Epidural Anesthesia: Mechanism of Action and Indications


Epidural anesthesia is a neuraxial procedure that involves delivering medication, most often local anesthetic, to the epidural space for analgesia or anesthesia. The epidural space is located superficial to the dura mater of the spinal cord and just deep to the ligamentum flavum of the vertebrae. The epidural space contains the roots of nerve fibers that transmit pain signals to the brain. The administration of local anesthetic in the epidural space blocks the transmission of the nerves’ signals.

The effect of local anesthetic epidural administration is based on the nerve’s size and degree of myelination. Small and unmyelinated sympathetic nerve fibers are blocked initially, followed by sensory nerve fiber blockade leading to loss of temperature sensation and touch, and lastly, motor fiber blockade. Recovery of the nerve blockade is regained in the reverse order.

Clinical Use: 

Performing the Block

Epidural procedures can involve a single injection into the epidural space, or more commonly, a thin catheter placed within the epidural space and connected to a medication pump to deliver a continuous supply of medication to the epidural space. To reach the epidural space, the provider uses a needle attached to a loss of resistance syringe filled with saline or air. The needle passes through the skin, subcutaneous tissue, the supraspinous and interspinous ligaments, and then the ligamentum flavum. Once through the ligamentum flavum, there is a loss of resistance on the syringe that indicates the epidural space. The syringe is then removed from the needle and a catheter is threaded through the needle and 4 to 6 centimeters of the catheter is left within the epidural space.

Mechanism of action

●      The primary anesthetic agents are local anesthetics that act directly on the nerve roots to block the electrical impulses that transmit sensation to the brain.

  • Epidural medication distributes widely with circumferential and longitudinal spread.
  • The effect of the epidural medication depends on the anatomical location of the epidural injection or infusion along the spine and may span several dermatomes.
  • Some administered medication may diffuse into the CSF. Medication removal occurs predominantly by vascular absorption.
  • Onset of analgesia/ anesthesia after injection: 10 to 20 minutes

●      Duration of anesthesia: A single bolus administration of local anesthetic results in only brief (up to several hours) anesthesia or analgesia, similar to spinal anesthesia. The choice of local anesthetic and the amount injected define the duration.

An epidural catheter may be left within the epidural space at the level that corresponds to the desired dermatomal coverage and connected to a medication pump to deliver boluses or a continuous infusion of medication to the epidural space for lasting analgesia up to several days.


Epidural medication administration can serve as a primary anesthetic if placed preoperatively or a supplemental resource for intraoperative or postoperative pain control depending on the medication used and the medication’s concentration. Epidurals can also be used in settings of labor and delivery and for intractable pain and palliation that is refractory to systemic analgesic medications.

Surgical procedures and scenarios that may benefit from epidural analgesia:

●      Major hip and knee surgery

  • Pelvic fractures
  • Cesarean delivery, labor analgesia
  • Hepatic, gastric, or colonic surgeries
  • Thoracotomy, esophagectomy
  • Nephrectomy, prostatectomy
  • Amputations of lower extremity, revascularizations
  • Trauma, rib fractures

●      Medical conditions such as myasthenia gravis, malignant hyperthermia and autonomic hyperreflexia


For further reading on epidural anesthesia, please see https://www.nysora.com/regional-anesthesia-for-specific-surgical-procedures/abdomen/epidural-anesthesia-analgesia/