Patient preparation and positioning, the use of monitors, and the needle approaches for epidural anesthesia are the same as for spinal anesthesia. However, unlike spinal anesthesia, epidural anesthesia may be performed at any intervertebral space.
Using the midline approach, the epidural needle is inserted into the interspinous ligament (“gritty” feel) and then advanced slowly until the ligamentum flavum is contacted (increased resistance).
The epidural needle must traverse the ligamentum flavum and stop in the epidural space (“loss of resistance”) before encountering the spinal meninges.
A glass syringe containing 2 to 3 mL of saline and 0.1 to 0.3 mL of air is attached to the epidural needle, and the plunger is pressed. If the needle is properly placed in the ligamentum flavum, it will be possible to compress the air bubble without injecting the saline. If the air bubble cannot be compressed without injecting fluid, then the needle tip is most likely not in the ligamentum flavum but instead in the interspinous ligament or off midline in the paraspinous muscles.
As the needle enters the epidural space, there will be a sudden and dramatic loss of resistance as the saline is rapidly injected. (The patient should be warned of possible pain.) If the needle is advancing obliquely through the ligamentum flavum, it is possible to enter into the paraspinous muscles instead of the epidural space. (Loss of resistance is less dramatic.)
When the syringe is disconnected from the needle, it is common to have a small amount of fluid flow from the needle hub (usually saline, which is at room temperature in contrast to CSF).
A test dose of local anesthetic solution is injected to help detect unrecognized intravenous (IV) or subarachnoid placement of the needle. After a negative test dose, the desired volume of local anesthetic solution should be administered in 5-mL increments. (This decreases the risk of pain during injection and allows early detection of adverse reactions.)