to say 'heartbeat', do I say 'pou kè a'?

Yes, you could say 'pou kè a', but to avoid confusion, you might consider the following H. Creole translations for heartbeat

Heartbeat
batman kè a
rit kè a
palpitasyon kè a
and even
woulman kè a
Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words