[RFI] Defibrillator and RF Exposure Query
ka5s at earthlink.net
Sun May 14 05:14:50 EDT 2006
The sensitivity of a device to RF upset is determined by how strong the field is in which it is immersed, how it responds to the field and its modulation, and how efficient the device (or its connecting wires) is at extracting energy from that field. An antenna distant enough that the victim device is in the far field will usually be affected less than a closer one. Since the near field's size is measured in wavelengths, not feet or meters, a lower frequency places more victims inside it than a higher frequency does.
Ground Fault Interrupters are connected to what amounts to a pretty efficient antenna anyway, so it's not a surprise they would be affected by an 80 meter Amateur installation with a nearby dipole. Implanted defibrillator/pacemakers have leads at most a meter long, an inefficient coupler at lower HF frequencies, immersed in a resistive medium, human flesh which is an absorber of RF at at the higher HF frequencies (remember 11 meter diathermy?). Between short parallel leads and the presence of flesh, an implanted device is a rather different proposition with regard to RF upset than a GFI.
That said, a manufacturer spends what is often a lot of money and effort hardening his medical devices to RF. There are regulatory requirements he do so; e.g.: EN/IEC 60601*, to which the FDA also subscribes. Because there is no regulatory inducement for a manufacturer to go beyond these, and because it is sometimes difficult merely to meet them, no manufacturer will warrant his product safe except when he knows or can believe that it is -- and can expect to escape liability if it isn't. There is a grey area; the real world is less forgiving than the tests and (for example) the RF field in an ambulance may far exceed the field 60601-1-2 requires. Other stresses may exist outside the scope of the test; 60601-1-2 requires a sizable magnetic field but doesn't test with a field of every frequency, and it has been found that European electric trains, which run off 16.667 Hz AC, a significant frequency when evaluating whether to defibrillate, can affect Automatic External Defibrillators**. Nevertheless, standards do change, and only a couple of years ago immunity requirements were tripled. (This caused a sudden increase in the number of EMC engineers working on medical products.)
Modern implanted pacemaker/defibrillators are safer than older ones. For one thing, they are now designed to return to a default constant pacing mode if RF affects their sensors. Still, no one will certify a pacemaker safe for Amateur Radio unless and until he has to, can do so at no cost, or has other competitive reasons. Those of us who have made the attempt can possibly inform the rest of us what the results were.
*See http://www.conformity.com/0503/0503basic.html and http://www.conformity.com/A05/A05_F03.html
More information about the RFI