Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
heart-to-heart [ˈhɑːtˈtəˈhɑːt] adj серде́чный
to have a heart-to-heart говори́ть (impf) по душа́м
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.
Русский: