eupnoeic vs breathing

eupnoeic is not a noun while breathing is a noun.

Word	Noun	Adjective	Verb	Adverb
eupnoeic	No	Yes	No	No
breathing	Yes	Yes	No	No

As adjectives, eupnoeic and breathing are synonyms defined as:

eupnoeic and breathing: passing or able to pass air in and out of the lungs normally; sometimes used in combination

Other synonyms of eupnoeic include eupneic.

eupnoeic (noun)	breathing (noun)
	the bodily process of inhalation and exhalation; the process of taking in oxygen from inhaled air and releasing carbon dioxide by exhalation

eupnoeic (adjective)	breathing (adjective)
passing or able to pass air in and out of the lungs normally; sometimes used in combination	passing or able to pass air in and out of the lungs normally; sometimes used in combination