Re: How does an electronic heart rate monitor work?

You didn't specify what type of monitor you were interested in learning 
about, so I am assuming that you are refering to the Electrocardiogram (ECG or 
EKG).  There are several good web sites you can take a look at to learn more 
about this technology and the interpretation of the test results.  A couple to 
get you started are as follows:

 The electrocardiogram and
 ECG library 

Before a muscle contracts a wave of depolarization sweeps along the muscle 
cell.  This wave is similar to that which causes nerve conduction and also 
causes potential differences outside the cell.  A unique feature of the muscle 
cells of the heart is that they remain depolarized for around 100 milliseconds. 
 Therefore, the change in potential outside the cell is much larger than that 
for other cells.  This change can be detected by electrodes attached to the body 
in a similar fashion to detecting potential changes in an electric circuit.

For a detailed discussion of the physics behind both the generation of the 
potential in the body and its detection I highly recommend the following 
text:
Intermediate Physics for Medicine and Biology, by Russel K. Hobbie, John 
Wiley & Sons, New York, 1988

It has a good chapter on the physics of the electrocardiogram.  The same 
author has written a couple of articles for the American Journal of Physics 
that may be more accessible to you.

     Hobbie, R.K., "The electrocardiogram as an example of electrostatics," 
        Am J Phys, 41:824-831, (1973)

     Hobbie, R.K., "Improved explanation of the electrocardiogram," 
        Am J Phys 52:704-705