How do I place the ECG isoelectric line on 0 volts level?

I'm working on a project for college and need to determine the onsets and offsets of ECG segments. I have detected all the peaks but need to do a zero-crossing, and I do not know how to put the isoelectric line on the 0 level. It can be noticed from the following pictures

that the isoelectric line isn't always a straight line.

Any suggestions are very welcome. Thank you

[Otto Max]

Use a low-pass filter with a frequency of 0.5 Hz

[Jaroslav Kopcan]

you need to use High-pass filter because respiration frequency 0.5-1Hz has to be filtered