WordNet records no synonym and no antonym for defibrillate. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
stop the fibrillation and restore normal contractions, usually by means of electric shocks
“The patient's heart had to be defibrillated to save his life”
Synonyms, antonyms and the broader and narrower relations for defibrillate come from Princeton University’s WordNet. Full sources and licences.