13.2 Neuraxial Anesthesia Setup & Techniques

13.2 Neuraxial Anesthesia Setup & Techniques

Neuraxial anesthesia encompasses spinal (subarachnoid), epidural, combined spinal-epidural (CSE), and caudal blockade. These techniques deliver local anesthetics, opioids, and adjuvants directly adjacent to the spinal cord and exiting spinal nerve roots, producing intense sensory analgesia, autonomic sympathetic blockade, and variable motor blockade. Anesthesia technologists are responsible for preparing sterile neuraxial trays, selecting specialized spinal and epidural needles, verifying local anesthetic baricity, setting up loss-of-resistance syringes, and maintaining immediate readiness for hemodynamic emergencies such as high or total spinal collapse.

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Functional Neuraxial Anatomy & Surface Landmarks

Understanding the cross-sectional and sagittal anatomy of the vertebral column is foundational to safe neuraxial needle placement.

NEURAXIAL ANATOMY & LANDMARKS

  Spinal Cord Termination (Conus Medullaris):  Adult = L1-L2   |   Infant = L3
  Dural Sac Termination:                        Adult = S2      |   Infant = S3-S4

  TUFFIER'S INTERCRISTAL LINE (Connects Superior Margins of Iliac Crests):
  ====================== Crosses Midline at L4 Spine or L4-L5 ======================

  Safe Lumbar Puncture Interspaces: L3-L4 or L4-L5 (Strictly Caudal to Conus Medullaris)

Critical Anatomical Terminations

1. Spinal Cord (Conus Medullaris): In normal adults, the spinal cord terminates at the lower border of L1 or upper border of L2. In neonates and infants, the cord terminates lower, at L3, before ascending with growth. To avoid traumatic mechanical needle injury to the spinal cord, spinal needle puncture must always be performed caudal to the conus medullaris, typically at the L3-L4 or L4-L5 intervertebral spaces.
2. Dural Sac & Subarachnoid Space: The dural sac and subarachnoid space (lumbar cistern) terminate at the level of the S2 vertebra in adults (S3-S4 in infants).
3. Tuffier's Line (Intercristal Line): The primary surface landmark in neuraxial anesthesia is Tuffier's line, an imaginary transverse line drawn between the highest palpable points of the superior iliac crests. Tuffier's line intersects the vertebral column at either the L4 spinous process or the L4-L5 interspace.

Anatomical Layers Traversed (Midline Approach)

When a spinal or epidural needle is advanced along the midline, it traverses seven distinct tissue layers before entering the subarachnoid space:

[1. SKIN]
    |
[2. SUBCUTANEOUS ADIPOSE TISSUE]
    |
[3. SUPRASPINOUS LIGAMENT]  ---> Dense fibrous cord connecting tips of spinous processes
    |
[4. INTERSPINOUS LIGAMENT]  ---> Thin membranous ligament between adjacent spinous processes
    |
[5. LIGAMENTUM FLAVUM]     ---> "Yellow ligament"; tough, dense elastic fibers; gritty resistance
    |
[6. EPIDURAL SPACE]        ---> Potential space (fat, Batson's venous plexus, nerve roots)
    |
[7. DURAL-ARACHNOID SAC]   ---> Dura mater ("tough mother") fused to underlying arachnoid
    |
    v
[SUBARACHNOID SPACE]       ---> Lumbar cistern containing CSF & Cauda Equina fibers

• Ligamentum Flavum Characteristics: The ligamentum flavum consists of dense, longitudinally arranged yellow elastic fibers that bridge the interlaminar spaces. It offers firm, gritty mechanical resistance during needle advancement, providing the tactile barrier used to identify the epidural space.
• Paramedian Approach: In elderly patients with profound osteoarthritis, bridging osteophytes, or severely calcified supraspinous and interspinous ligaments, a paramedian approach is utilized. The needle enters 1.0 to 1.5 cm lateral to the superior aspect of the inferior spinous process and is directed 10-15° medially and cephalad. This trajectory bypasses the supraspinous and interspinous ligaments entirely, traversing only skin, subcutaneous fat, and paraspinous musculature before penetrating the lateral ligamentum flavum.

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Spinal Anesthesia: Needle Designs & Baricity Physics

Spinal anesthesia involves injecting small volumes (typically 1.5 to 4.0 mL) of local anesthetic directly into the cerebrospinal fluid (CSF) of the subarachnoid space.

SPINAL NEEDLE BEVEL GEOMETRIES

  1. QUINCKE (Cutting Bevel):
     ========================\       <--- Sharp cutting edge transects longitudinal dural
     ========================/            fibers; leaves patent dural hole (High PDPH rate)

  2. WHITACRE (Pencil-Point Non-Cutting):
     =======================(\.      <--- Conical solid tip spreads dural fibers;
     =======================("       <--- Lateral side-port injects local anesthetic
                                          Elastic fibers snap back (Low PDPH rate <1%)

Spinal Needle Design: Cutting vs. Pencil-Point

• Cutting Needles (Quincke-Babcock): Feature a sharp, beveled, knife-like tip. As the cutting edge penetrates the dura mater, it transects longitudinal dural and arachnoid fibers. This creates a persistent, patent defect that allows continuous leakage of CSF into the epidural space, resulting in a high incidence of Post-Dural Puncture Headache (PDPH rates of 10% to 30% with 20G-22G needles).
• Pencil-Point Non-Cutting Needles (Whitacre, Sprotte): Feature a solid, rounded conical tip resembling a pencil point, with a separate lateral injection orifice located 1 to 2 mm proximal to the tip. As the conical tip traverses the dura, it non-traumatically separates and spreads the longitudinal dural and elastic fibers rather than cutting them. Upon needle withdrawal, the stretched elastic fibers recoil and close the defect, dramatically reducing CSF leakage and lowering the PDPH rate to <1% to 2%.
• Introducer Needles: Because small-gauge pencil-point needles (24G, 25G, 27G) are highly flexible, they bend when contacting tough skin and supraspinous ligaments. An introducer needle (typically 20G, 1.25 inches long) is inserted first into the interspinous ligament to act as a rigid guide cannula through which the fine spinal needle is advanced.

Local Anesthetic Baricity Physics

Baricity is defined as the ratio of the density of a local anesthetic solution to the density of human cerebrospinal fluid at body temperature (37°C). Normal CSF density ranges from 1.0003 to 1.0008 g/mL.

$\text{Baricity} = \frac{\text{Density of Local Anesthetic at 37°C}}{\text{Density of Human CSF at 37°C}}$

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Epidural Anesthesia: Equipment, Mechanics & Test Dose

Epidural anesthesia involves identifying the potential space surrounding the dural sac and depositing larger volumes (10 to 20 mL) of local anesthetic to bathe spinal nerves as they exit through the intervertebral foramina.

EPIDURAL SETUP: TUOHY NEEDLE & LOSS-OF-RESISTANCE (LOR)

       Huber Curved Tip (30°)
             |\_____
             |      ====|  <--- 16G-18G Tuohy Needle with 1 cm depth markings
             |/-----

  Advancement through Interspinous Ligament & Ligamentum Flavum:
  - Dense mechanical resistance on LOR syringe plunger ("rebound bounce")

  Traversing into Epidural Space:
  - SUDDEN, COMPLETE LOSS OF RESISTANCE -> Plunger glides freely
  - Negative / subatmospheric pressure allows free saline/air injection

Tuohy Needle & Loss-of-Resistance (LOR)

• Tuohy Needle Design: Standard epidural needles are 16-gauge or 18-gauge, 3.5 inches (9 cm) in length, equipped with 1 cm depth markings along the shaft. The needle tip features a blunt, curved Huber point (30° curve). This curve serves two functions: it pushes the tough dura mater away rather than puncturing it, and it directs the flexible epidural catheter cephalad (or caudad) along the longitudinal axis of the epidural space.
• Loss-of-Resistance (LOR) Syringe: A specialized, ultra-low-friction glass or plastic syringe filled with 2 to 3 mL of preservative-free normal saline (with or without a 0.5 mL air bubble). When the needle tip is embedded within the dense fibrous matrix of the interspinous ligament and ligamentum flavum, pressure applied to the syringe plunger encounters unyielding mechanical resistance (the plunger bounces back). The instant the beveled Huber tip traverses the ligamentum flavum and enters the epidural space, resistance abruptly vanishes—the plunger advances effortlessly without resistance, confirming entry.
• Epidural Catheter Placement: The catheter (19G or 20G) is threaded through the Tuohy needle into the epidural space. Technologists must know the depth rule: the catheter is advanced 3 to 5 cm into the epidural space. Threading less than 3 cm increases the risk of dislodgement, while threading more than 5 cm increases the risk of catheter curling, knotting, unilateral blockade, or migration into an epidural vein or through the dura.

The Epidural Test Dose Protocol

Accidental intravascular injection (into the engorged epidural venous plexus) or accidental intrathecal injection (into the subarachnoid space) are life-threatening hazards of epidural placement. To detect catheter misplacement before therapeutic dosing, a standardized epidural test dose is administered:

+-----------------------------------------------------------------------------+
|                   THE EPIDURAL TEST DOSE FORMULATION                        |
|      3 mL of 1.5% Lidocaine with Epinephrine 1:200,000 (15 mcg Epi)         |
+-----------------------------------------------------------------------------+
                                      |
        +-----------------------------+-----------------------------+
        |                                                           |
        v (If Intravascular)                                        v (If Intrathecal / Subarachnoid)
  - Epinephrine bolus enters systemic veins                 - 45 mg Lidocaine deposited in CSF
  - Heart Rate rises ~10-20+ bpm within 30-60 s             - Rapid onset of dense spinal sensory block
  - Systolic BP rises about 15 mmHg or more                 - Rapid lower extremity motor paralysis
  - Patient reports palpitations / tinnitus                   within 2 to 4 minutes

• Intravascular Reaction: If the catheter tip resides in an epidural vein, the 15 mcg epinephrine bolus enters the systemic circulation, producing a prompt increase in heart rate (commonly about 10 to 20 beats per minute or more) or a rise in systolic blood pressure of about 15 mmHg or more within 30 to 60 seconds. The patient may also experience metallic taste, perioral tingling, or palpitations. (Caveat: In patients taking beta-blockers or laboring parturients with active contractions, heart rate changes may be blunted; a rise in systolic blood pressure of ≥15 mmHg or T-wave amplitude changes on the ECG serve as secondary indicators).
• Subarachnoid Reaction: If the catheter resides in the subarachnoid space, the 45 mg of lidocaine acts as a dense spinal anesthetic, producing warm lower-extremity numbness, sensory blockade, and complete motor block (inability to lift legs) within 2 to 4 minutes.

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Combined Spinal-Epidural (CSE) & Caudal Blockade

COMBINED SPINAL-EPIDURAL (CSE): NEEDLE-THROUGH-NEEDLE TECHNIQUE

  [16G Tuohy Needle in Epidural Space]
        \==========================
         \   [127 mm Spinal Needle Advanced through Tuohy Lumen]
          \  ====-----------------------------------------> Punctures Dura into CSF
           \==========================

  Sequence: 1. Confirm CSF in spinal needle -> 2. Inject spinal dose ->
            3. Remove spinal needle -> 4. Thread epidural catheter through Tuohy

• Combined Spinal-Epidural (CSE): Utilizes a specialized "needle-through-needle" kit. The 16G or 18G Tuohy needle is introduced into the epidural space using LOR. A long, small-gauge spinal needle (typically 25G-27G, 127 mm long) is advanced through the lumen of the Tuohy needle until its pencil-point tip punctures the dura. Once free-flowing clear CSF confirms subarachnoid entry, a rapid-acting spinal local anesthetic dose is administered. The spinal needle is withdrawn, and an epidural catheter is inserted through the Tuohy needle into the epidural space. This combines the rapid onset and sensory intensity of spinal anesthesia with the flexibility of a continuous epidural infusion for extended surgery and postoperative analgesia.
• Caudal Anesthesia: Caudal blockade accesses the epidural space via the sacral hiatus, an anatomical opening created by the failure of the S5 laminae to fuse in the midline. The hiatus is bounded laterally by the sacral cornua and roofed by the sacrococcygeal ligament. A 22G needle or angiocatheter is inserted at a 45° angle to penetrate the sacrococcygeal ligament (feeling a distinct "pop"), lowered to 20°, and advanced 1 to 2 cm into the sacral canal. Widely utilized in pediatric anesthesia for infraumbilical, inguinal hernia, and hypospadias surgery.

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Critical Neuraxial Complications: Total Spinal & PDPH

Total Spinal Anesthesia

Total spinal anesthesia is a catastrophic emergency resulting from the accidental injection of a large, epidural-volume local anesthetic dose (e.g., 10 to 20 mL) into the subarachnoid or subdural space.

TOTAL SPINAL PHYSIOLOGICAL PROGRESSION:

  High Thoracic Block (T1-T4)  -->  Intercostal paralysis; Loss of Cardioaccelerators
                                    - Severe Bradycardia & Profound Vasodilation / Shock
        |
        v
  Cervical Block (C3-C5)       -->  PHRENIC NERVE PARALYSIS
                                    - Acute Dyspnea, Inability to Phonate, APNEA
        |
        v
  Brainstem / Cranial Nerves   -->  Dilated, Unreactive Pupils; Loss of Consciousness

• Cardiovascular Collapse: Blockade of sympathetic preganglionic fibers from T1 to L2 eliminates systemic vascular resistance, precipitating massive venous pooling. Simultaneous blockade of the cardioaccelerator fibers (T1-T4) strips sympathetic tone from the sinoatrial and atrioventricular nodes, leaving unopposed vagal parasympathetic tone. The patient develops profound, life-threatening bradycardia and severe hypotension.
• Respiratory Arrest: Cephalad spread to C3, C4, and C5 paralyzes the phrenic nerves, halting diaphragmatic excursion. The patient complains of difficulty breathing, difficulty swallowing, and inability to speak above a whisper, progressing rapidly to complete apnea.
• Emergency Resuscitation Protocol:
  1. Airway & Breathing First: Immediately call for help, administer 100% FiO2 via bag-valve-mask, and perform immediate endotracheal intubation to secure the airway and prevent anoxic brain injury.
  2. Cardiovascular Support: Administer rapid intravenous crystalloid boluses through large-bore IV access to counter profound venous pooling.
  3. Vasoactive Pharmacotherapy: Mild hypotension/bradycardia may respond to Ephedrine (5 to 10 mg IV) or Phenylephrine (100 to 200 mcg IV). However, when high sympathetic blockade causes profound bradycardia, Epinephrine is the drug of choice (initial IV boluses of 10 to 50 mcg, escalating to full ACLS resuscitation if cardiac arrest ensues).

Post-Dural Puncture Headache (PDPH)

PDPH results from continuous CSF leakage through a dural defect exceeding the rate of choroid plexus CSF production (normal CSF production = 0.35 mL/min, ~500 mL/day).

• Pathophysiology: Loss of CSF volume reduces intracranial CSF pressure, causing the brain to sag downward in the cranium when upright. This exerts painful gravitational traction on pain-sensitive meninges, bridging veins, and cranial nerves (especially Cranial Nerve VI [Abducens], leading to lateral rectus palsy and diplopia, as well as CN VIII causing tinnitus and hearing loss).
• Clinical Presentation: Characteristically postural: a throbbing frontal or occipital headache that worsens within minutes (commonly within 15 minutes) of sitting or standing upright and is completely or substantially relieved by lying flat (supine).
• Definitive Treatment: Epidural Blood Patch (EBP): If conservative measures (bed rest, hydration, oral analgesics, caffeine benzoate 500 mg IV) fail after 24 to 48 hours, an Epidural Blood Patch is the definitive treatment, with high reported success rates. Under strict sterile conditions, an epidural needle is introduced into the epidural space at or one space below the prior dural puncture. An operator draws 15 to 20 mL of autologous sterile whole blood from the patient's antecubital vein and injects it slowly through the epidural needle. The injected blood spreads cephalad and caudad, forming a gelatinous fibrin clot over the dural defect to seal the CSF leak, while simultaneously compressing the thecal sac to immediately restore intracranial CSF pressure.