WordNet records no synonym and no antonym for cardiac cycle. 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.
the complete cycle of events in the heart from the beginning of one heart beat to the beginning of the next; an electrical impulse conducted through the heart muscle that constricts the atria which is followed by constriction of the ventricles
“the cardiac cycle can be shown on an electrocardiogram”
Synonyms, antonyms and the broader and narrower relations for cardiac cycle come from Princeton University’s WordNet. Full sources and licences.