Friday, June 22, 2007

The MRI-Safe Pacemaker

Magnetic resonance imaging (MRI) scanners are one of the most effective ways to follow soft-tissue and vascular diseases in the body. Unfortunately, sometimes these patients also have heart disease that requires that a permanent pacemaker or implantable defibrillator be installed for control of heart rhythm abnormalities. For these patients, follow-up of their soft tissue tumors is impaired because MRI-safe pacemakers and defibrillators are not available on the market, especially for patients who are dependent on their pacemaker for their heart to beat at all. Fortunately, some investigators (in the US and Europe) have devised special protocols for non-pacemaker dependent patients to undergo MRI scans when they are clinically required. The FDA still frowns on this practice, however.
Studies presented by Sommer et al and Nazarian et al in this issue of Circulation offer further promising evidence in this regard. As with the previous studies, however, the authors acknowledge a multitude of limitations that prevent broad applicability of the results. Furthermore, one of these studies was not entirely free from concerning outcomes with the potential for serious clinical events. We view these results as consistent with our previous message that, on a case-by-case basis, the diagnostic benefit from MRI outweighs the presumed risks for some pacemaker and ICD patients. However, the FDA remains firm in its belief that those risks have not yet been characterized and mitigated sufficiently to justify the routine use of MRI in those populations.
In February of this year, Medtronic began a trial overseas with a new MRI-safe pacemaker called Enrhythm MRI. The was trial recently announced in Canada as well. It will probably be a bit longer before we see the trial spread to the United States, although it looks like the centers have been selected that will participate.

There are several issues with MRI's that can cause problems with conventional pacemaker and defibrillator systems:
  • Usually the most common is interference with sensing the patient's underlying rhythm (oversensing, usually of noise, causes the pacemaker to inhibit its output, or undersensing causes the pacemaker or defibrillator to not be able to see the underlying rhythm and may cause pacing at inappropriate times)

  • Alteration in programming (like reverting to a power-on reset mode of pacing or actual damage to the circuitry)

  • Changes in pacing thresholds, perhaps due to lead movement (torque) or local heating.
A short review of these issues (with references) can be found from this piece from the University of Utah.

Medgadget also has a an even better overview of Medtronic's study and the technology in place to counteract these issues.

I just hope that someday MRI-safe capabilities will eventually become a standard feature in all cardiac devices marketed. After all, patients may have their pacemakers for years before the need for an MRI arises.


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