The best way would be to apply a (time,voltage) dataset of ECG samples to Multisim's PWL (piece-wise-linear) source. Ofcourse you would have to find this dataset somewhere.
I happen to have some sort of ECG signal on my computer that you can try.
There is the NI Biomedical Startup kit:
Also there is some additional processing details here:
It is possible to use these to create a LabVIEW VI and/or export signals that will generate ECG signals into Multisim.
As such I did a simple concept VI (based on some of these signals) here: