J. G. Webster's Design of Cardiac Pacemakers has given a detailed electrode model with explanation.
For a pacemaker the values are Cd is around 10s of uF, Rd is 34.1Kohm, Rs is 25 ohms. I am not exactly sure about Ecell, I think it is in 10s of mV range.
I am not sure if ECG/EKG have same/similar values.