Magnetic resonance imaging safety in nonconditional pacemaker and defibrillator recipients: A meta-analysis and systematic review.

BACKGROUND: Recommendations regarding performance of magnetic resonance imaging (MRI) in non-MRI conditional pacemaker and defibrillator recipients are evolving. Previous studies have suggested low adverse event rates with MRI in nonconditional cardiac implantable electronic device (CIED) recipients, but low power limits optimal characterization of risk. OBJECTIVE: The purpose of this study was to perform a systematic review and meta-analysis to characterize the clinical risk associated with MRI in CIED recipients in order to improve power. METHODS: PubMed and CINAHL indexed articles from 1990 to 2017 were queried. A random effects model was used for meta-analysis of continuous variables. Safety outcomes were evaluated with descriptive statistics. RESULTS: Seventy studies of non-MRI conditional devices undergoing MRI were identified, allowing for analysis of 5099 patients who underwent a total of 5908 MRI studies. Heterogeneity in lead parameter changes was observed within studies, although smaller variances were noted between studies. All lead characteristics and battery voltages showed very small, clinically insignificant changes when assessed as a pooled cohort, although cases of clinically relevant outcomes were also noted (lead failure 3, implantable cardioverter-defibrillator shock 1, electrical reset 94). Electrical resets were found only in older devices. Defibrillator function was unchanged, and inappropriate shocks were avoided with pre-MRI programming changes. CONCLUSION: This review demonstrated low lead failure and clinical event rates in non-MRI conditional pacemaker and defibrillator recipients undergoing MRI. Observed changes were small and interstudy variance was low, suggesting that the composite event rates offer a reasonable estimate of true effect. The observed adverse events reinforce the need for ongoing vigilance and caution, particularly with older devices.

Radiation tolerance of contemporary implantable cardioverter-defibrillators.

BACKGROUND: Implantable cardioverter-defibrillators (ICDs) are complex instruments using integrated circuit technology. Previous studies suggested risk to the device when exposed to a radiation environment. Little data is available on contemporary ICD systems. OBJECTIVES: The purpose of the present study was to assess the ability of contemporary ICD designs to resist the damaging effects of direct exposure to therapeutic doses of radiation. METHODS: Four contemporary ICDs and four legacy ICDs devices were exposed to escalating doses of photon irradiation (XRT) from a 6-MV linear accelerator. Escalating doses were administered over 8 days to a maximum cumulative dose of 131.11 Gy or catastrophic failure. RESULTS: Each legacy device had catastrophic failure following the 6th XRT session, characterized by failure to deliver shock therapy. All four contemporary devices remained fully functional following the 8th and final XRT session (P = 0.03). The cumulative, survived radiation dose was significantly different between the contemporary and legacy groups (131.11 vs. 41.11 Gy, P = 0.01). Changes seen in the legacy devices were sudden and not anticipated by trends in prior sessions. CONCLUSION: The results of this study suggest that contemporary ICD designs may be more robust than earlier designs in a radiation environment.

Spectral methods to distinguish ventricular fibrillation from artefact in implantable cardioverter-defibrillators.

BACKGROUND: Despite the proven benefit of implantable cardioverter-defibrillators (ICDs), inappropriate shocks remain a significant problem. Recent trends have shown an increased incidence of lead failure and an increased exposure of devices to extreme electromagnetic interference environments. AIMS: The goal of the current study is to evaluate the spectral characteristics of ventricular fibrillation (VF) detected in an ICD at time of defibrillation threshold testing and use of the findings to predict event types from a population of clinical VF and artefact events. METHODS AND RESULTS: A modelling group was created from induced VF and artefact events at time of ICD implantation and DFT testing. Power spectral density evaluation was performed on each event and used to calculate an energy ratio (ER; the ratio of energy under the first three harmonics to the entire spectrum). The model was then applied to a database of clinical VF and artefact events to determine its sensitivity and specificity. The far-field ER of the modelling group was significantly larger for VF (0.888 ± 0.110) than artefact (0.265 ± 0.156, P < 0.0001). In the test group, the far-field ER of VF (0.882 ± 0.088) was also significantly larger than artefact (0.344 ± 0.128, P < 0.0001). At a cut-off of >0.526, the far-field ER had a sensitivity of 100% [confidence interval (CI) 100-100%] and a specificity of 92.4% (CI 84.9-98.5%) to distinguish clinical VF from clinical artefact. CONCLUSION: Far-field signal during VF detected by an ICD has a distinct spectral pattern that can distinguish VF from artefact.

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

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 AND RESULTS: One-hundred and three patients with a total of 240 leads underwent a total of 127 scans of any body landmark using 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=NS], though the sensed amplitudes [6.7 (2.9, 11.5) vs. 6.1 (2.9, 11.2) mV, P<0.0001] and pacing impedances [500 (440, 609) vs. 491 (437, 593) Omega, P<0.0001] 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.

Ectopy in patients with permanent pacemakers and implantable cardioverter-defibrillators undergoing an MRI scan.

BACKGROUND: Recent series suggest that magnetic resonance imaging (MRI) scanning can be performed safely in select patients with pacemakers or implantable cardioverter-defibrillators (ICDs). Limited data have been reported on ectopy during MRI scans in patients with pacemakers or ICDs. This study evaluated increased ectopy observed in patients with permanent pacemakers or ICDs undergoing MRI scanning of any landmark without peak specific absorption rate (SAR) limit. METHODS: Fifty-two patients with a total of 119 leads underwent a total of 59 MRI scans of any landmark using usual protocols with standard peak SAR settings for the scan. No patient was pacemaker dependent. All devices were programmed to single-chamber demand mode (VVI) or dual-chamber demand mode (DDI) with a lower rate of 40 bpm.Both telemetry and pulse oximetry plethysmographic waveform were observed continuously throughout the scans for ectopy. RESULTS: Increased ectopy was observed during seven scans. The ectopy in four scans was ventricular and had fixed coupling intervals of 1,500 and 3,000, and was likely due to device noise rejection behavior. The etiology of ectopy observed during the other three scans could not be determined. Ectopy could not be predicted by peak SAR, scan time duration, or landmark. No significant changes in pacing thresholds were seen postscan. CONCLUSIONS: The current series suggests that a minority of patients with implanted pacemakers may have MRI-related ectopy. A significant proportion of this ectopy may arise from normal device behavior within the MRI environment.

Cardiac biomarkers in patients with permanent pacemakers and implantable cardioverter-defibrillators undergoing an MRI scan.

BACKGROUND: Recent series suggest that magnetic resonance imaging (MRI) scanning can be performed safely in select patients with pacemakers or implantable cardioverter-defibrillators (ICDs). Limited data have been reported on cardiac biomarker release following MRI scans in patients with pacemakers. The current study evaluated cardiac biomarkers pre- and postscan in patients with permanent pacemakers or ICDs undergoing MRI scanning of any body region without peak specific absorption rate (SAR) limit. METHODS: Thirty-seven patients with a total of 75 leads underwent a total of 40 MRI scans of both truncal and nontruncal regions using usual protocols with standard peak SAR settings for the scan. No patient was pacemaker dependent. Pacemaker magnet mode and ICD therapy were disabled during the scan. Baseline cardiac troponin-I and myoglobin levels were obtained immediate pre- and 6-12 hours postscan. Pacemaker capture thresholds were measured immediately pre- and postscan. RESULTS: The median peak SAR was 2.4 (1.3, 3.2) W/kg for all scans. Cardiac troponin-I was unchanged following an MRI scan (0.01 (0.01, 0.02) versus 0.01 (0.01, 0.02) ng/mL, P = 0.90). Capture thresholds were no different pre- and postscan (0.67 (0.50, 0.80) versus 0.70 (0.50, 0.79) V at 0.5 ms, P = 0.50). CONCLUSIONS: The current series suggests that an MRI scan may be performed safely in carefully selected patients with close monitoring during the scan without limitation on peak SAR level or body landmark. Furthermore, it is unlikely that an MRI scan will produce sufficient tissue heating to cause enough myocardial cell necrosis to result in cardiac biomarker release.

A randomized comparison of defibrillation thresholds in the right ventricular outflow tract versus right ventricular apex.

OBJECTIVE: To determine whether the placement of an implantable cardioverter-defibrillator (ICD) lead in the right ventricular outflow tract (RVOT) has the same defibrillation threshold (DFT) as the right ventricular apex (RVA). BACKGROUND: Right ventricular ICD leads have usually been placed in the RVA. Development of active fixation technology has allowed the placement of these leads in alternate locations such as the RVOT. METHODS: At time of device implantation, 26 patients with either ischemic or dilated cardiomyopathy underwent DFT testing in both the RVA and RVOT using a binary search algorithm. RESULTS: Placement of the lead in the RVA had a mean DFT of 7.6 +/- 2.8 J while the placement of the lead in the RVOT had a mean DFT of 10.3 +/- 3.0 J. The median (25th and 75th percentiles) DFTs in the RVA and RVOT were 7.5 J (6 and 11 J) and 11 J (9 and 14 J), respectively (p = 0.0002). CONCLUSIONS: Placement of the right ventricular lead in the RVA has a significantly lower DFT than placement of the lead in the RVOT.

Myocardial stunning following defibrillation threshold testing.

Implantable cardioverter-defibrillators (ICDs) have a proven mortality benefit in appropriately selected people. Several retrospective studies, though, have postulated that appropriate ICD therapy may lead to fatal, pulseless electrical alternans (PEA). This case report describes an episode of transient PEA from myocardial stunning and standstill following defibrillation threshold testing. The risk of post-shock myocardial stunning and standstill should be kept in mind as a potential complication of testing. With rapid recognition and prompt intervention, this complication is potentially reversible.