Spinal Anesthesia

Spinal Basics & Anatomy - Backbone Block Blueprint

• Definition: Local anesthetic (LA) injection into subarachnoid space (intrathecal).
• Mechanism: Blocks nerve roots in CSF → sensory, motor, sympathetic blockade.
• Pros: Rapid onset, dense block. Cons: Hypotension, Post-Dural Puncture Headache (PDPH).
• Key Layers Pierced (Midline Approach):
  • Skin
  • Subcutaneous tissue
  • Supraspinous ligament
  • Interspinous ligament
  • Ligamentum flavum
  • Dura mater
  • Arachnoid mater 📌 Mnemonic: Skinny Sailors Seek Islands Like Distant Atolls.
• Target Space: Subarachnoid space (contains Cerebrospinal Fluid - CSF: clear, colorless).
• Vertebral Level (Adults): L3-L4 or L4-L5 interspace. (Spinal cord typically ends L1-L2).

  ⭐ Tuffier's line, connecting the highest points of iliac crests, typically crosses the L4 vertebral body or L4-L5 interspace. oka

Spinal Pharmacology - Potions & Properties

• Local Anesthetics (LAs): Primarily amides.
  • Bupivacaine (0.5% Heavy): 7.5-15 mg (1.5-3 mL). Long duration.
  • Lidocaine (5% Heavy): 50-100 mg. Fast onset. ⚠️ Risk of Transient Neurological Symptoms (TNS).
  • Ropivacaine: Similar to bupivacaine, less cardiotoxic.
• Key LA Properties:
  • Baricity: Governs CSF spread.
    • Hyperbaric (LA + Dextrose): Sinks. Most common.
    • Isobaric (LA in Saline): Limited spread by injection volume.
    • Hypobaric (LA in Distilled Water): Floats.
  • $pK_a$: Affects onset (lower $pK_a$ closer to physiological pH = faster onset).
  • Lipid Solubility: ↑ solubility = ↑ potency & ↑ duration.
• Adjuvants: Enhance block quality & duration.
  • Opioids: Fentanyl (10-25 mcg), Morphine PF (100-200 mcg). Enhance analgesia.
  • Clonidine: (15-75 mcg). Prolongs block, provides sedation.

⭐ Bupivacaine is the most commonly used spinal anesthetic due to its long duration of action and dense sensory block, making it suitable for a wide range of surgical procedures.

Spinal Technique & Block Management - Perfecting the Poke

• Prep: IV access, monitors (ECG, NIBP, SpO2), consent.

• Positioning: Sitting or lateral decubitus (L3-L4, L4-L5).

• Needles: Quincke (cutting), Whitacre/Sprotte (pencil-point, ↓PDPH). Sizes: 22-27G.

• Procedure:

• Factors Affecting Block Height:

  • Major: Baricity (hyperbaric > isobaric > hypobaric), patient position post-injection, LA dose/volume, injection site.
  • Minor: CSF volume, needle bevel direction, speed of injection, age, pregnancy.
  • 📌 Mnemonic: Baricity, Position, Dose, Site (BPDS - "Block Placement Determines Spread")

• Physiological Effects:

  • CVS: Sympathetic block (T1-T4) → ↓BP, ↓HR. Treat hypotension with fluids, vasopressors (phenylephrine, ephedrine).
  • Resp: Usually minimal. High block (>T4) → intercostal paralysis. Apnea if brainstem hypoperfusion.

⭐ PDPH risk is significantly reduced by using smaller gauge, pencil-point needles (e.g., Whitacre 25-27G) and orienting the bevel of cutting needles parallel to dural fibers.

Spinal Complications & Troubleshooting - Navigating Pitfalls

• Immediate:
  • Hypotension (most common): IV fluids, vasopressors (phenylephrine 50-100 mcg; ephedrine 5-10 mg).
  • Bradycardia: Atropine 0.4-0.6 mg IV.
  • High/Total Spinal: Airway support, O₂, cardiovascular support.
  • Nausea/Vomiting.
• Early (days):
  • PDPH: Postural headache. Conservative (fluids, analgesics, caffeine). Epidural Blood Patch (EBP) if severe. 📌 PDPH: Posture Dependent Painful Headache.
  • Urinary Retention: Monitor; catheterize if needed.
  • Transient Neurological Symptoms (TNS).
• Late (weeks+):
  • Neurological Injury: Direct trauma, hematoma, infection (meningitis, abscess).
  • Cauda Equina Syndrome (CES): ⚠️ Urgent MRI! Bowel/bladder dysfunction, saddle anesthesia.
  • Arachnoiditis.

⭐ Using smaller gauge (e.g., 25-27G) pencil-point needles (Whitacre, Sprotte) significantly ↓ PDPH risk compared to cutting needles (Quincke).

High‑Yield Points - ⚡ Biggest Takeaways

• Spinal anesthesia involves injection into the subarachnoid space, usually at L3-L4 or L4-L5.
• Hypotension is the most frequent complication from sympathetic blockade.
• Post-dural puncture headache (PDPH) risk is reduced by atraumatic (pencil-point) needles.
• Key absolute contraindications: Local infection, raised ICP, severe hypovolemia, coagulopathy.
• Baricity of local anesthetic (e.g., hyperbaric bupivacaine) governs its spread.
• Sequence of nerve fiber blockade: Sympathetic (B) → Sensory (Aδ, C) → Motor (Aα).