Spinal Anesthesia: Subarachnoid Block

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SAMPLE EXCERPT
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PRE-PROCEDURE
INDICATIONS
  • Operations below the umbilicus, such as hernia repairs, gynecologic or urologic procedures, and any lower extremity surgeries
  • Patients with congestive heart failure, except those with stenotic valvular heart disease or hypertrophic cardiomyopathy
  • Cesarean section
CONTRAINDICATIONS

Absolute Contraindications

  • Patient refusal or inability to cooperate and remain still
  • Coagulation defects, including intraoperative anticoagulation
  • Infection at the injection site

Other Contraindications

  • Sepsis
  • Neurologic disease, particularly involving the spinal cord, such as myelitis
  • Intracranial hypertension
  • Severe spinal or spinal cord deformity
  • Spinal cord tumor
  • Stenotic heart valve lesions
  • Severe hypertrophic cardiomyopathy
  • Lack of anesthesiologist experience
EQUIPMENT
  • Patient monitors: ECG, pulse oximeter, blood pressure cuff
  • Resuscitation equipment, including oxygen, bag and mask, and suction
  • Sterile gloves and mask: Some institutions may require sterile gown.
  • Patient must have intravenous access for intravenous fluid and medication delivery.
  • Sterile prep equipment: Betadine or non-iodine (for patients with iodine allergy) scrub
  • Spinal needle of small gauge (24-26 gauge)
  • Sterile drape with fenestration or sterile towel drapes
  • Local anesthetic for infiltration of the skin and subcutaneous tissues
  • Small (3-5 mL) syringe for local anesthetic for skin infiltration with small (25 or 30 gauge) needle
  • 3- to 5-mL syringe (usually glass) for spinal anesthetic agent, with markings for 0.2 mL increments on the barrel
  • Anesthetic for subarachnoid injection
  • Additives for subarachnoid injection, such as epinephrine, Duramorph, fentanyl
  • Bandage
ANATOMY
  • Spinal anesthetics have their effects at the spinal cord, which originates at the foramen magnum of the skull and the brainstem and extends caudally to the conus medullaris. The distal termination varies from about the level of the 3rd lumbar vertebrae (L3) in infants to the lower border of L1 in adults. The spinal cord is surrounded by three membranes (from central to peripheral): the pia mater, arachnoid mater, and dura mater. It is believed that the arachnoid mater is responsible for up to 90% of the resistance to drug migration in and out of the CSF. Inside the subarachnoid space are the CSF, spinal nerves, a network of trabeculae between the two membranes, and blood vessels supplying the spinal cord. Although the spinal cord ends at about L1 in adults, the subarachnoid space continues to about the second sacral vertebrae (S2).
  • Posterior to the epidural space is the ligamentum flavum, which extends from the foramen magnum to the sacral hiatus. Immediately posterior to the ligamentum flavum are the lamina and spinous processes of the vertebral bodies or the interspinous ligaments. Posterior to these structures is the supraspinous ligament, which joins the vertebral spines.
  • Anatomic landmarks most important to performance of spinal anesthesia are the iliac crests, the midline of the back, and the vertebral spinous processes. Palpation of the midline of the back identifies the spinous processes and vertebral interspaces in most patients but may be difficult in obese patients. A line drawn between the upper borders of the iliac crests across the midline of the back identifies the approximate level of L4 or the L4-L5 interspace.
  • Spinal anesthesia is usually performed at the level of the L3 or L4 vertebrae in the adult patient, because the spinal needle is introduced below the level at which the spinal cord ends.

PROCEDURE
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  • Links to medical evidence and related procedures

POST-PROCEDURE
CARE

Assess the Block

  • Sensory level
  • Motor blockade

Treat Problems with the Block

  • Block not high enough: Tilt the table head down or up, depending on baricity of the solution of anesthetic used.
  • Block too high: Treat hypotension, bradycardia, and support respirations. Sedation may be necessary for patient comfort.
  • Block is one-sided or not high enough on one side: Position the patient on the side likely to facilitate solidification of the block in areas where it is inadequate, depending on the baricity of the solution of anesthetic used and on the surgical site.
  • No block: Repeat the block if there is no motor or sensory block after waiting at least 10 minutes after injection.

Monitoring Post-Procedure

  • Blood pressure, pulse, and respiratory function (oxygen saturation as well as the quality and depth of respiratory efforts) must be monitored continually and frequently (every 2-5 minutes) after block injection for at least 20 to 30 minutes.

Sedation

  • Many patients want to be unaware of surgery, even if they have no sensation of it. Sedation is a common adjunct to spinal anesthesia.

Postsurgical Care

  • Care in the postanesthesia care unit (PACU) is focused on monitoring and maintaining hemodynamic and respiratory stability of the patient after surgery, and frequent assessment of block level should be documented.
  • Hypotension in the PACU is seldom due to increasing level of spinal blockade and should lead to a diligent search for other causes, such as postoperative hemorrhage, inadequate intraoperative fluid resuscitation, or postoperative myocardial ischemia.
  • Treat increasing discomfort from the surgical site as the spinal block recedes.
  • Consider the possibility of late respiratory depression after intrathecal narcotics, particularly in elderly patients or those with underlying respiratory compromise.
  • Patients should remain in bed until full sensation and motor strength have returned.
COMPLICATIONS

Early Complications

  • Hypotension
  • Bradycardia
  • Cardiac arrest
  • Total spinal block

Late Complications

  • Neurologic injury
  • Post–dural puncture headache
  • Backache
  • Infection
  • Urinary retention
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