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Cardiac implantable devices that aren't specifically approved for use with MRI may still be safe for it under certain circumstances, according to a prospective registry.

"In this study, in any patient with a non–MRI conditional pacemaker or ICD who underwent clinically indicated non-thoracic MRI at 1.5 Tesla, was appropriately screened, and had the device reprogrammed in accordance with the prespecified protocol," , of the Scripps Research Institute in La Jolla, Calif., and colleagues reported in the Feb. 23 issue of the New England Journal of Medicine.

While MRI has long been recommended against for patients with an implanted cardiac device, the researchers noted that at least half of the estimated 2 million Americans and additional 6 million around the world have devices that don't meet the criteria considered safe for MRI can be expected to need this imaging during their lifetime.

The MagnaSafe registry, undertaken in consultation with the FDA, included 1,000 pacemaker cases and 500 implantable cardioverter-defibrillator (ICD) cases – all "non–MRI-conditional" -- referred for clinically indicated nonthoracic MRI at 19 U.S. centers.

The standardized protocol before MRI for asymptomatic cases with a heart rate of at least 40 bpm was to program the device to a no-pacing mode. For symptomatic patients or those with a slower intrinsic heart rate considered pacing-dependent, the cardiac device was reprogrammed to an asynchronous pacing mode. For ICD patients, those dependent on the device for pacing were excluded and the others had all bradycardia and tachycardia therapies inactivated. Settings were restored after MRI and reprogrammed again after the full device interrogation as necessary.

During the MRI, four patients had generator-site discomfort, of whom one with an ICD was taken out of the scanner due to a sensation of heating.

No deaths, lead failures, losses of capture, or ventricular arrhythmias occurred as a result of MRI in these patients. In the one case in which an ICD generator could not be interrogated after MRI and had to be replaced, the pre-MRI programming protocol had not been followed.

There were six cases each of self-terminating atrial fibrillation or flutter (lasting less than 49 hours) and of partial electrical reset – all had older pacemakers implanted 5.7 to 9.7 years prior. There were also a "small number" of cases exceeding prespecified thresholds for lead impedance, pacing threshold, battery voltage, or P-wave and R-wave amplitude. The most common was a 3-ohm change in ICD shock lead impedance (16.4%).

The risks were not clinically significantly correlated with time since lead placement.

The researchers concluded that "device removal and replacement seem unlikely to be safer than proceeding with scanning for patients with a pacemaker or an ICD who require a nonthoracic MRI, provided a protocol similar to the one used in our study is followed."

They cautioned, though, that there were limitations to their data, including the heterogeneous range of generators and leads from multiple manufacturers and initial as well as repeat examinations at 1.5 Tesla in the study. Also, exclusion of children, thoracic MRI, and pacing-dependent ICD patients as well as the low number of left ventricular leads may limit extrapolation.

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