cantering rhythm vs arrhythmia

cantering rhythm and arrhythmia both are nouns.

Word	Noun	Adjective	Verb	Adverb
cantering rhythm	Yes	No	No	No
arrhythmia	Yes	No	No	No

As nouns, arrhythmia is a hypernym of cantering rhythm; that is, arrhythmia is a word with a broader meaning than cantering rhythm:

cantering rhythm: cardiac rhythm characterized by the presence of an extra sound; can indicate a heart abnormality
arrhythmia: an abnormal rate of muscle contractions in the heart

Other hypernyms of cantering rhythm include cardiac arrhythmia.

cantering rhythm (noun)	arrhythmia (noun)
cardiac rhythm characterized by the presence of an extra sound; can indicate a heart abnormality	an abnormal rate of muscle contractions in the heart