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]