orthopnea vs dyspnea

orthopnea and dyspnea both are nouns.

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

As nouns, dyspnea is a hypernym of orthopnea; that is, dyspnea 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
dyspnea: difficult or labored respiration

Other hypernyms of orthopnea include dyspnoea.

orthopnea (noun)	dyspnea (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