For an epidural anesthetic, a Tuohy needle is usually used by the Vascular Surgeon Melbourne. It has a slightly curved Huber point. This tends to blunt the end and allows a catheter to be threaded easily into the epidural space. Once the correct site has been located. the local anaesthetic is injected. The subarachnoid space is recognized by a free flow back of cerebrospinal fluid. The epidural space is more difficult to locate and several methods have been devised. The most commonly used are loss of resistance with air or saline and the hanging drop method.
The latter relies on the fact that the epidural space is under negative pressure, so that when the space is entered a drop in the hub of the needle is sucked into the space. For an epidural anaesthetic much larger doses are injected. Therefore a test dose is given prior to the definitive dose to ensure the needle or catheter is not in the subarachnoid space or in a vessel.
