Apparently conv(ecg,h) returns a vector, meaning that you can't store it in ye(i).
My guess would be that you would be willing to use a cell array for ye instead.
Without your variables it is difficult to test your code and to see what you mean.
N = [2 4 8 16 32];
b = 1./N;
ye=cell(size(b));
figure(4);
for n = 1:numel(b)
ye{n} = b(n)*(conv(ecg,h));
subplot(3,2,n+1);
stem(ye{n});
end
subplot(3,2,1);
stem(ecg);
