Cite the UpToDate topic as a chapter in a book titled UpToDate, edited by Denise S. Basow, published by UpToDate in Waltham, MA. There are no page numbers to cite, and the publication year for any topic should be the current year.
For example:
Marion, DW. Diaphragmatic pacing. In: UpToDate, Basow, DS (Ed), UpToDate, Waltham, MA, 2012.
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