Thomas McGahee wrote:
>
> Soon Lee,

<snip>
> 2) Electrical. An "instrumentation" amp built from op amps can be used to
> directly monitor the small electrical signals from the heart. Besides the two
> differential input electrodes, a "guard" electrode at ground potential is placed
> between the two differential electrodes (which are placed on the skin, on either
> side of the heart). It is generally a good idea to follow the output of the
> instrumentation amplifier with a low-pass filter and simple opamp amplifier to
> boost the useable gain. The filter helps eliminate a lot of unwanted noise that
> is otherwise present.
> Pros: Can be made very selective by proper placement of electrodes.
> Cons: Requires that electrodes be attached directly to the skin surface using
> electrode sets that provide a firm electrical connection via a conductive gel.
<snip>

What you are building is basically an Electro Cariogram, but without the
waveform readout. Another con is that it will also pick up the impulses
from muscle movement, so your patient would have to be still. Also if
you look at an ECG, you will notice there are a few waves for each
heartbeat. You would have to filter or interpret these somehow, because
if you just count all the deflections off of the isoelectric baseline,
you will end up with a heart rate that could be 3 or 4 times the actual
heart rate. If you design an ECG machine, let me know as I'd love to
build one for myself :)

Hope this helps.
Josh Koffman
joshy@mb.sympatico.ca