That causes heartbreak; = heartbreaking, adj. Obsolete.

That breaks the heart; causing intense sorrow or overwhelming emotional distress; extremely upsetting. Later also: deeply moving; having an…

Of an action, emotion, etc.: resulting from or characteristic of heartbreak; accompanied by or expressive of anguish, despair, or grief.

Causing intense sorrow or emotional anguish; evoking deep compassion; deeply distressing or moving. Also (of a painful emotion): deeply or acutely…

Aching, hurt; (also) sad, heart-rending.