First, read it and assign the variables:
D = readmatrix('ECG_10.11.20_11.57.59.txt', 'HeaderLines',6);
EKG = -D(:,1); % EKG Trace (mV)
t = D(:,2)*1E-3; % Time Vector (s)
There are a lot of noise and other problems with that signal, so it is going to require a significant amount of processing. It is not regularly-sampled, so the Signal Processing Toolbox resample function will be necessary to assign it to a consistent sampling interval before further processing. (I would resample it to a sampling frequency of 130 Hz.) Note that the leads were apparently reversed, the resons for negating the EKG signal.
I leave all that to you.
