October 28, 2011 by dailybolusoflr
Pacemakers and Magnets
The ever present question about this on the In-training exam will just never go away. . .
When placing a magnet over a pacemaker, it eliminates the sensing mode of the pacemaker. Sensing detects the native electrical activity of the heart and suppresses firing of the pacemaker when activity is adequate based on the settings of the pacemaker device. Eliminating the sensing mode of the pacemaker thus causes the pacemaker to fire at its fixed backup rate. This firing of the pacemaker with no coordination with the intrinsic electrical activity of the heart puts the patient at risk for “R on T phenomenon” or the delivery of an impulse during the refractory phase of the cardiac cycle. This is a risk for the development of VF. It is a rarely seen occurrence in this setting.
If every you have a patient with a “runaway pacemaker” (rapid firing of the pacemaker) you should try to see if magnet application will reset the device to a slower pace setting.
It should also be noted that some ICDs can be de-activated with a magnet temporarily or permanently if the magnet is kept there for over 30 seconds. This can be useful in codes or when inappropriate shocks are being delivered.
Ref: Am J Emerg Med. 2007 Sep;25(7):812-22.