The data you uploaded has size 1200x18000, but your code:
ECG_l=[ones(1,l1)*ECG(1) ECG ones(1,l1)*ECG(N)];
is unclear, what you expect to occur. ECG(1) and ECG(N) will be scalars, but because N is the number of columns which you then use as a linear index it will select... element (80,15). Very unusual, and most likely not what you intended.
Then you concatenate horizontally, but with two row vectors: what do you expect to occur when you concatenate such arrays horizontally together?:
[1x18000, 1280x18000, 1x18000] % not possible: the number of rows are different
PS: why do you need three different names for exactly the same thing?:
ecg_i = ecg_struct.(fn{1});
s=ecg_i;
ECG=s;
