You are going to need to build a small differential preamp. You do not need really good freq response but you do need a very high common mode rejection ratio.
Look in some analog design manuals for the common 3-opamp design for starters. You are looking for millivolts swamped in volts of common mode hum (noise). Be safe - power your thingie from batteries. You do not want a very good electrical connection right to your heart if something shorts out.
Put the ground under your belt or so. Put the other two differential inputs at various places across the chest to see the differential across that specific plane. ( the professional EKG machines have quite a few planes they simultaneously monitor to form a more accurate image of the pattern, but for the simple quickie pulse, two will do. There is nothing special about the electrodes - the old silver dimes worked great but a
cleaned penny will work too. ( you will have to solder to it. A simple mechanical connection will corrode and inject all sorts of corrosion noise into the amplifier.) For experimental purposes darned near anything, including saliva, works as electrode paste. It just has to be liquid and conduct. If you wanna get fancy, make some gelatin solution with salt in it. Yup, it will corrode the electodes in time ( thats why the silver dimes worked better - but they are hard to get these days. ). Use solid metal electrodes - no plated stuff. Two metals together cause problems - its bad enough to have the copper-lead (solder) "battery" inadvertantly formed when the back connection is made - for maximum noise reduction, cover the connection with silicone so it does not get wet during use, leaving only the copper face exposed.
Your preamp will probably put out a volt or so ( for gains in the 100 to 1000 range ) which would feed right in to the digitizer. You can play around with it to find out. If you get nothing but hum, you did not get your Common-Mode balanced.