The spinal canal contains the spinal cord and its coverings (or meninges), fatty tissue, and the venous plexus. It extends from the foramen magnum to the sacral hiatus. The meninges are composed of three layers: the pia mater, the arachnoid mater, and the dura mater. The pia mater is adherent to the spinal cord, whereas the arachnoid mater is adherent to the tough, fibroelastic dura mater. The subarachnoid space, located between the pia mater and the arachnoid mater, contains cerebrospinal fluid. The epidural space is a potential space, containing fat, alveolar tissue, nerve roots, and a network of arteries and venous plexus. No fluid is present in the epidural space. The epidural space extends cephalad from the foramen magnum and caudad to the sacral hiatus. The space is bound anteriorly by the posterior longitudinal ligament, laterally by the interverterbral foramen, and posteriorly by the ligamentum flavum. The spinal cord extends from the foramen magnum to L1 in adults and L3 in children. The anterior and posterior nerve roots at each spinal level exit the intervertebral foramina forming spinal nerves from C1 to S5.

Epidural anesthesia is produced by injection of a local anesthetic solution into the epidural space. Continuous epidural anesthesia is achieved by placement of a catheter into the epidural space. The epidural space is identified by the “loss of resistance” technique, reflecting passage from an area of high resistance in the ligamentum flavum to an area of low resistance in the epidural space. After the epidural needle is positioned in the ligamentum flavum, a glass syringe with a freely movable plunger is attached to the needle and continuous pressure is applied to the plunger. If the needle is positioned correctly in the ligamentum flavum, the plunger should not inject when pressure is applied. When the needle passes into the epidural space, a loss of resistance is achieved, and the plunger will easily inject.