The principles of the 'loss of resistance' technique for epidural localisation are shown in this diagram.
The Tuohy needle is inserted into the interspinous ligament with the stylette in place. Once in the ligament, the stylette is removed and the needle connected to a syringe containing either air or saline. The needle is now advanced slowly through the interspinous ligament into the ligamentum flavum using either a continuous or intermittent movement. As the needle is advanced, pressure is applied to the plunger of the syringe. While in the ligament, it is not possible to inject the air/saline. As soon as the needle enters the epidural space, the air/saline can be freely injected. Recent studies suggest that saline may be preferable to air as the loss-of-resistance indicator.

Once the space has been localised, various methods can be used to initiate the block and place an epidural catheter. Some operators prefer to inject a small amount of saline (or local) before insertion of the catheter, while others thread the catheter first and administer all solutions through the catheter. Both techniques have their merits. Whichever technique is chosen, it is important not to thread more than ~3-4 cms of catheter into the space.