Pacemaker may skip a beat during routine CT scan

A trip through the MR scanner canwreak havoc with implantable cardiacdevices such as pacemakers and defibrillators.But this kind of interferencemay not be limited to MR imaging,according to Cynthia McCollough,Ph.D.

Her research found that x-rays canalso interact with these devices,McCollough said at the 2008 StanfordInternational Symposium onMultidetector-Row CT."We know that semiconductortypedevices are sensitive to radiationtherapy, but historically no one hasbelieved that there's been any effect atdiagnostic levels," said McCollough,director of the CT Clinical InnovationCenter at the Mayo Clinic in Rochester,MN.

But a report from Japan correlatedinterference with a pacemaker at theexact time that the patient was havinga CT scan (Circ J 2006:7[3]:190-197). The pacemaker was reset to itsdefault setting at the time of CT, shesaid.

"While (the interference) was atransient effect, the possibility of arrythymiacannot be ignored," saidMcCollough, who is also an associateprofessor of radiologic physics at theMayo Clinic College of Medicine."That is attention-catching. So I beganlooking into this."

Teaming up with implantabledevice maker Medtronic of Minneapolis,McCollough conducted a seobstructions.ries of experiments that prospectivelymeasured the response of 21implantable cardiac rhythm managementdevices to the radiation deliveredby CT for both maximum andtypical dose levels (Radiology 2007:243[3]:766-774). They found oversensingoccurred most often.

"Oversensing means (the device) issensing an event that didn't comefrom the heart," she said. "The currentfrom the ionizing radiation foolsthe pacemaker into thinking that theheart had activity."

McCollough's group observedoversensing in 20 of 21 devices atmaximum doses and in 17 of 20devices at typical doses. Oversensingmost often manifested as inhibition,although it occasionally manifested astracking or safety pacing.

They also found that two devicesinhibited for more than four secondsin spiral mode at clinical dose levels.Oversensing was transient and ceasedas soon as the device stopped movingthrough the x-ray beam or the beamwas turned off, according to resultsfrom McCollough's group.

"What mechanism would do this?"McCollough said. "Basically, you areinducing current across the p-n junctionof the semiconductor. This is aninterference effect much like [the way]a microwave oven interferes with apacemaker."

She said that permanent damage tothe device is not likely at the standardradiation doses given for common CTexams, such as coronary CT angiography,chest CT, or pulmonary embolismstudies.

She added that 63 million CTscans were performed in the U.S. lastyear, but there were no reports of anyserious events involving implantablecardiac devices and CT scanners.

McCollough cautioned, however,that radiologists should be aware thatsome interference occurs, especiallyduring CT studies-dynamic perfusion,and interventional-when thedetector may dwell over the device forlonger than four seconds.

"This [effect] definitely needs morein vitro and in vivo testing to see ifthere is something further that weshould be doing for these patients,"she said.

-By Shalmali Pal