Use of a catheter for epidural anesthesia affords greater flexibility than the single-shot technique but introduces the risk of catheter migration (subarachnoid space, intervertebral foramen) and increases the likelihood of a unilateral epidural block.
Epidural catheters are usually inserted through a curved-tip needle to help direct the catheter away from the dura mater. The catheter typically encounters resistance as it reaches the curve of the needle, but using steady pressure usually results in its passage into the epidural space. One explanation for the inability to thread an epidural catheter is that the tip of the epidural needle was bent during bony contact and now partially occludes the needle lumen.
The catheter should be advanced only 3 to 5 cm into the epidural space. This minimizes the risk of forming a knot, entering a vein, puncturing dura mater, exiting via an intervertebral foramen, and wrapping around a nerve root.
After the catheter is appropriately positioned, the needle is slowly withdrawn with one hand as the catheter is stabilized with the other. The length of the catheter in the epidural space is confirmed because this distance is important when trying to determine if a catheter used in the postoperative period has been dislodged.
A test dose of the local anesthetic solution is injected before the initial injection and any subsequent top-up dose (typically 50% of the initial dose at an interval equal to two thirds the expected duration of the block).