heartbeat has 12 distinct synonyms spread across 2 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
a very short time (as the time it takes the eye to blink or the heart to beat)
“if I had the chance I'd do it in a flash”
an animating or vital unifying force
“New York is the commercial heartbeat of America”
Synonyms, antonyms and the broader and narrower relations for heartbeat come from Princeton University’s WordNet. Full sources and licences.