Please post you schematic for the sensor hookupWhat instrument Amp are you using?thanks
Hopefully you are using precautions like 1 Mohm series resistor on all leads going to your skin. If you've got the Arduino plugged in via USB, that ground is connected to your PC ground, which is connected to your wall outlet AC ground. And the electrodes are taped right over your heart so the lethal current threshold to trigger ventricular fibrillation is much lower than normal... accidents can happen! I think commercial EKG equipment always uses optoisolators for complete electrical isolation.
I did a battery-powered heartrate monitor almost 20 years ago, and the completed unit is still on my shelf. Your post inspired me to take a few photos of my lab notebook, and the unit itself. I posted them here:http://bealecorner.org/best/measure/EKG/Since it was battery powered and transmitted the output via RF there was no problem with electrical hazards from ground loops, etc.I was just measuring heartrate so I just did one pulse per beat. To actually record the full EKG waveform you'd have to encode that somehow, probably as a FM tone-encoded signal.