The Collaborative International Dictionary of English v.0.48:

Pitapat \Pit"a*pat`\, adv. [An onomatopoetic reduplication of
   pat a light, quick blow.]
   In a flutter; with palpitation or quick succession of beats.
   --Lowell. "The fox's heart went pitapat." --L'Estrange.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pitapat \Pit"a*pat`\, n.
   A light, repeated sound; a pattering, as of the rain. "The
   pitapat of a pretty foot." --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pit-a-pat
    adv 1: as of footsteps; "he came running pit-a-pat down the
           hall" [syn: pit-a-pat, pitty-patty, pitty-pat,
           pitter-patter]
    2: describing a rhythmic beating; "his heart went pit-a-pat"
       [syn: pit-a-pat, pitty-patty, pitty-pat, pitter-
       patter]

Moby Thesaurus II by Grady Ward, 1.0:

107 Moby Thesaurus words for "pitapat":
   arrhythmia, barrage, beat, beating, bump, chink, click, clink,
   clop, clump, clunk, crump, dance, didder, disquiet, disquietude,
   dithers, drum, drum music, drumbeat, drumfire, drumming, dull thud,
   fidgetiness, fidgets, flap, flick, flicker, flit, flitter, flop,
   flump, flutter, heartbeat, heartthrob, heaving, inquietude, pad,
   palpitant, palpitation, panting, paradiddle, pat, patter,
   pitter-patter, plump, plunk, pop, pound, pounding, pulsatile,
   pulsating, pulsation, pulsative, pulsatory, pulse, pulsing,
   quaking, quaver, quavering, quiver, quivering, rap, rat-a-tat,
   rat-tat, rat-tat-tat, rataplan, rattattoo, restlessness, rhythm,
   rhythmic, roll, rub-a-dub, ruff, ruffle, shake, shakes, shaking,
   shiver, shivers, shudder, spatter, splutter, sputter, staccato,
   tap, tat-tat, tattoo, tempo, throb, throbbing, thrum, thud, thump,
   thumping, tick, tinkle, tom-tom, tremble, trembling, tremor,
   trepidation, trepidity, tunk, twitter, unrest, waver