Definition of "agonizingly"

Leisurely the long hours ticked into nothingness, yet there was no sound from him. Would he go in the night, as he did before from Stein's, without a word? Ursula's heart quickened agonizingly, though she lay still, tingling with the thought.

1907, Barbara Baynton, edited by Sally Krimmer and Alan Lawson, Human Toll (Portable Australian Authors: Barbara Baynton), St Lucia: University of Queensland Press, published 1980, page 261