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
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