Spinal Anesthesia – Anatomy & Physiology

Understanding the Spine: The Foundation of Spinal Anesthesia

Spinal anesthesia is a cornerstone technique in regional anesthesia, but its safe execution requires deep understanding of spinal anatomy, physiological principles, and appropriate patient selection. Let's explore the critical anatomical structures, physiological principles, and clinical applications that form the basis of this technique.

Key Anatomical Landmarks

  • Vertebral Levels: The spinal cord typically ends at L1-L2 in adults, making the lower lumbar region (L3-L4, L4-L5) the safest for needle insertion.
  • Ligamentous Structures:
    • Supraspinous ligament
    • Interspinous ligament
    • Ligamentum flavum (key tactile landmark)
    • Dura mater
    • Arachnoid mater
  • Subarachnoid Space: Contains cerebrospinal fluid (CSF) and spinal nerve roots. Target for local anesthetic injection.

Physiological Principles

  1. CSF Dynamics:
    • Volume: 120-150 mL in adults
    • Production: 0.35 mL/min (500 mL/day)
    • Specific gravity: 1.0003-1.0008 at 37°C
  2. Local Anesthetic Spread:
    • Baricity (density relative to CSF) determines spread
    • Hyperbaric solutions spread with gravity
    • Isobaric solutions spread more predictably
    • Hypobaric solutions rise against gravity
  3. Physiological Effects:
    • Sympathetic blockade (T1-L2): Vasodilation, hypotension
    • Sensory blockade: Loss of pain/temperature sensation
    • Motor blockade: Loss of motor function

Clinical Implications

Understanding these fundamentals helps predict:

  • Onset time of blockade
  • Level of anesthesia achieved
  • Duration of effect
  • Hemodynamic consequences

Indications for Spinal Anesthesia

Spinal anesthesia is particularly suitable for:

  1. Lower Abdominal and Pelvic Surgery:
    • Hernia repairs
    • Prostatectomies (TURP)
    • Hysterectomies
    • Cesarean sections
    • Bladder procedures
  2. Lower Extremity Surgery:
    • Knee arthroscopy
    • Hip replacements
    • Amputations below umbilicus
    • Vascular procedures on legs
  3. Perineal and Anorectal Surgery:
    • Hemorrhoidectomy
    • Fistula repairs
    • Anal sphincter procedures
  4. Special Patient Populations:
    • Patients with respiratory compromise (COPD, asthma)
    • Patients at risk for malignant hyperthermia
    • Outpatient surgery (faster recovery)
    • Patients with difficult airways

Contraindications for Spinal Anesthesia

Proper patient selection is crucial for safe spinal anesthesia:

Absolute Contraindications:

  • Patient Refusal: Lack of informed consent
  • Infection at Injection Site: Cellulitis, abscess, or sepsis
  • Coagulopathy:
    • INR >1.5
    • Platelets <75,000/mm³
    • Thrombocytopenia or bleeding disorders
    • Therapeutic anticoagulation (warfarin, heparin, DOACs)
  • Increased Intracranial Pressure: Risk of brain herniation
  • Severe Hypovolemia: Uncontrolled hemorrhage or dehydration

Relative Contraindications (Require Risk-Benefit Analysis):

  • Spinal Deformities:
    • Scoliosis
    • Previous spinal surgery
    • Ankylosing spondylitis
  • Neurological Disorders:
    • Pre-existing neuropathy (diabetic, alcoholic)
    • Multiple sclerosis
    • ALS
  • Cardiovascular Disease:
    • Severe aortic stenosis
    • Fixed cardiac output states
    • Severe coronary artery disease
  • Special Considerations:
    • Pregnancy (higher risk of PDPH)
    • Elderly patients (more hemodynamic instability)
    • Chronic back pain
    • Psychiatric disorders (severe anxiety, claustrophobia)

Special Considerations in Anticoagulated Patients

Follow ASRA guidelines for neuraxial procedures:

  • Warfarin: Stop 4-5 days prior, INR normal
  • LMWH:
    • Prophylactic dose: Wait 12 hours
    • Therapeutic dose: Wait 24 hours
  • Fondaparinux: Wait 42-48 hours
  • Antiplatelets:
    • Aspirin/NSAIDs: No delay needed
    • Clopidogrel: Stop 7 days prior
    • Ticagrelor/Prasugrel: Stop 5-7 days prior

Next Steps

Ready to apply this knowledge? Check out our Step-by-Step Guide to Performing Spinal Anesthesia for practical application. And don't miss our Complications and Management guide to complete your understanding.

Key References

  1. Neal JM, et al. ASRA Practice Advisory on Neurologic Complications in Regional Anesthesia and Pain Medicine. Reg Anesth Pain Med. 2015
  2. Griffin RP, et al. Local anesthetic pharmacology. Continuing Education in Anaesthesia Critical Care & Pain. 2005
  3. Horlocker TT, et al. Regional Anesthesia in the Patient Receiving Antithrombotic or Thrombolytic Therapy. ASRA Guidelines. 2018

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