Epidural Anesthesia

Anatomy and Physiology

The Spinal Cord and Epidural Space

The adult spinal cord is approximately 45 cm shorter than the spinal canal; the cord typically ends at the L1 vertebra in 50% of adults and extends to the L2 vertebra in about 40%. Recent studies have controverted previously held notions regarding the termination of the spinal cord in the newborn; current evidence indicates that the conus medullaris of the neonate also terminates at the L2 vertebra. Below this anatomical level, the lumbar and sacral nerves converge to form the cauda equina. The spinal cord is suspended in cerebrospinal fluid and surrounded by the arachnoid mater. The arachnoid mater and subarachnoid space extend caudally to S2 in adults, S3 in children, and S4 in newborns. The arachnoid mater closely approximates the dura mater, which is attached to the spine by its outer endosteal portion. The arachnoid mater envelopes the brain intracranially and the spinal cord and extends through the intervertebral foramina to the epineural connective tissues of the spinal nerves.[2]

The spinal epidural space extends from the foramen magnum at the base of the skull to the sacral hiatus and contains fatty and connective tissues, blood vessels, and lymphatics. The blood vessels may dilate in the presence of ascites or during pregnancy, increasing the risk of a traumatic or bloody puncture. The epidural space is further divided into the anterior and posterior epidural spaces. The anterior epidural space contains the ventral spinal nerves, the basivertebral veins, and the internal vertebral venous plexus. The posterior epidural space contains the dorsal spinal nerves, the intervertebral veins, and the sinuvertebral nerves.[5] 

The boundaries of the epidural space are formed by the dura mater and arachnoid mater internally, the ligamentum flavum and the vertebral periosteum externally, and the intervertebral foramina laterally.[5] The dorsal border of the spinal epidural space is the ligamentum flavum. The remaining layers, moving from the ligamentum flavum toward the skin, are the interspinous ligament located between the spinous processes, the supraspinal ligament situated on the surface of the spinous processes, the subcutaneous tissue, and the skin.[2] The distance between the skin and the spinal epidural space widely varies with age and subcutaneous adiposity; the distance commonly measures 4 cm in adults with a normal body mass index but may increase to more than 8 cm in patients with obesity.

The anatomy of the epidural space is of paramount importance to the administration of epidural anesthesia. By targeting specific spinal segments and structures within the epidural space, clinicians can achieve targeted pain relief and minimize the risk of complications. In addition, understanding the anatomical variations and the location of blood vessels, spinal nerves, and other structures within the epidural space is essential for safe and effective epidural catheter placement.[6]

Techniques to Locate the Epidural Space

Various techniques have been developed to locate and access the epidural space safely and accurately. The traditional landmark-based approach relies on identifying anatomical landmarks, including the spinous processes, interspinous spaces, and the midline, to guide epidural needle insertion.[6] The loss-of-resistance technique is commonly employed once the epidural needle is placed into the interspinous ligament. This technique relies on the practitioner feeling resistance during the gentle injection of air or saline through the epidural needle when it is outside the epidural space. A sudden loss of this resistance signals that the needle has successfully entered the epidural space.[6]

The caudal epidural block is a specific technique that involves the administration of local anesthetic agents into the caudal epidural space located at the sacral hiatus.[5] This technique is often employed for pediatric patients and specific lower extremity and perineal procedures in adults. The most common shape of the sacral hiatus in men and women is an inverted U; the distance from the apex of the sacral hiatus to the first sacral spine is slightly longer in women, but the width of the sacral cornu does not differ. The sacral canal is formed by the union of the pedicle and laminae of the 5 sacral vertebrae and contains important structures, eg, the cauda equina and sacral nerves.[7] 

Understanding these morphometric parameters is crucial for successful epidural anesthesia procedures. Variations in the sacrum can contribute to lower backache and may impact surgical procedures. Identifying these anatomic landmarks can guide needle placement. However, even when identifying anatomical landmarks, clinicians fail to place a caudal epidural block in one-third of patients, secondary to anatomical variations. Therefore, clinicians must have a good understanding of the anatomy of the sacral region to have any chance of successfully administering a caudal epidural block. An ultrasound-guided technique may help to eliminate these failed placements.[8] 

Indications

The primary indications for epidural anesthesia include obstetrical anesthesia during labor and surgical anesthesia for thoracic, major intraabdominal, or spine surgeries that do not require muscle relaxation. Epidural anesthesia may be considered for patients with difficult airways or other concerns for respiratory compromise secondary to the effects of general anesthesia. Epidural anesthesia may also be used as an adjunct in intraoperative or postoperative pain management. Patients at higher risk for postoperative complications, such as those with ischemic heart disease, have demonstrated increased benefits with epidural anesthesia, including decreased postoperative pulmonary complications and faster intestinal return to function.[1][9]

A meta-analysis comparing intravenous and epidural routes of patient-controlled analgesia (PCA) in abdominal surgery demonstrated that epidural PCA was associated with better postoperative pain relief, reduced opioid consumption, and fewer postoperative complications than intravenous PCA.[10] Additionally, combined epidural anesthesia and analgesia reduced postoperative cognitive impairment in general anesthesia patients.[11] Furthermore, investigations have been conducted on the effects of perioperative epidural analgesia on cancer recurrence and survival, suggesting a potential association between epidural analgesia and improved survival outcomes.[12] However, recent studies have shown decreased benefits of epidural analgesia during surgery compared to catheter wound infusions and peripheral nerve blocks with a higher risk of complications.[4] In limited studies, epidural anesthesia demonstrated slightly more effective postsurgical pain control than systemic medications. Therefore, other than for obstetrical indications, epidural anesthesia may be more useful as an adjuvant therapy; further research is needed before definitive conclusions can be reached.[13]

Contraindications

Epidural anesthesia is considered relatively safe for most patients but may not be appropriate in all circumstances. Contraindications to epidural anesthesia may be absolute or relative.

Absolute Contraindications

Epidural anesthesia is contraindicated if any of the following factors are present:

• Patient refusal
• Local infection at the puncture site
• Increased intracranial pressure
• Traumatic spinal cord injury [14]

Relative Contraindications

Alternatives to epidural anesthesia should be considered in patients with the following:

• Hemodynamic instability
• Obstructive cardiomyopathy
• Uncorrected coagulopathy or therapeutic anticoagulation
• Thrombocytopenia
• Inability to maintain positioning for epidural placement
• Anatomic spinal abnormalities [15][14]

The American Society of Regional Anesthesia provides recommendations for clinicians regarding epidural anesthesia in patients using anticoagulants or antiplatelet drugs. Clinicians should review up-to-date information about these medications.[16]

Equipment

The equipment required to perform epidural anesthesia typically includes:

• Epidural needle: A 3.5-inch, 17- or 18-gauge Tuohy, Hustead, Crawford, or Weiss epidural needle may be used; the Tuohy epidural needle is most often employed. Epidural needles of up to 6 inches in length are available and typically used for patients with obesity.[17]
• Loss of resistance syringe: A syringe made of glass or plastic with very low friction between the plunger and the barrel is used to detect the change in resistance at the epidural space. Air, saline, or both may be loaded into the syringe. The injected medium does not affect the success of identifying the epidural space, nor does it change the complication rate.[18][17]
• Epidural catheter: A catheter is required to administer continuous epidural anesthesia or analgesia. The catheters may be flexible or stiff and have single or multiple perforations for perfusing the analgesic agent. In most commercially available kits, syringe-catheter connections, such as Luerlock or Luer-slip connectors, are included.[19][17]
• Epidural procedure site preparation supplies: An aseptic technique must be employed when placing an epidural catheter and dressing at the insertion site to prevent infection using the following:
  • Sterile gloves
  • Surgical cap and mask
  • Sterile draping
  • Individualized skin preparation solution packets, such as chlorhexidine with or without alcohol or povidone-iodine with or without alcohol
  • 25-gauge needle 
  • Topical local anesthetic, typically 1% or 2% lidocaine
  • Sterile occlusive dressing [20]
• Medications: A combination of a local anesthetic (eg, bupivacaine or ropivacaine) and an opioid (eg, fentanyl or sufentanil) is typically utilized; exact formulas vary with clinical preference.[14] 
• Intravenous fluids: Fluids are typically administered before an epidural is performed to prevent hypotensive episodes.
• Adrenergic agonist: Intravenous ephedrine or phenylephrine should be available to treat hypotension during neuraxial anesthesia.[21]

Ultrasound-Guided Epidural Anesthesia

Clinicians may choose to place an epidural catheter with ultrasound guidance. Ultrasound technology allows real-time visualization of the relevant anatomical structures, potentially improving the accuracy and safety of epidural catheter placement. The advantages of using ultrasound guidance for epidural anesthesia include better identification of the epidural space, fewer needle passes and attempts, and a lower incidence of complications such as unintentional dural puncture, infection, and hematoma formation. However, further research is needed to establish the superiority of ultrasound-guided epidural anesthesia over traditional landmark-based techniques.[22]

Personnel

Epidural catheter placement requires a qualified clinician, typically an anesthesiologist or nurse anesthetist. Other specialized clinicians, such as physical medicine and rehabilitation practitioners or neurologists, perform the procedure within their scope of practice and training. A clinical staff member is commonly present to provide intraprocedural support.

Preparation

A comprehensive medical history should be obtained before epidural anesthesia, and a physical examination should be performed. Elements of the medical history requiring thorough investigation include but are not limited to past medical history, medication allergies, prior anesthesia exposures and reactions, current medications, other drug and supplement use, and family anesthesia history. The physical examination should evaluate the airway, neurologic functioning, and the structure and function of the back and spine.

Preprocedural laboratory investigations should include coagulation studies and a platelet count. Patients taking anticoagulants may require intervention before undergoing epidural anesthesia. For example, patients receiving unfractionated heparin doses of 7500 to 10,000 units twice daily should have neuraxial anesthesia delayed 4 to 12 hours. Patients taking warfarin must be reversed or the medication withheld for 5 days; an immediate preprocedural international normalized ratio (INR) of ≤1.4 must be documented.

Shared decision-making should include a discussion of the procedure, indications, complications, potential risks, and alternatives. A procedural time-out is required. Standard American Society of Anesthesiologists (ASA) monitoring equipment should be placed before positioning the patient in either the sitting or lateral decubitus position. Spinal flexion and neutral rotation should be achieved to establish a straight path for the needle insertion between the spinous processes. 

The patient should have the back of their knees as close to the table edge as possible if seated. The patient should touch their chin to their chest, rest their hands on their thighs, and slouch, pushing the umbilicus back towards the administering clinician to achieve spinal flexion. A supporting staff member should assist and stabilize the patient safely, preventing a forward lean and possible fall from the table. Positional aids such as pillows and stands may be helpful.

If the patient is in the lateral decubitus position, laterality is chosen based on patient comfort while ensuring adequate room for support personnel and the proceduralist. The patient's back should be closest to the proceduralist. The patient should draw their thighs towards their torso to facilitate spinal flexion while maintaining a neutral rotational position. Once correctly positioned, the patient should be stabilized. 

Procedural equipment should be opened and prepared in a sterile fashion. Premedication can be titrated as needed for patient anxiety. Caution should be taken to achieve minimal sedation, given the need for patient positioning, cooperation, and feedback throughout the procedure.[23][20]

Technique or Treatment

The procedural approach to epidural anesthesia is dictated by the area of the spine where the epidural space will be accessed. Due to the difficult angulation of the thoracic spinous processes, epidural anesthesia performed above the T11 vertebra generally employs the paramedian approach; procedures performed below T11 typically employ a midline approach.[24][25] Once the target level is identified by palpating the 2 spinous processes corresponding with the desired spinal level for placement, the skin is cleansed in a sterile fashion. Once the cleanser is dry, drape the target area in a sterile fashion.

Medial and Paramedian Approach

In the medial (ie, midline) approach, the needle insertion site is midline between the spaces created by the vertebral spinous processes. Lidocaine 1% must be injected into the skin and underlying tissues at the desired level for the epidural needle insertion to decrease the discomfort with the advancement of the epidural needle. Once localized anesthesia is achieved, the epidural needle is advanced with a stylet in place and with its bevel point cephalad, ultimately guiding the epidural catheter to the proper location. The epidural needle must be advanced through the skin, subcutaneous tissue, supraspinous, and interspinous ligaments. Once there, the stylet is removed, and the loss of resistance syringe that is filled with saline, air, or both must be attached to the needle. As the needle advances, the clinician applies pressure to the plunger so that once the ligamentum flavum is pierced, a loss of resistance is noted, confirming entry into the epidural space. On average, the distance from the skin to the ligamentum flavum is approximately 4 cm.[25] The anesthesiology clinician will usually inject 5 to 10 mL of saline to expand the epidural space and decrease the risk of vascular injury.[26]

In the paramedian approach, the needle insertion site is 1 cm lateral to the vertebral interspace. Local anesthetic is injected along the predicted path of the epidural needle as described for the medial approach. The epidural needle is then advanced through the paraspinal tissues. Given this location, the needle will not transverse the supraspinous or interspinous ligaments. The advancement of the needle must stop upon feeling the engagement in the ligamentum flavum. The loss-of-resistance syringe is attached, and the epidural space is located similarly to the median approach.[25]

Once the epidural space is reached by either the midline or paramedian approach, the epidural catheter is advanced through the introducer needle after removing the loss-of-resistance syringe. The epidural catheter should be advanced to the 20 cm mark on the outside of the catheter. The epidural needle is removed after noting the distance from the skin to the epidural space as indicated by the marked epidural needle. The epidural catheter is then partially withdrawn, leaving approximately 5 to 6 cm of the tip in the epidural space. However, no consensus exists on the precise length of the catheter that should be left within the epidural space.[27] The clinician can calculate the length by adding 5 to 6 cm to the distance previously measured by the epidural needle; the resulting total is the number up to which the epidural catheter should be withdrawn.[28]

Once the catheter is in its final position, the catheter connector is attached to the end of the catheter to enable attachment of syringes or epidural pump tubing. The catheter should be aspirated with an empty and dry 3 mL syringe to ensure no return of cerebrospinal fluid (CSF). If CSF is not aspirated, proceed to administer the test dose. If CSF is aspirated, remove the catheter and attempt reinsertion at a different level. 

The test dose aims to identify unintentional intrathecal or epidural vein catheterization. Intrathecal catheterization could result in a total or high spinal, while that of a vein could result in local anesthetic toxicity. A 3 mL solution consisting of lidocaine 1.5% and 1:200,000 epinephrine is commonly used. An increase in a patient's heart rate of 20 to 30 bpm or 15 to 20 mm Hg in systolic blood pressure may indicate intravascular positioning. Clinicians should be aware of the potential for a blunted response in patients receiving beta-adrenergic blocker medications. If the test dose is negative, a transparent sterile dressing should be placed over the catheter site, and the remaining catheter can be secured with tape.[23][29]

Caudal Anesthesia

Caudal anesthesia is a variation of epidural anesthesia widely used in children for procedures performed on areas under the umbilicus (eg, circumcision, herniotomy, or orchiopexy). Typically, patients are placed in a lateral decubitus position, and after the site is sterilely prepped, the sacral hiatus is identified. The sacrococcygeal membrane is pierced with a 22 to 25 G venous catheter at a 45-degree angle relative to the body's longitudinal axis, and the epidural catheter advanced until a loss of resistance is felt. Additional advancement should not be attempted since the distance between the sacrococcygeal membrane and the caudal end of the dural sac may be <10 mm. If CSF or blood is noted, the catheter is repositioned. Once properly located, a common test dose with epinephrine 0.5 µg/kg is given to exclude a malpositioned catheter.[23][30][27]

Complications

Spinal cord injuries (SCI) resulting from anesthesia procedures are rare but significant concerns for surgical patients. These injuries can have devastating effects on the quality of life and may even lead to mortality. Complications associated with anesthesia-induced SCI include temporary or permanent neurological symptoms, epidural hematoma or abscess, direct traumatic spinal injury, and adhesive arachnoiditis. These complications can manifest as motor deficits, sensory loss, pain, paraesthesia, and even permanent paralysis or anesthesia.[14]

High-risk patients for anesthesia-induced SCI include those with spinal canal malformations, extremes of age, obesity, diabetes, immunocompromised or critically ill individuals, and those with previous neurological diseases. Careful evaluation of patients preoperatively to identify any clinical conditions that may increase the risk of complications associated with neuraxial techniques is essential.[15] Other complications associated with epidural anesthesia include:

• Hypotension
• Nausea and vomiting
• Intravascular injection
• Local anesthetic systemic toxicity
• Bronchoconstriction
• Postdural puncture headache [31]
• Transient neurological syndrome 
• Nerve injury with possible neuropathy; paralysis is rare
• Epidural hematoma
• Epidural abscess
• Meningitis
• Accidental intrathecal injection with total spinal anesthesia
• Osteomyelitis [23]

Clinical Significance

The epidural technique is one of the earliest performed in the field of anesthesia. When correctly performed, epidural anesthesia is a safe technique that provides multiple benefits, including decreasing the need for general anesthesia, exposure to volatile anesthetics, and the associated risks. In addition, this technique can reduce the opioid requirement during and after a procedure, lowering the incidence of adverse effects associated with these medications. Epidural anesthesia is particularly relevant in the pediatric population, where the potential negative impact of certain anesthetic drugs on neurodevelopment is being actively researched. The epidural technique is also valuable for postoperative pain management as part of a multimodal approach.[2] A recent benefit of epidural anesthesia was providing an alternative to aerosol-generating general anesthesia, which was restricted during the COVID-19 pandemic.

Studies have critically examined the balance between the benefits and risks associated with epidural analgesia for postoperative pain management, concluding that while epidural analgesia offers superior pain relief and improved postoperative outcomes, it also carries inherent risks. Therefore, careful patient selection and close monitoring are essential to minimize complications.[4] Despite potential complications, neuraxial techniques, including epidural anesthesia, remain among the best opioid-sparing pain prevention and management options. These techniques can reduce patients' morbidity, improve outcomes, shorten hospital stays, and decrease the risk of chronic pain, ultimately providing economic benefits. Consequently, epidural anesthesia is a valuable technique for pain management, but it is not without risks. Careful patient selection, close monitoring, and timely interventions are essential to minimize the risk of anesthesia-induced SCI and associated complications. Further research and improvement of anesthesia protocols are necessary to enhance patient safety in neuraxial procedures.[15]

Enhancing Healthcare Team Outcomes

A growing body of evidence demonstrates that epidural anesthesia has positive effects, such as faster return in bowel function, decreased hormonal stress response to pain, decreased postoperative pulmonary complications, shorter length of in-hospital stay, and increased patient satisfaction. However, the safe and effective use of epidural anesthesia requires an interprofessional team approach, including surgeons, anesthesia clinicians, specialty-trained nurses, and pharmacists collaborating across disciplines to monitor and manage patients during the perioperative period.

Collaboration between interprofessional and interprofessional teams is essential in the choice of the locoregional anesthesia technique as the chosen technique must take into account various factors, in particular, the opinion of the patient, the surgeon, and the preferences of the anesthesiology clinician who must be able to perform the nerve block without difficulty for patient safety it is advisable to follow specific guidelines and protocols to conduct adequate regional anesthesia procedures. Furthermore, maintaining continuous closed-loop communication between all members of the perioperative care team regarding the need, technique, and potential management issues associated with the regional anesthetic utilized is essential to optimizing patient outcomes. In addition to obtaining thorough informed consent from either the patient or their authorized legal guardian before the placement of any regional anesthesia, all health team members are responsible for voicing their concerns as needed throughout the perioperative period based on their professional discretion.