heartbeat vs split second

heartbeat and split second both are nouns.

Word	Noun	Adjective	Verb	Adverb
heartbeat	Yes	No	No	No
split second	Yes	No	No	No

As nouns, heartbeat and split second are synonyms defined as:

heartbeat and split second: a very short time (as the time it takes the eye to blink or the heart to beat)

Other synonyms of heartbeat include blink of an eye, flash, instant, jiffy, New York minute, trice, twinkling, wink.

heartbeat (noun)	split second (noun)
an animating or vital unifying force	a very short time (as the time it takes the eye to blink or the heart to beat)
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)