Synonyms for Heartbeat (same or very similar meaning)
WordNet sense 1 (a very short time (as the time it takes the eye blink or the heart to beat)):
flash, instant, jiffy, trice, twinkling, wink, blink of an eye, New York minute, split secondWordNet sense 2 (the rhythmic contraction and expansion of the arteries with each beat of the heart):
beat, pulsation, pulseFrom the ODE community, based on WordNetadd/edit