluth, MN 55805, USA Received 18 October 2009; accepted after revision 4 March 2010; online publish-ahead-of-print 30 March 2010 Aims The purpose of the current study is to evaluate the safety profile of patients with pacemakers or implantable cardioverter-defibrillators (ICDs) undergoing a medically necessary magnetic resonance imaging (MRI) scan without limitation on peak specific absorption rate (SAR). Recent series suggest that MRI scanning can be performed safely in select patients with pacemakers or ICDs. These studies, though, limited peak SAR.... Methods One-hundred and three patients with a total of 240 leads underwent a total of 127 scans of any body landmark using and results usual protocols with standard peak SAR settings for the scan. No patient was pacemaker dependent. Thresholds were obtained immediately before and after the scan. For all scans, the median (25th and 75th percentiles) peak SAR was 2.5 (1.3, 3.2) W/kg whereas the median scan time was 1650 (1236, 2099) s. Pre- and post-scan pacing thresholds were unchanged [0.7 (0.5, 0.8) vs. 0.6 (0.5, 0.8) V at 0.5 ms, P ¼ NS], though the sensed amplitudes [6.7 (2.9, 11.5) vs. 6.1 (2.9, 11.2) mv, P, ] and pacing impedances [500 (440, 609) vs. 491 (437, 593) V, P, ] both decreased significantly.... Conclusion The current series suggests that MRI scans may be performed safely in appropriately selected patients up to a peak SAR of 3.2 W/kg. Furthermore, peak SAR level poorly predicts the safety profile of patients with pacemakers or ICDS who are exposed to an MRI environment Keywords Pacemaker Specific absorption rate Magnetic resonance imaging Introduction Magnetic resonance imaging (MRI) is a diagnostic technique that has become the imaging modality of choice for many neurological and musculoskeletal disorders. MRI makes use of both static and gradient magnetic fields as well as pulsed radiofrequency fields. Pacemakers have been relatively contraindicated in MRI scans because of potential adverse effects including tissue heating, failure of capture, runaway pacemaker function, unpredictable reed switch behaviour, asynchronous pacing, or damage to pacemaker circuitry. 1 To date, no known deaths have been reported of patients with pacemakers or implantable cardioverter-defibrillators (ICD) undergoing an MRI scan under the supervision of a physician while on telemetry. Irnich et al., 2 though, did raise the possibility of six deaths of patients with pacemakers while undergoing MRI scans, but the decision criteria by which the coroner made the diagnosis were not listed. Several large, prospective series have been published demonstrating the relative safety of scanning patients with pacemakers in a controlled environment. 3 5 The peak specific absorption rate (SAR) in these studies, though, was 2.0 W/kg in nearly all of the scans. In order to limit peak SAR, techniques including increasing repetition time, adjusting flip angles, and changing matrix size are employed but may result in less than optimal studies. The purpose of the current study is to evaluate the safety profile of patients with pacemakers or ICDs undergoing a medically necessary MRI scan without limitation on peak SAR. * Corresponding author. Tel: ; fax: , mike@mollerus.org Published on behalf of the European Society of Cardiology. All rights reserved. & The Author For permissions please journals.permissions@oxfordjournals.org.

2 948 M. Mollerus et al. Methods Study subjects Eligible subjects consisted of patients with implanted permanent pacemakers or ICDs referred by their health-care providers for medically necessary MRI scans. Devices must have been in place for at least 6 weeks at the time of the scan and have a battery status that was beginning of life (BOL). Patients who had a native ventricular rate of,40 bpm, had an epicardial pacing lead, had a known or suspected fractured lead, had a generator with battery status that was at elective replacement indicator (ERI) or end of life (EOL), or had a device with known increased risk to exposure to an MRI scan 6 were excluded. Study subjects were enrolled between July, 2005 and September Study protocol The study was a prospective, non-randomized, unblended cohort study of patients undergoing a medically necessary MRI scan. The study population included those patients reported earlier in two smaller studies. 7,8 Prior to the scan, each patient received counselling regarding potential risks of an MRI scan and gave written consent to participate. Underlying rhythm was confirmed by temporarily programming the device to VVI with a lower rate of 40 bpm. Baseline sensing, impedance, and pacing threshold measurements at a fixed pulse width of 0.5 ms were obtained. Sensing and impedance measurements were made using the automated features of the manufacturers programmers. The devices were programmed to either DDI or VVI with a lower rate of 40 bpm for the first 97 scans. Subsequently, devices were programmed either to ODO/OVO mode or to subthreshold pacing. 8 No changes were made to the sensing parameters. If possible, magnet mode was disabled. If an ICD was present, therapy features were disabled. Histograms and event logs were saved. Each patient underwent an MRI scan in a Siemens Symphony 1.5 T scanner using an AS39T gradient coil and NUMARIS/4 (ver. syngo MR A30 DHHS) software. No limitations were placed on peak SAR level or body landmark. Scans were performed using usual protocols with standard peak SAR settings for the scan. Turbo spin echo, FLAIR, and diffusion sequences were permitted. Continuous monitoring was performed with an Invivo 3155MVS non-invasive monitoring system including telemetry and continuous pulse oximetry with plethysmographic waveform. Blood pressure measurements were obtained every 3 min. A cardiac electrophysiologist was present for the entire scan and resuscitation equipment was available in the MRI suite. Following the scan, the device was reinterrogated for any changes to pre-scan settings. Histograms and event logs were reinterrogated. Thresholds were repeated using the same protocol as pre-scan. Prior to discharge from the MRI suite, each device was reprogrammed to its pre-scan settings. Patients were followed for at least 3 months following the scan for any adverse events including loss of capture, unexpected generator malfunction or failure, device resetting, early battery depletion, or death. Significant changes in battery status were defined as a change from BOL to ERI or EOL status immediately following the scan. A threshold change or difference was defined as the pre-scan threshold value minus the post-scan threshold value. A significant increase in pacing threshold was defined as an increase the pacing amplitude by 1 V or more at a pulse width of 0.5 ms following the scan. The study complied with the Declaration of Helsinki. All participants gave written, informed consent to participate in the trial. The study was approved by the local Institutional Review Board. Statistics Pacing, sensing, and impedance thresholds were evaluated for right atrial, right ventricular, and coronary sinus leads both individually and as an aggregate. Scans were categorized as either truncal (cervical spine, thoracic spine, lumber spine, pelvis, abdomen, or chest) or non-truncal (head and extremities), and as high SAR (peak SAR.2.0 W/kg) or low SAR (peak SAR 2.0 W/kg). If a scan included both truncal and non-truncal landmarks, that scan was recorded as a truncal scan. Analysis was performed of all leads in aggregate, as well as subdivided into lead locations of right atrium, right ventricle, and coronary sinus. Because of the small number of coronary sinus leads, thresholds from these leads were not evaluated separately during subanalysis of high- vs. low-sar scans. Statistical analysis was performed using R software, version (R Foundation for Statistical Computing, Vienna, Austria). The Shapiro Wilk test was used to assess normality. Because the data were not normally distributed, results are expressed as median (25th, 75th percentiles). Analysis was performed using the paired Wilcoxon rank sum test with continuity correction for continuous variables, and the Kruskal Wallis test for categorical data. Comparison between pre- and post-scan samples was performed using the paired data. A two-sided P-value of,0.05 was considered significant. Results One hundred and three patients with a total of 240 leads underwent a total of 127 scans. One hundred and four right atrial, 127 right ventricular, and 9 coronary sinus leads were scanned. Twenty-two ICDs and 105 pacemakers were exposed to the MRI scanner. All patients had a documented, native ventricular rhythm.40 bpm. For all scans, the median peak SAR was 2.5 (1.3, 3.2) W/kg, whereas the median scan time was 1650 (1236, 2099) s. Sixty-two scans included at least one truncal landmark. The median peak SAR was significantly higher for truncal than nontruncal scans [3.2 (2.9, 3.3) vs. 1.3 (1.1, 1.7) W/kg, P, ]. The median scan time was similar between truncal and non-truncal scans [1686 (1268, 2248) vs (1236, 2032) s, P ¼ NS]. One patient experienced the onset of atrial fibrillation during a scan. 8 One pacemaker had a device reset which required reprogramming. One ICD had its arrhythmia log erased during a scan. No significant changes in battery status were seen immediately following a scan. No significant study-related events were seen at the 3-month follow-up. When evaluated as an aggregate, pacing thresholds were unchanged pre- from post-scan, though sensed amplitudes and pacing impedances both decreased significantly (see Table 1). When analysed by lead location, the right atrial leads showed only a decrease in pacing impedances, whereas the right ventricular leads showed a decrease in both sensed amplitudes and pacing impedances. There were no differences in coronary sinus lead thresholds, though the numbers may have been too small to detect a difference. The largest increase in pacing thresholds immediately following a scan was 0.5 V at a pulse width of 0.5 ms of an atrial lead in a patient undergoing a brain MRI. The peak SAR for that scan was 1.0 W/kg. Sixty-nine scans of 60 right atrial, 69 right ventricular, and 7 coronary sinus leads were high-sar scans whereas 58 scans of 44 right atrial, 58 right ventricular, and 2 coronary sinus leads

3 MRI and pacemakers without SAR restriction 949 Table 1 Thresholds pre- and post-scan Pre-scan Post-scan P-value All leads (240) Pacing (V) 0.7 (0.5, 0.8) 0.6 (0.5, 0.8) NS Sensing (mv) 6.7 (2.9, 11.5) 6.1 (2.9, 11.2), Impedance (V) 500 (440, 609) 491 (437, 593), RA Leads (104) Sensing (mv) 2.7 (1.8, 3.6) 2.6 (1.8, 3.5) NS Impedance (V) 479 (430, 540) 466 (427, 542), RV leads (127) Sensing (mv) 11.2 (8.0, 15.7) 10.8 (7.8, 14.9), Impedance (V) 530 (462, 620) 522 (440, 610), CS leads 9 Pacing (V) 1.0 (0.8, 2.0) 1.0 (1.0, 2.0) NS Sensing (mv) 10.5 (9.5, 17.7) 10.2 (9.5, 14.9) NS Impedance (V) 688 (496, 989) 676 (483, 966) NS NS, not significant. Comparison of pacing, sensing, and impedance thresholds preand post-scan. Pacing thresholds were obtained at a fixed pulse width of 0.5 ms. Data are expressed as median (25th, 75th percentiles). Lead totals are placed in parentheses. were low-sar scans. The median peak SAR [3.2 (2.9, 3.2) vs. 1.3 (1.0, 1.5) W/kg, P, ] was larger and the median scan time [1678 (1346, 2329) vs (1187, 1947) s, P ¼ 0.008] was longer for the high-sar group than the low-sar group. The pacing thresholds of all leads evaluated as an aggregate were unchanged between pre- and post-scans for both high- and low-sar groups, whereas sensed amplitudes and pacing impedances decreased significantly (see Table 2). During low-sar scans, the right atrial leads showed only a decrease in pacing impedances whereas the right ventricular leads showed a decrease in pacing impedances but a slight increase in median sensed amplitude. Though the median RV sensed amplitude was smaller pre-scan, there was no difference in the median change in sensed amplitude from pre- to post-scan (see Table 3). During high-sar scans, right atrial leads showed only a decrease in pacing impedances whereas right ventricular leads showed a decrease in both sensed amplitudes and pacing impedances. When high-sar threshold changes were compared with low-sar threshold changes, no differences between SAR level groups were seen in pacing, sensing, or impedances changes when the leads were analysed by lead location. When the leads were analysed as an aggregate, the changes in pacing thresholds were greater in the low-sar group than in the high-sar group (see Table 3). Discussion Several series have been published demonstrating the relative safety of MRI scanning of patients with pacemakers and ICDs in a controlled environment. In the first large series published, Martin et al. 3 reported on 54 patients undergoing 62 scans using Table 2 Pre- and post-scan thresholds by SAR level Pre-scan Post-scan P-value Low SAR RA leads (44) Sensing (mv) 2.5 (1.7, 3.3) 2.4 (1.6, 3.1) NS Impedance (V) 457 (415, 532) 443 (415, 521) RV leads (58) Pacing (V) 0.7 (0.5, 0.9) 0.7 (0.5, 1.0) NS Sensing (mv) 9.7 (7.7, 14.1) 9.9 (7.4, 14.1) Impedance (V) 539 (440, 659) 530 (440, 610), All leads (104) Sensing (mv) 6.2 (2.7, 11.2) 5.9 (2.8, 10.4) Impedance (V) 500 (430, 603) 489 (430, 590), High SAR RA leads (60) Pacing (V) 0.6 (0.5, 0.9) 0.6 (0.5, 0.8) NS Sensing (mv) 2.8 (1.9, 4.4) 2.9 (2.0, 4.0) NS Impedance (V) 490 (438, 563) 475 (430, 560), RV leads (69) Sensing (mv) 11.2 (8.6, 15.7) 11.2 (8.1, 15.0) Impedance (V) 523 (470, 612) 520 (460, 607), All leads (136) Sensing (mv) 6.7 (3.2, 11.9) 6.4 (3.1, 11.2) Impedance (V) 503 (449, 610) 500 (440, 602), NS, not significant. Comparison of pacing, sensing, and impedance thresholds preand post-scan by peak SAR level. Pacing thresholds were obtained at a fixed pulse width of 0.5 ms. Data are expressed as median (25th, 75th percentiles). Lead totals are placed in parentheses. a GE Signa CV/i 1.5 T scanner. In their series, 9.4% of patients had significant pacing threshold changes, though only 1.9% required device reprogramming. The peak SAR attained during any scan was 2.0 W/kg. Nazarian et al. 4 evaluated 55 patients undergoing 68 scans in a GE Signa CV/i 1.5 T scanner. No significant adverse events were seen in their series, though peak SAR was kept to,2.0 W/kg during nearly all the scans. Sommer et al. 5 reported a series of 82 patients with Medtronic devices undergoing 115 scans in a Philips Intera 1.5 T scanner. In their series, significant increases in pacing thresholds were seen in 3.1% of leads. Peak SAR was limited to 1.5 W/kg and chest scans were avoided. To our knowledge, the current series is the first study to date with a large number of devices exposed to a high-sar environment.2.0 W/kg. In this series, no significant decreases in pacing thresholds were seen either in a high- or low-sar environment. With the exception of coronary sinus leads, a decrease in impedances was seen for right atrial and right ventricular leads both in a high- or low-sar environment. The small coronary sinus lead sample size was most likely underpowered to detect any differences. A decrease in sensed amplitude was inconsistently

4 950 M. Mollerus et al. Table 3 Comparison of high- and low-sar threshold changes High SAR Low SAR P-value RA leads D Pacing (V) 0.00 (0.00, 0.03) 0.00 (20.1, 0.00) NS D Sensing (mv) 0.0 (20.3, 0.2) 0.0 (0.0, 0.2) NS D Impedance (V) 9 (0, 12) 3 (0, 10) NS RV leads D Pacing (V) 0.00 (0.00, 0.00) 0.00 (0.00, 0.00) NS D Sensing (mv) 0.2 (0.0, 0.8) 0.0 (0.0, 0.6) NS D Impedance (V) 10 (0, 20) 10 (0, 20) NS All leads D Pacing (V) 0.00 (0.00, 0.01) 0.0 (20.04, 0.00) D Sensing (mv) 0.00 (20.1, 0.6) 0.0 (0.0, 0.4) NS D Impedance (V) 10 (0, 19) 7 (0, 17) NS NS, not significant. Comparison of high- and low-sar threshold changes. A positive value signifies that the pre-scan value was greater than the post-scan value. Pacing thresholds were obtained at a fixed pulse width of 0.5 ms. Data are expressed as median (25th, 75th percentiles). seen for right atrial and right ventricular leads in high- or low-sar environments. When the leads, though, were evaluated as an aggregate, a significant decrease in sensed amplitude was seen in both high- and low-sar environments. The larger sample size of the aggregate data most likely increased the power to detect a significant decrease in sensed amplitude. The only difference seen in threshold changes between high- and low-sar group was the pacing threshold change when leads were evaluated as an aggregate, where the pacing threshold change was greater in the low-sar group than in the high-sar group. Radiofrequency fields during an MRI scan can lead to resistive heating at the lead tip-tissue interface. 9 Phantom gel and animal models have detected local tissue heating from 5.7 to 23.58C during exposure of a pacing system to the MRI environment. 6,10 12 Luechinger et al. 9 evaluated local tissue heating in a swine model with peak SAR set to 3.8 W/kg, while Roguin et al. 6, evaluated the effect of worse-case scenario MRI protocols on thresholds in a canine model. Neither study found evidence of tissue necrosis at post-scan necropsy, though Roguin et al. reported on a failure to capture for 12 h following the scan in one animal. The decrease in both lead impedance and sensed amplitudes seen in the current series suggests the possibility of some local lead tip-tissue interface changes such as local tissue heating or oedema or of changes within the device circuitry. These changes occurred irrespective of SAR levels and may have no clinical significance. The current scientific statement from the American Heart Association does not comment on SAR limitations, 13 though the European Society of Cardiology recommends a SAR limit of 2 W/kg. 14 The current series would suggest that a 2 W/kg SAR limit would be unduly restrictive, and that the safety profile of an MRI scan is poorly predicted by peak SAR level. The MRI environment is dynamic with multiple interrelated factors that may influence risk in the MRI scanner including not only peak SAR level and body landmark, but also MRI manufacturer, generator and lead system manufacturer and model, 6 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, 15,16 and lead system integrity. The present study represents a real-world situation where patients may be exposed to uncontrolled MRI environments and where attempts to limit SAR levels may lead to suboptimal scans. Adjusting flip angles may adversely affect the signal-to-noise ratio. Reducing matrix size may decrease scan resolution. Increasing repetition time may prolong scan time and so increase the possibility of scan artifact from patient motion. This series suggests that MRI scans can be performed safely in patients up to a peak SAR of 3.2 W/kg and may include truncal landmarks. Nevertheless, consistent decreases in lead impedances indicative of possible local lead tip-tissue changes, as well as the results of Luechinger et al. and Roguin et al. suggest that a level of caution and vigilance must be maintained when submitting a patient with a pacemaker or ICD to an MRI scan irrespective of peak SAR level, and that preparations must be in place to address potential device failure during or following a scan. Conclusions The current series suggests that MRI scans may be performed safely in appropriately selected patients up to a peak SAR of 3.2 W/kg. Furthermore, peak SAR level poorly predicts the safety profile of patients with pacemakers or ICDs who are exposed to an MRI environment. Limitations Because of the sample size, the current study may not have enough power to detect uncommon complications related to MRI scanning. This study is underpowered to determine the safety profile of coronary sinus leads. Measurements reported by the MRI console may be imprecise and may vary between manufacturers and models. 11,17 The current study cannot be extrapolated to 3 T MRI scanners or to other models of 1.5 T MRI scanners. Because of limited battery data acquired during the study, no conclusions about the affect of MRI scanning on battery performance or longevity can be drawn. The absence of clinically significant pacing threshold changes does not predict long-term device function or battery longevity. Furthermore, because pre- and post-scan defibrillation threshold testing was not performed, no conclusions about defibrillation thresholds can be made. The current study cannot be used to risk stratify recently implanted systems or to generators at ERI or EOL. Caution should be observed in applying this study to pacemaker-dependent patients. Conflict of interest: M.M. received research grants from Boston Scientific and Medtronic. Funding This study was funded by the Duluth Clinic Foundation and by the Department of Cardiology, St. Mary s Duluth Clinic.

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