PhD, St Paul’s Hospital, Vancouver, Canada
Modern pacemakers and implantable cardioverter-defibrillators (ICDs) have been designed to reduce the potential risks associated with magnetic resonance imaging (MRI). These devices are labelled MRI-conditional by the Food and Drug Administration (FDA) centre for Devices and Radiological Health. The potential risks posed by scanning non-MRI-conditional devices include myocardial thermal injury and damage to pacing properties. Many millions of individuals worldwide have non-MRI-conditional devices. The FDA has not approved these devices for MRI scanning, and yet approximately 50% of patients with a non-MRI-conditional device will develop a clinical indication for MRI during their lifetimes (majority brain or spine).
A recent report from the MagnaSafe Registry published in the NEJM examined device related events among patients with non-MRI-conditional devices undergoing non-thoracic MRI at a magnetic field strength of 1.5 Tesla.(1) Patients with pacemakers (1000 cases) and ICDs (500 cases) were referred for clinically indicated MRI using a standardised protocol. Devices were reprogrammed before and after the scan. No deaths, lead failures, losses of capture or ventricular arrhythmias occurred during MRI. There were six cases of self-terminating AF/atrial flutter and six cases of partial electrical reset. The importance of pre-programming according to the protocol was emphasised by one case where the ICD generator was not adequately pre-programmed (antitachycardia therapy left in the active mode) and required immediate replacement when it could not be interrogated after the scan. Small changes in device settings were common but few were considered clinically important.
A previous prospective non randomised trial by Nazarian et al. reported similar results in patients with pacemakers and ICDs undergoing 555 MRI studies.(2) As part of their protocol pacing mode was changed to asynchronous for pacing dependent patients and to demand for others. Tachyarrhythmia functions were disabled. The authors recommended that devices should have been in place for at least six weeks, no epicardial or non-functioning leads should be present and patient monitoring and follow up should be carefully planned.
Providing the device is pre-programmed according to protocol, it appears safe for selected patients with non-MRI-conditional pacemakers and ICDs to undergo non-thoracic MRI at 1.5 Tesla. However, there are caveats. The MagnaSafe Registry excluded thoracic MRI, devices implanted prior to 2001, patients under 18 years and pacing dependent patients with ICDs. There were a small number of LV leads included. Therefore, it may not be possible to extrapolate these results to paediatric and CRT populations, patients with older devices and those undergoing thoracic MRI. Risk benefit assessments for individual patients with non-FDA approved devices should be performed. These studies have not demonstrated that MRI in these patients is risk free, but the risks appear small. It is important that any planned MRI scans are clinically indicated and that equivalent information cannot be obtained more safely by using other methods.
References
- Russo et al. Assessing the risks associated with MRI in patients with a pacemaker or defibrillator. NEJM 2017;376:755-764.
- Nazarian et al. A prospective evaluation of a protocol for magnetic resonance imaging of patients with implanted cardiac devices. Ann Intern Med 2011;155(7):415-24.
The MagnaSafe Registry represents an important step forward in our understanding of the effects of performing non-thoracic MRI scans in patients with non-MR conditional cardiac devices. It supports data from earlier (smaller) studies that suggest the risk of device-related complication is extremely low provided specific device programming measures are undertaken. In the absence of a randomised study (which will not be feasible), this represents the best quality data we have to date. The strength of the available data is not however, sufficient to assume the universal safety of scanning patients with devices implanted since 2001 at 1.5T. The clinical indication for the scan (and presence of any alternative imaging modality), needs to be carefully weighed against the small possibility of device related complication and as such, the decision to scan should be made in consultation with clear local protocols written jointly by radiologists and cardiac device specialists.
Despite the relative reassurance provided by the MagnaSafe data, the development of MR conditional cardiac devices is a welcome and necessary step forward within the field. These technologies have been prospectively evaluated within an MRI environment thus allowing more confidence in their safety. Many of the devices are also suitable for thoracic scanning and this is likely to be increasingly important given the expanding indications for cardiac MRI scanning.
In summary, non-thoracic scanning of most non-MR conditional devices at 1.5T is likely to be safe but each scan should be sanctioned by a radiologist and cardiac device specialist and where possible, de-novo implants should utilise readily available MR-conditional technology.
