Yes, HEARTBEAT is a Scrabble word!

Letter values in Scrabble US: H 4, E 1, A 1, R 1, T 1, B 3, E 1, A 1, T 1.

Is it a valid word?

HEARTBEAT is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the rhythmic contraction and expansion of the arteries with each beat of the heart
a very short time (as the time it takes the eye to blink or the heart to beat)
an animating or vital unifying force

Also: blink of an eye, split second, new york minute.