The Collaborative International Dictionary of English v.0.48:

Heartrending \Heart"rend`ing\ (-r[e^]nd`[i^]ng), a.
   Causing intense grief; overpowering with anguish; very
   distressing.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heartrending
    adj 1: causing or marked by grief or anguish; "a grievous loss";
           "a grievous cry"; "her sigh was heartbreaking"; "the
           heartrending words of Rabin's granddaughter" [syn:
           grievous, heartbreaking, heartrending]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "heartrending":
   affecting, afflictive, agonizing, calamitous, consuming,
   desolating, dire, distressing, doleful, excruciating, grievous,
   harrowing, heartbreaking, heartsickening, heartwounding,
   lamentable, moving, pathetic, piteous, pitiable, pitiful, racking,
   regrettable, rending, tormenting, torturous, touching,
   unfortunate