How safe is the EKG shield to use? Specifically, is there any current being delivered to the body, and if so how much is it?
Do I need to be worried about any lasting effects?
SHIELD-EKG-EMG is considered electronics development board - it hasn't passed IEC601 (the standard that each electrical medical device should pass before being used on humans). This is why I usually make sure to warn that CONNECTING A DEVICE VIA ELECTRODES TO HUMANS OR ANIMALS IS POTENTIALLY HAZARDOUS AND MAY RESULT IN ELECTRIC SHOCK AND/OR SEIZURE.
That being said I believe SHIELD-EKG-EMG is a pretty safe-to-use device if it is in good condition (altogether with the board it is mounted on). I have used it multiple times without any electrical feedback or disturbance. Furthermore, we have never received reports of even slight electrical disturbance from a customer. The design doesn't suggest any electrical output on the contact electrodes.
Thanks for the clarification. You mentioned that the design does not suggest any electrical output on the leads. I used a multimeter on the electrodes (the left & right clipped onto the positive prong of the multimeter, and the ground clipped onto the negative prong). It gave me a reading of ~.8 volts. Is this normal or something of a concern?
This is not correct way to measure! DRL is not GND. You are short-circuiting the shield. Good there is some protection on the board.
You should take measures when negative prong of multimeter goes to GND of the board (not the DRL, since the DRL actually provides the reference voltage) and positive prong goes alternatively to RIGHT/LEFT/DRL electrodes(not when they are all connected simultaneously).