cardiac output vs rate of flow

cardiac output and rate of flow both are nouns.

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

As nouns, rate of flow is a hypernym of cardiac output; that is, rate of flow 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
rate of flow: the amount of fluid that flows in a given time

Other hypernyms of cardiac output include flow, flow rate.

cardiac output (noun)	rate of flow (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