The Collaborative International Dictionary of English v.0.48:

Heedful \Heed"ful\, a.
   Full of heed; regarding with care; cautious; circumspect;
   attentive; vigilant. --Shak. -- Heed"ful*ly, adv. --
   Heed"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heedfully
    adv 1: in a careful deliberate manner [syn: mindfully,
           heedfully, advertently] [ant: inadvertently,
           unknowingly, unmindfully, unwittingly]