The Collaborative International Dictionary of English v.0.48:

palpitating \palpitating\ adj.
   Beating irregularly; -- of the heart.

   Syn: palpitant.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Palpitate \Pal"pi*tate\, v. i. [imp. & p. p. Palpitated; p.
   pr. & vb. n. Palpitating.] [L. palpitare, palpitatum, v.
   intens. fr. pappare. See Palpable.]
   To beat rapidly and more strongly than usual; to throb; to
   bound with emotion or exertion; to pulsate violently; to
   flutter; -- said specifically of the heart when its action is
   abnormal, as from excitement.
   [1913 Webster]

WordNet (r) 3.0 (2006):

palpitating
    adj 1: having a slight and rapid trembling motion; "palpitant
           movements rather than violent eruptions"; "my palpitating
           heart" [syn: palpitant, palpitating]