Hi there...
Funny you're asking about heart rates. I'm experiencing fast heartbeat, myself, but I just had my first round of chemo. Nothing affected me, until I did the very last "drip" of ABVD...it was the "D" that seemed to raise my heart rate. I've had fast heartbeat before then, but mostly it was due to anxiety. Are you feeling anxious? I would imagine you are, as any disease is a bit unnerving...especially not knowing fully what stage you're at, etc.
I had my doctors call something in to calm me and help me sleep. Maybe that would help.
To be on the safe side, you should ask your doctor.

S.