[Amps] Pacemakers

[G3rzp at aol.com]

Professionally, I am somewhat on the periphery of this, being very heavily  
involved with designing radios to go in pacemakers
 
Firstly, there was an article a good few years ago (late 1980's) in RF  
Design magazine. They reported on some work at Georgia Tech, and that was  
interesting in that anything made after about 1982 or so didn't act up until the  
field strength at HF was higher than is safe - from memory, they were talking  
well over 100 V/m.
 
Secondly, the leads into the apcemakers go through some extremely expensive  
feedthroughs with low pass filters in them - 4 of them in something about 3/16 
 inch diameter and 3/16 long.
 
The international (certainly European - EN45500) standards call  for at least 
10V/m immunity, although it is subject to risk assessment. In  equipment 
associated with the pacemaker, if failure is considered to be life  threatening, 
then 10V/m is a minimum ( IEC60601-1 is the relevant  standard).
 
As I understand it from my discussions with people who've designed  pacemaker 
chips ( I work with a number!), most, if not all pacemakers go into a  
'fallback' mode if an EMC problem is detected, where it goes to a  straight 'pacing' 
mode. I'm not sure about de-fibrillators, which deliver a  healthy (?) 700 or 
so volts across the heart when they go offt. I have heard of  problems in 
Belgium, although it's only hearsay, where the railway is  electrified at about 
3kV and 16-2/3Hz. A thyristor controlled electric loco puts  out intense low 
frequency magnetic pulses, which can get picked up on the leads  into the 
pacemaker and cause problems. Again, this is anecdotal.
 
The best bet is to contact the pacemaker manufacturer directly. They will  
know the critical level in volts/meter, and you can do some fairly simple  
calculations: they'll almost certainly have a much better idea than any tech at  
the clinic, unless he happens to either be a ham or experienced in EMC. At HF,  
the body loss is unlikely to be significant, but as a guide, allow 10 to 20dB 
at  432MHz. A lot there depends on how deep the pacemaker is implanted. I've 
done  some measurements on this: a strange effect appeared to be that deeper 
implants  showed a much greater degree of polarisation of the 400MHz transmitted 
 signal from the implant than shallower ones.
 
One problem that can occur is that there are few EMC standards for radiated  
field below 80MHz. This is because of the size of the antennas required and 
the  spacings and powers to get a uniform  far field. However, small items can  
be measured in a suitable chamber or even with a 'slab' line. Pacemakers do 
have  leads on them, which can alter enormously the pick up, depending on how 
their  installed, wrapped around etc.
 
Hope this helps
 
vy 73
 
Peter G3RZP