dear heart noun used as a term of address UK Often conveys sarcasm, or may be affectedly theatrical.
“Dear heart, I said [...] “how nice of you to call. — Martin Waddell, Otley, p. 135, 1966
Yep, she walked in about the time he mounted one of his lady patients, and asked him, “What’s this, dear heart, your famous meat injection?” — Ken Weaver, TexasCrude, p. 108, 1984