Resonancia magnética nuclear y marcapasos

Posted at — cardiocirugia — diciembre 20th, 2009 — 14:18 under noticias,tecnología

Silver Spring, MD – MRI scans may interfere with the electrical stimulation from pacemakers with abandoned leads or inadequately sealed lead connections, an in vitro study by FDA engineers suggests.

While the findings will not change current clinical practice, they may ultimately lead to better methods for manufacturers to develop and test devices.

Study authors—FDA engineers Howard Bassen and Gonzalo Mendoza—note that MRI is generally contraindicated for patients with pacemakers because of the potential of the magnetic field to cause localized heating of leads and/or nerve stimulation at the lead fixation point. However, as reported by heartwire, Medtronic’s EnRhythm MRI SureScan pacemaker system, designed to be MR-compatible, has performed safely in clinical trials, and recently published research led by Dr Claas Philip Naehle (University of Bonn, Germany) shows that ICDs can be programmed to avoid conflicts with MRI in patients who are not pacemaker-dependent.

“However, there is little information published on the magnitude and spatial distribution of the [electrical] field surrounding the tips of electrodes of medical implants such as cardiac and neural stimulation leads in bodies of patients exposed to MRI gradient fields,” Bassen and Mendoza point out.

For their study, they assessed the risk of MRI scans causing unintended pacemaker stimulations by measuring the electric fields induced near lead tips by a simulated MRI gradient system in a saline tank. They used two versions of a two-electrode probe (0.5 mm and 0.2 mm in width) made with solid-conductor copper wires to map magnetically induced electrical fields within 0.1 mm resolution as close as 1 mm from the lead tips. The study found that the fields created by MR may exceed the fields produced by a pulse generator such as a pacemaker and therefore may interfere with the pulse delivered by the pacemaker to pace the heart.

The results of their study are published December 15, 2009 in Biomedical Engineering Online.

The researchers found that MRI will induce an electrical field at the distal tip of a unipolar implantable pacemaker lead usually only during or immediately following a pulse from the pacemaker but can significantly alter (either increase or decrease) the waveform of the pacemaker pulse. An MRI system operating under normal conditions with a 30- to 100-T/s gradient field rate of change can cause significant changes in pacemaker stimulation voltages when the MRI’s gradient pulse is delivered within about 30 milliseconds of the pacemaker pulse.

Different types of pacemaker leads produced different magnetically induced electrical fields in the study. Active-fixation leads and tined-tip leads produced similar strength fields, while the field from a lead made of insulated wire with a blunt cut tip was much weaker. The cause of this discrepancy is not known, the authors write, but it could be due to electrical properties of the wire, a smaller surface area, or some other factor.

Although the authors measured only the electrical fields near pacemaker leads, their noninvasive methods could be applied to leads of other stimulation devices, including neural stimulators, they explain. The magnitude of the magnetically induced electrical field depends on the impedance of the device’s electronics, and the duration and timing of the gradient pulse from the MRI system determines the probability that the heart will receive an unintended stimulation. Some MRI sequences, such as echo planar imaging, could produce continuous magnetic gradient pulses with a very high duty cycle, so the chances of the magnetic pulse coinciding with the pacemaker pulse are high. This could interfere with several pacemaker stimuli in a row and effectively cancel the therapy.

MRI-pacemaker conflict can be avoided by simply turning off the pacemaker during the MRI procedure, but this will not be an option for pacemaker-dependent patients, the authors explain.

The method matters more than the results

Commenting on the FDA researcher’s study, Roger Lüchinger (Swiss Federal Institute of Technology, Zurich), who was one of the paper’s original reviewers, told heartwire, “The important part of the paper is, in my opinion, not the results itself, but the proposed measurement setup. This setup allows comparison of the induced voltages directly with the voltages of the stimulation of the pacemaker. Now the research can start, and manufacturers have a nice test setup.

“The results are less important, because they are for special leads and . . . an artificial magnetic field was used and not one of a scanner, which may be larger,” he says. He points out that the only time the MRI system could interfere with the pacemaker is during the short periods during cardiac stimulation and during charging of the pacemaker, because those are the only times the device’s impedance is low.

“Overall, in my opinion, the risk [of MR interfering with pacing] is still very limited, but it cannot be excluded. Perhaps with this new method somebody could more carefully evaluate the risk [with each device],” he said. “New devices may need a redesign of the input circuit to reduce the risk of induced voltages and stimulation. And this has to be tested, for example, with the new method.” However, he said, the new findings will not change the guidelines for MRI with pacemakers, which call for ECG and/or arterial blood monitoring during MRI.

Naehle agrees that the findings will have little direct impact on clinical practice but open up new avenues for further research. “While numerous pacemaker and ICD publications have demonstrated that asynchronous stimulation can induce malignant ventricular arrhythmia, so far this has not been demonstrated to have happened as a result of gradient-field-induced voltages in the lead during MR imaging.”

The risks of MRI causing asynchronous stimulation leading to a ventricular arrhythmia or inhibition of pacemaker output are already fully addressed in all major guidelines, Naehle pointed out. “Hence, to fully evaluate and understand the risks associated with gradient-induced voltages in pacemaker/ICD leads, further studies are again needed, evaluating more than two pacemakers and more than two leads using a ‘real’ MRI scanner,” he concluded.

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