dyspnoea vs orthopnea

dyspnoea and orthopnea both are nouns.

Word	Noun	Adjective	Verb	Adverb
dyspnoea	Yes	No	No	No
orthopnea	Yes	No	No	No

As nouns, orthopnea is a hyponym of dyspnoea; that is, orthopnea is a word with a more specific, narrower meaning than dyspnoea:

dyspnoea: difficult or labored respiration
orthopnea: form of dyspnea in which the person can breathe comfortably only when standing or sitting erect; associated with asthma and emphysema and angina pectoris

Other hyponyms of dyspnoea include breathlessness, shortness of breath, SOB.

dyspnoea (noun)	orthopnea (noun)
difficult or labored respiration	form of dyspnea in which the person can breathe comfortably only when standing or sitting erect; associated with asthma and emphysema and angina pectoris