orthopnea vs dyspnoea

orthopnea and dyspnoea both are nouns.

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

As nouns, dyspnoea is a hypernym of orthopnea; that is, dyspnoea is a word with a broader meaning than orthopnea:

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
dyspnoea: difficult or labored respiration

Other hypernyms of orthopnea include dyspnea.

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