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ANATOMIC BOUNDARIES

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Epidural anesthesia is typically implemented in the clinical realms of surgery, obstetrics, and in the subspecialty of pain management. The epidural space is considered a potential space, which is filled with nerve roots, blood vessels, lymphatic vessels, and fat. The anatomic boundaries of the epidural space are:

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  • foramen magnum (rostrally)

  • sacrococcygeal ligament (caudally)

  • posterior longitudinal ligament (anteriorly)

  • ligamentum flavum and vertebral lamina (posteriorly)

  • vertebral pedicles (laterally)

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The epidural space varies in width from the cervical to lumbar region and is 2-3 mm wide at C3-C6, 3-5 mm wide in the thoracic spine, and widest (5-6 mm) in the lumbar spine.

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MECHANISM OF ACTION

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In the epidural space, the primary site of action for local anesthetics is the spinal nerve roots. Sodium channel blockade occurs in the dural sleeve, the region where nerves travel through the intervertebral foramen. Secondary and minimal influence occurs from diffusion of local anesthetic from the epidural space into the subarachnoid space, which invariably affects the nerve roots and spinal cord tracts.

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With implementation of a successful epidural anesthetic, several physiologic changes occur in the order of sympathectomy first, then sensory blockade, and finally motor blockade. Sympathectomy is induced by epidural anesthesia, which may lead to profound hypotension in individuals predisposed to reduced preload. A reduction in afterload also contributes to hypotension. In response to decreased systemic vascular resistance, tachycardia has been well documented. Upper thoracic sympathectomy (T1-T4) inhibits cardioaccelerator fibers, thus causing decreased cardiac contractility and heart rate. Sympathectomy at T8 and above may inhibit sympathetic afferent neurons to the adrenal medulla leading to a decreased stress response. With phrenic nerve paralysis (C3-C5), ventilation and airway protection may be compromised. However, cranial nerves are unaffected because the foramen magnum serves as the rostral boundary of the epidural space. It is important to differentiate between high epidural blockade and a total spinal anesthetic. With the latter, oculomotor nerve function is compromised as pupillary dilation is present accompanied by an absent light reflex.

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Other manifestations of sympathectomy include increased bowel motility and contraction, urinary retention, and increased propensity for decreased core body temperature secondary to peripheral vasodilation if external warming measures are ignored. Advantages of sympathectomy include decreased probability of ileus from unopposed parasympathetic tone and decreased blood loss during procedure from hypotension. Disadvantages include increased risk for decreased perfusion, resulting in ischemia to vital organs (brain—stroke, spinal cord—myelopathy, heart—myocardial infarction).

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CONTRAINDICATIONS

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Absolute contraindications to epidural anesthetics are patient refusal, sepsis with hemodynamic instability, hypovolemia, and coagulopathy. Once a patient has been properly informed about regional anesthesia and subsequently expresses disapproval, avoid further attempts to convince. Sepsis with hemodynamic instability presents a vasodilated patient at baseline predisposed to further reductions in systemic vascular resistance and afterload with administration of local anesthetics. Epidural anesthesia may contribute to hemodynamic instability; sepsis increases the possibility of ...

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