Neuraxial Anesthesia and Spinal Anatomy

Neuraxial Anesthesia

• Studies suggest that postoperative morbidity and mortality may be reduced when neuraxial blockade is used alone or with general anesthesia.
• Neuraxial blocks may reduce complications such as venous thrombosis, pulmonary embolism, cardiac issues in high-risk patients, bleeding, transfusion requirements, vascular graft occlusion, pneumonia, and respiratory depression following upper abdominal or thoracic surgery in patients with chronic lung disease.

Spinal Column and Meninges

• The spinal column forms a double C shape, being convex anteriorly in the cervical and lumbar regions.
• The meninges consist of three layers: pia, arachnoid, and dura mater.
• Cerebrospinal fluid (CSF) is found between the pia and arachnoid mater in the subarachnoid space.
• The epidural space is a potential space within the spinal canal, bounded by the dura and the ligamentum flavum.

Spinal Cord Extent

• The spinal cord in adults extends from the foramen magnum to the level of L1.
• In children, the spinal cord ends at L3 and moves upward with age.

Neuraxial Blockade Mechanism

• The principal site of action for neuraxial blockade is the nerve root.
• Local anesthetic is injected into CSF (spinal anesthesia) or the epidural space (epidural and caudal anesthesia), blocking nerve roots in the subarachnoid or epidural space, respectively.
• Direct injection of local anesthetic into CSF for spinal anesthesia allows for a relatively small dose and volume to achieve dense sensory and motor blockade.
• The same local anesthetic concentration within nerve roots is achieved with much larger volumes during epidural and caudal anesthesia.

Indications for Neuraxial Blocks

• Neuraxial blocks may be used alone or in conjunction with general anesthesia for procedures below the neck.
• Neuraxial blocks are useful for lower abdominal, inguinal, urogenital, rectal, and lower extremity surgery.
• Lumbar spinal surgery can be performed under spinal anesthesia.
• Although upper abdominal procedures (like gastrectomy) have been performed with spinal or epidural anesthesia, these techniques aren't commonly used due to difficulty achieving a sufficient sensory level for patient comfort.

Types of Blocks: Somatic

• Sensory blockade generally affects both somatic and visceral pain stimuli.
• Differential blockade often leads to sympathetic blockade (influenced by temperature sensitivity) at a higher level than sensory (pain, light touch) blockade, which is in turn usually higher than motor blockade.

Types of Blocks: Autonomic

• Interruption of efferent autonomic transmission during neuraxial blocks results in sympathetic blockade.
• Sympathetic nerve fibers exit the spinal cord via spinal nerves from T1 to L2.
• Neuraxial anesthesia does not block the vagus nerve, the main parasympathetic supply.
• Physiological responses to neuraxial blockade result from decreased sympathetic tone and/or unopposed parasympathetic tone.
• Neuraxial blocks may cause variable decreases in blood pressure and heart rate.
• A high sympathetic block can prevent compensatory vasoconstriction and also block the sympathetic cardiac accelerator fibers that arise in T1-T4.
• Hypotension may result from arterial dilation, venous pooling, and bradycardia.
• Sudden cardiac arrest, sometimes seen during spinal anesthesia, might be related to unopposed vagal tone.
• Many factors (hydration, volume depletion, pregnancy) should be considered to minimize hypotension.
• Excessive or symptomatic bradycardia is treated with atropine; hypotension with vasopressors. Ephedrine increases heart rate/contractility and produces vasoconstriction.
• Surgical trauma can produce a systemic neuroendocrine response featuring increased ACTH, cortisol, epinephrine, norepinephrine, vasopressin, and renin-angiotensin-aldosterone system activation.
• Clinical manifestations can include intraoperative/postoperative hypertension, tachycardia, hyperglycemia, and altered renal function.
• Neuraxial blockade can partially or completely suppress the neuroendocrine stress response.
• Neuraxial blocks can lead to a small, contracted gut with active peristalsis.
• Postoperative epidural analgesia with local anesthetics and minimal systemic opioids can improve gastrointestinal function after open abdominal surgery.
• Autonomic blockade can cause urinary retention until the block wears off.
• If not using a urinary catheter, use the shortest duration regional anesthetic practical for the procedure and give the minimum safe volume of intravenous fluid.

Contraindications

• Major contraindications to neuraxial anesthesia include patient refusal, bleeding tendencies, severe hypovolemia, elevated intracranial pressure, and infection at the injection site.
• Other relative contraindications include severe valvular heart disease, sepsis and bacteremia (potentially spreading infectious agents into the epidural or subarachnoid space), and preexisting neurological deficits (which may worsen post-block).
• Patients with dementia, psychosis, or emotional instability may also be at risk.

Anticoagulants & Antiplatelets

• Documenting a normal prothrombin time (PT) and INR for oral anticoagulants (like Warfarin) is crucial before neuraxial blockade.
• Aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) do not usually increase the risk of spinal hematoma.
• More potent antiplatelet agents (like Clopidogrel or Ticlopidine) should be discontinued, and neuraxial blockade administered only after the drug effects have fully worn off.
  • For (Ticlid), that should be 14 days before the procedure.
  • For (Plavix), that should be 7 days or more before the procedure.
• Prophylactic "minidose" subcutaneous heparin is not a contraindication to neuraxial anesthesia.
• Neuraxial anesthesia should be avoided in patients on therapeutic doses of heparin and with elevated partial thromboplastin time (PTT).
• For patients on LMWH, neuraxial anesthesia is preferable 12 hours after the last dose.

Technical Considerations

• Neuraxial blocks should only be performed in facilities prepared for intubation, resuscitation, and general anesthesia with readily available equipment and drugs.
• Standard monitoring should be implemented.
• Non-pharmacological patient preparation (explaining the procedure) can help minimize anxiety.
• Use supplemental oxygen with a face mask or nasal cannula as needed to prevent hypoxemia during sedation.
• Strict adherence to sterile techniques is essential.

Surface Anatomy

• Spinous processes are typically palpable and help define the midline.
• The most prominent cervical spinous process is C7.
• The spinous process of T7 is level with the inferior angle of the scapula.
• A line between the highest points of both iliac crests generally crosses the body of L4 or the L4-L5 interspace.

Patient Positioning

• Sitting: Anatomical midline is easier to establish in a sitting position, particularly with obese patients. Patients sit with elbows on thighs or the side of the bed, or resting on a pillow to maximize the "target" spinal area for the procedure.
• Lateral Decubitus: Many clinicians prefer a lateral position with knees flexed and pulled up towards the chest (fetal position). An assistant can be helpful in ensuring stable positioning.
• Buie's (Jackknife): This position is used for anorectal procedures. Patient positioning is the same as the surgical procedure (no moving between procedures). Disadvantage is that CSF flow might be impaired, so subarachnoid needle placement needs to be verified by aspirating CSF.

Spinal Anesthesia

• Spinal anesthetic solutions inhibit nerve conduction as they traverse the subarachnoid space.
• During procedure, the needle is advanced from the skin until two "pops" are felt during the procedure.
• The first pop signifies penetration of the ligamentum flavum, while the second signifies entry into the dura-arachnoid membrane.
• Penetration into the dura-arachnoid membrane is confirmed by verifying free flow of CSF after removing the stylet.

Spinal Needles

• Spinal needles come in various sizes, lengths, and designs (bevel/tip).
• All needles have a removable stylet that prevents tracking in the subarachnoid space.
• Needles are divided into sharp or blunt types (Quincke and pencil tip).
• Needle design selection (gauge and tip) can influence post-procedure headache risk.

Factors Influencing Spinal Block Level

• Local anesthetic solution baricity influences its migration in the CSF (higher density, higher cephalad migration).
• Patient position during and after injection influences the spread of the local anesthetic within the CSF (head-down to cephalad, lateral to non-dependent side).
• Drug dosage determines the cephalad level of anesthesia.
• Other factors such as weight, height, age, pregnancy, and injection site can also affect the spinal block level.
• Sitting position can result in "saddle-block" anesthesia, aiming for blocking nerves only in the lower lumbar and sacral regions.

Spinal Anesthetic Agents

• Preservative-free local anesthetic solutions are used.
• Adding vasoconstrictors (like epinephrine) and opioid analgesics can enhance and prolong the effects of spinal anesthesia.
• Hyperbaric bupivacaine and tetracaine are common agents with slower onset and prolonged duration.
• Lidocaine and procaine offer relatively rapid onset and shorter lasting effects.

Epidural Anesthesia

• Continuous epidural anesthesia is a broader application technique than typical single-dose spinal anesthesia.
• Epidural blocks can be performed at lumbar, thoracic, or cervical levels.
• Sacral epidural anesthesia is a caudal block.
• Indications include surgical anesthesia, obstetric analgesia, postoperative pain control, and chronic pain management.
• Epidural blocks can be single-shot or catheter-assisted with bolus or continuous infusions.
• Epidurals have a slower onset compared to spinal anesthesia and may not achieve as profound blockade.

Epidural Needle and Catheter

• Typical epidural needles are 17-18 gauge, approximately 3-3.5 inches long, with a blunt bevel and a gentle 15-30° curve at its tip (Tuohy needle).
• The blunt tip helps propel the needle through the ligamentum flavum without penetrating the dura mater during insertion.
• Catheters in the epidural space facilitate continuous infusion.

Toxic Epidural Effects and Mitigation

• Severe side effects are likely if a "full dose" of epidural anesthetic is administered intrathecally or intravascularly.
• Safeguards against these side effects include using test doses and administering the local anesthetic in increments.
• Test doses containing a local anesthetic and epinephrine are commonly used (to determine whether subarachnoid space was reached).
• Thorough aspiration before injection helps prevent intravenous injection.
• Incremental dosing is a strategy to prevent severe complications.

Factors Affecting Epidural Block Level

• Factors influence epidural anesthesia level less predictably than spinal anesthesia.
• Guidelines suggest 1-2 mL of local anesthetic per segment for adult patients. Achieving a particular level (such as T4 sensory) requires determining the correct volume of local anesthetic.
• Additives (particularly opioids) influence epidural anesthesia quality rather than duration.

Epidural Anesthetic Agents

• Common short- to medium-acting agents include chloroprocaine, lidocaine, and mepivacaine for surgical anesthesia.
• Longer-acting agents include bupivacaine, levobupivacaine, and ropivacaine.
• Preservative-free local anesthetics are essential.

Caudal Anesthesia

• Caudal anesthesia is the sacral portion of the epidural space.
• Popular in pediatric patients and anorectal surgery for adults.
• Often combined with general anesthesia (in children).
• Used for procedures below the diaphragm (urogenital, rectal, inguinal, lower extremity surgeries).

Sacral Hiatus

• The sacral hiatus can be located as a notch above the coccyx.
• It is between two bony prominences, the sacral cornua.

This quiz covers essential concepts related to neuraxial anesthesia, including its benefits in reducing postoperative complications. Additionally, it explores the anatomy of the spinal column and meninges, describing the spinal cord's extent and its protective layers.