Pace Makers
Pacemakers and EMF
From
Question:
Can any patients with implanted cardiac pacemakers be safely scanned in any MR scanners?
Answer:
(Note: Although more accurately referred to as pulse generators, I will utilize the more generic and commonly used term, “pacemaker”, throughout this response.) Pacemakers present potential problems to MR scanning from several points:
1) Motion of the pacemaker in strong static magnetic fields (e.g., 1.0 T) 2) Modification of the function of the pacemaker, temporarily and/or permanently, by the static magnetic field of the MR imager 3) Heating induced in/of the pacemaker leads due to the time varying (RF) magnetic fields of the imaging system during the MR imaging process
4) Voltages/currents induced in the pacemaker leads and/or myocardium during the MR imaging process by the time varying RF (and gradient?) magnetic fields
There have been numerous patients (at least two dozen to date) that have been placed in MR imaging systems (either intentionally or inadvertently). There appears to be documentation that at least five of these to date have died. It should be stressed that (in my opinion) the cause of death is NOT KNOWN. Much has been written in especially the radiologic literature about the static field of the MR imager closing the reed switch of many pacemakers.
Indeed, these switches close in static fields as low as 15 gauss, with the newest pacemakers having reed switch activation occur in static magnetic fields as low as 5 to 7 gauss. See note below
Nevertheless, all this accomplishes is placing the pacemaker into asynchronous mode (and a pre-determined fixed pacing rate takes over) during the time period that the reed switch is activated. This does not explain why patients would experience cardiac distress or death in MR imagers, however. In fact, thousands of patients are placed into asynchronous mode each day in outpatient visits to cardiologist offices as their pacers are interrogated as part of their routine pacemaker maintenance program. So I do not believe that this often quoted reed switch activation by the static magnetic field of the MR scanner is to blame for patient adverse outcomes in MR scanners.
I would like to point out an interesting potential complication in this discussion. At least one study (Hayes DL, Holmes DR Jr, Gray JE. Effect of 1.5 Tesla nuclear magnetic resonance imaging scanner on implanted permanent pacemakers. Journal of the American College of Cardiology, 1987;10:782-786) demonstrated that in one pacemaker (Cordis 415A), after the cessation of RF pulsing there appeared to be a malfunction of the reed switch such that the device neither switched into asynchronous mode nor generated its programmed pulses. Thus the device appeared to have been entirely inhibited by exposure to MR imaging. Another study was performed on pacemakers from Pacesetter model 283, Intermedic model 283-01, and Medtronics model 7000A (Erlebacher JA, Cahill PT, Pannizzo F, Knowles JR. Effect of magnetic resonance imaging on DDD pacemakers. American Journal of Cardiology, 1986; 57:437-440). This study reported no apparent electrocardiographically detectable output from these pacemakers when studied in a 0.5 Tesla MR scanner (using 10 kW RF pulses at a 20.91 MHz carrier frequency). This apparent total pacemaker inhibition in one study and pacemaker malfunction in the other have serious safety implications for studying pacemaker dependent patients in MR imaging systems.
There have also been several studies where dogs as well as humans have been tachyarrhythmic and/or hypotensive during the MR imaging process. It is entirely possible that what is at play here is the induction of voltages/currents within the pacemaker/lead/myocardial loop that is sufficient to induce action potentials/contraction of myocardium and an electrical, as well as physiologic, systole. In fact, some cardiologists have maintained in peer reviewed literature that they observed cardiac pacing at the selected TR. I do not believe that this can yet be considered reliable (RF cycling rates in spin echo imaging sequences are greater than that determined by the selected TR even in single slice acquisitions, for example). Nevertheless, such tachypacing at rates that yield cardiac outputs that are not compatible with sustaining life is, in my opinion, a very likely possibility as the cause of death in some of the pacemaker patients that were scanned in MR imagers.
(Interestingly, with the RF turned on and the gradients turned off, such tachypacing was still observed, but with the RF inactivated and the gradients left on this tachypacing was no longer observed in one experiment (Hayes DL, Holmes DR Jr, Gray JE. Effect of 1.5 Tesla nuclear magnetic resonance imaging scanner on implanted permanent pacemakers. Journal of the American College of Cardiology, 1987;10:782-786).)
Heating of the pacemaker/pacing leads during MR imaging is also potentially problematic, and thermal injury to the endocardium/myocardium must be considered a potential adverse outcome if RF power is transmitted in the vicinity of the pacemaker and/or its leads.
In any case, it is fair to say that at this point and in my opinion, cardiac pacemakers should be considered an “absolute” contraindication to MR scanning until further notice. I do believe, however, that this will likely change as more is known about this issue and when more information becomes available as to when such patients may be safely imaged with MR imaging systems.
For Today:
Without an integrated and co-ordinated approach together with Cardiology and knowledgeable personnel from Radiology (and perhaps Institutional Review Board and/or Human Rights Committee) involvement and informed consent from the patient that includes possible arrhythmias, pacemaker malfunction, and death, and without exceptionally strong indications that there is indeed a powerful clinical NEED for scanning this particular patient using MR technology, today it should still be considered contraindicated to scanning any patient with a cardiac pacemaker in an MR scanner.
This said, theory suggests that it may perhaps be possible to safely scan certain patients (such as patients who are not pacemaker dependent) with (certain) pacemakers, for studies in which the body RF coil is NOT used for RF excitation, where active monitoring is being performed throughout the examination, etc. etc. etc. There is a protocol already designed that may be used to attempt to scan certain very specific patients with pacemakers in MR imaging systems. My compliments to Dr. Rod Gimbel (cardiology fellow at the Cleveland Clinic) who is spearheading this effort, and who has set up what is referred to as the SAFE trial. You can contact him at the Clinic or me (Dr. Kanal) at the University of Pittsburgh Medical Center or Dr. Shellock to learn more about this study, designed to determine which patients, if any, may be safely studied with MR despite the presence of cardiac pacemakers. Nevertheless, this should in no way be treated cavalierly at this point. Remember that of the at least 5 confirmed deaths to date from scanning pacemaker patients in MR scanners, at least one has apparently been confirmed as not having been pacemaker dependent.
Keep your eyes out for this issue; it is an actively changing one, and will likely be progressing over the next few years as more information in this area is gained.
E. Kanal 8/7/95 Updated 1/2/96
Note: I have observed readings as high as 48mg in homes, due to the internal wiring and some from external sources M Parks.