The NET Bible includes this textual criticism note:
Several important Greek mss (Ì75 א1 A B N T W 579 1071*) along with diverse and widespread versional witnesses lack 22:43-44. In addition, the verses are placed after Matt 26:39 by Ë13. Floating texts typically suggest both spuriousness and early scribal impulses to regard the verses as historically authentic. These verses are included in א*,2 D L Θ Ψ 0171 Ë1 Ï lat Ju Ir Hipp Eus. However, a number of mss mark the text with an asterisk or obelisk, indicating the scribe’s assessment of the verses as inauthentic. At the same time, these verses generally fit Luke’s style. Arguments can be given on both sides about whether scribes would tend to include or omit such comments about Jesus’ humanity and an angel’s help. But even if the verses are not literarily authentic, they are probably historically authentic. This is due to the fact that this text was well known in several different locales from a very early period. Since there are no synoptic parallels to this account and since there is no obvious reason for adding these words here, it is very likely that such verses recount a part of the actual suffering of our Lord. Nevertheless, because of the serious doubts as to these verses’ authenticity, they have been put in brackets. For an important discussion of this problem, see B. D. Ehrman and M. A. Plunkett, “The Angel and the Agony: The Textual Problem of Luke 22:43-44,” CBQ 45 (1983): 401-16.
In plain English, Luke's original manuscript probably didn't include these verses. Early Christian scribes may have had the verses in some form (perhaps a fragment of a larger document or perhaps an annotation to a copy of the text) and fit them in were they thought they belonged.
The passage being a later addition breeches the question of whether Jesus sweated blood in historical fact. In my opinion, this is a pious, but misguided, addition and not historically authentic. I base this opinion largely on the work of Bart D. Ehrman and his popularization, Misquoting Jesus: The Story Behind Who Changed the Bible and Why. The first four chapters are particularly useful.