Active driven guard is not particularly required in ECG due to the limited frequency range and fair source impedance.
Common mode feedback respectively 3-electrode configuration is commonly used in ECG applications, as far as I know. It's also
a "serial" topic at edaboard. Apart from avoiding common mode overload of the input amplifier, it also reduces interferences
from power supply system. But ECG is your business rather than mine I guess, you should know the respective literature.