The usual way of filtering EKG signals is to use a bandpass filter with a passband frequency of 2 to 100 Hz, and a stopband of 2 to 110 Hz. That should produce a stable filter. My filter design procedure is here: How to design a lowpass filter for ocean wave data in Matlab?
This designs a stable filter that should do what you want (eliminate base line wander and d-c offset, and high-frequency noise):
Fs = 1000; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
Wp = [1.5 100]/Fn; % Normalised Passband
Ws = [0.1 120]/Fn; % Normalised Stopband
Rp = 20; % Passband Ripple (dB)
Rs = 30; % Stopband Ripple (dB)
[n,Wn] = buttord(Wp,Ws,Rp,Rs); % Filter Order
[b,a] = butter(n,Wn); % Filter Coefficients
[sos,g] = tf2sos(b,a); % Second-Order-Section For Stability
figure(1)
freqz(sos, 4096, Fs) % Filter Bode Plot
Use the filtfilt function with ‘sos’ and ‘g’ to filter your EKG signal.
