ECG signal reconstruction data processing

Hi everyone ¡¡¡

I'm working now in a proyect for the school. I'd like to make a ECG signal generator from the data of physionet of MIT database.

My first idea was take the ecg data and make fast furier transform to have the most important frequency harmonics.... but all of my data are a number collection and when I make fft(data) y don't have any good result....

what can I use to reconstruction the signa and get a mathematical function similar to an ideal ecg plot...?

Thaks a lot '' I'm very lost now and need some orientation