Entry of the needle into the epidural space is recognized by a sudden loss of resistance to injection of air. Source: Chapter 16. Combined Spinal–Epidural Anesthesia, NYSORA Textbook of Regional Anesthesia and Acute Pain Management Citation: Hadzic A. NYSORA Textbook of Regional Anesthesia and Acute Pain Management; 2007 Available at: http://accessanesthesiology.mhmedical.com/DownloadImage.aspx?image=/data/books/hadz/hadz_c016f002.jpg&sec=39830049&BookID=413&ChapterSecID=39828163&imagename= Accessed: December 26, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved