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Re: [RFI] Defibrillator and RF Exposure Query

To: RFI List <rfi@contesting.com>
Subject: Re: [RFI] Defibrillator and RF Exposure Query
From: Cortland Richmond <ka5s@earthlink.net>
Reply-to: Cortland Richmond <ka5s@earthlink.net>
Date: Sun, 14 May 2006 05:14:50 -0400 (GMT-04:00)
List-post: <mailto:rfi@contesting.com>
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 Defi
 brillators**.  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 


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