cardiac output vs flow rate

cardiac output and flow rate both are nouns.

Word	Noun	Adjective	Verb	Adverb
cardiac output	Yes	No	No	No
flow rate	Yes	No	No	No

As nouns, flow rate is a hypernym of cardiac output; that is, flow rate is a word with a broader meaning than cardiac output:

cardiac output: the amount of blood pumped out by the ventricles in a given period of time
flow rate: the amount of fluid that flows in a given time

Other hypernyms of cardiac output include flow, rate of flow.

cardiac output (noun)	flow rate (noun)
the amount of blood pumped out by the ventricles in a given period of time	the amount of fluid that flows in a given time