Central Neuroaxial Block

Neuraxial anesthesia pertains to local anesthetics placed around the nerves of the central nervous system to produce anesthesia sufficient for surgical procedures to be performed. Examples include:

• Spinal anesthesia (also called subarachnoid anesthesia)
• Epidural anesthesia (cervical, thoracic, lumbar, sacral)

It can also produce highly effective pain relief for a wide variety of indications and requires adequate knowledge of the relevant anatomy, physiology, and pharmacology.

The spinal cord runs along a canal formed by successive vertebrae, their intervertebral discs, and ligaments. The canal extends from the medulla oblongata to the coccyx.

Contents of Epidural Space

• Venous plexuses
• Fatty tissue continuous with the fat in the paravertebral space
• Lymphatics
• Arteries
• Loose areolar connective tissue
• Spinal nerve roots

A formal pre-anaesthetic assessment should be carried out.

• The back should be examined, and any lesions or abnormalities noted.
• Special attention should be given to the patient’s cardiovascular status, with emphasis on:
• Valvular lesions
• Other conditions that might impair the ability to increase cardiac output in response to the vasodilation that inevitably follows sympathetic blockade.
• Laboratory assessment of the patient’s coagulation status is necessary where there is any doubt regarding coagulopathy or anticoagulation therapy.
• Inform the patient about possible risks and complications associated with Neuraxial anaesthesia.
• Obtain informed consent prior to performing the block.
• A work area that is equipped for airway management and resuscitation (oxygen, equipment, and drugs).
• Facilities for monitoring blood pressure and heart rate must be available.
• Procedure table that can be adjusted for height or Trendelenburg positioning.
• Cushions and pillows for support and pressure point padding.
• Step stool to support the legs.
• Check all equipment.
• Apply standard monitors.
• Establish Intravenous access, preferably with a large bore cannula (e.g. 16G).

Pre-Loading

• Given via a large bore cannula immediately before the block.
• Purpose: to increase blood volume ahead of vasodilatation caused by sympathetic blockade.
• Volume of fluid given varies with the age and the extent of the proposed block.
• Fluid:
  • Preferably normal saline or Hartmann’s solution.
  • 5% dextrose is readily metabolized and so is not effective in maintaining blood pressure.

• Position:
  • Sitting or
  • Lateral decubitus position
  • Prone for block in chronic pain management
• Patient should be encouraged to adopt a curled up position.
• Locating the level:
  • An imaginary line joining the two iliac crests crosses over the spinous process of L4.

Antiseptic Procedures

• The operator (Anaesthetist):
  • Should take full sterile precautions
  • Thorough hand washing with surgical scrub solution
  • Use of barrier precautions:
    • Wearing of a cap, mask, sterile gown and gloves
    • Use of a large sterile drape.
• The patient:
  • Prepare the skin with alcohol or iodine-containing sterilizing solution (Chlorhexidine in alcohol or povidone)
  • Drape the back in a sterile fashion.
• Select the desired level and infiltrate the site with local anesthetic.

Structures that Needle will pass through in CNB

Spinal anaesthesia

1. Skin
2. Subcutaneous fat
3. Supraspinous ligament
4. Interspinous ligament
5. Ligamentum flavum
6. Epidural space
7. Dura
8. Subarachnoid matter
9. Subarachnoid space

Epidural anaesthesia

1. Skin
2. Subcutaneous fat
3. Supraspinous ligament
4. Interspinous ligament
5. Ligamentum flavum
6. Epidural space
7. Dura

Technique of SAB

• The spinal cord usually ends at the level of L1 in adults and L3 in children. However, in 10% of adults, the cord terminates lower than L1/2.
• Dural puncture above these levels is associated with a slight risk of damaging the spinal cord and is best avoided.
• Any level in the Lumbar region below the termination of spinal cord can be used.
• An important landmark to remember is that a line joining the top of the iliac crests is at L4 to L4/5.
• In 50% of cases, the level of the chosen space may be incorrectly identified.
• Advisable not to insert a spinal needle any higher than the L3/4 interspace.

Lateral Position

Males tend to have wider shoulders than hips and so are in a slight “head up” position when lying on their sides, whilst for females with their wider hips, the opposite is true.

Sitting Position

Injecting a small volume of local anesthetic under the skin with a disposable 25-gauge needle at the proposed puncture site.

Confirmation of Correct Needle Placement: CSF Return

• Clear
• Free flow
• Aspiration into syringe
• Fluid Test:
• Litmus Paper
• Urine dip stick
• Temperature

Technique of Paramedian Approach to SAB

Indications:

1. Difficult anatomy
2. Narrowed interlamina space
3. Narrowed interspinous space
4. Morbidly obese patients
5. Calcified or ossified ligaments – as in the elderly
6. Inability to adequately flex due to pain

Needle Path:

Skin >> superficial fascia >> fat >> erector spinae muscle >> ligamentum flavum >> subarachnoid space

Advantages:

1. Larger target through the interlaminar space
2. Avoidance of the supraspinous and interspinous ligaments
3. Fewer attempts at needle insertion
4. Lower incidence of PDPH
• Can be performed in the neural spine position. Flexing the patient is not necessary using this approach
5. Less painful and ideal in elderly patients with calcified ligaments

Taylor Paramedian Approach to SAB

The Taylor, or lumbosacral approach to spinal anaesthesia is a paramedian approach directed toward the L5-S1 interspace. It is used when other approaches are not successful or cannot be performed, typically due to calcification or fusion of higher intervertebral spaces.

Indications:

• Calcified or fusion of higher intervertebral spaces

Needle Placement:

The needle should be inserted at a point 1cm medial and inferior to the posterior superior iliac spine, then angled cephalad 45° - 55° and medially.

Technique for Confirming Correct Needle Placement

Identifying the Epidural Space

1. Loss of resistance technique
   • To air
   • To liquid (saline or local anaesthetic)
2. Hanging drop technique
3. Saline infusion technique (Compression of a small air bubble in saline)
4. Macintosh epidural balloon
5. Ultrasonography guided identification of epidural space

L.A (Amides)

• Bupivacaine
• Lidocaine
• Ropivacaine
• Mepivacaine
• Prilocaine
• Cinchocaine

L.A. (Esters)

• Chloroprocaine cocaine
• Novocaine
• Tetracaine
• Amethocaine

Additives to Local Anaesthetics

1. Epinephrine (or phenylephrine):
   • Action: causes vasoconstriction
   • Decreased absorption
   • Increases neuronal uptake

   Effects:

   • Prolong duration of block
   • Increase intensity of block
   • Decrease systemic absorption, thus limiting toxic side effects
2. Analgesics:

   A. Opioids: e.g.

   • Fentanyl
   • Morphine
   • Tramadol

   B. Non-Opioids: e.g.

   • Clonidine
   • Midazolam
   • Neostigmine

Three classes of nerves:

• Autonomic: control the caliber of blood vessels, heart rate, gut contraction, and other functions not under conscious control.
• Sensory: transmit sensations such as touch and pain to the spinal cord and from there to the brain.
• Motor: when they are blocked, muscle paralysis results.

Autonomic and sensory (Pain, Temperature, Touch, Pressure) fibers are blocked first and Motor fibers last.

Order of Blockade:

1. Pain
2. Temperature (cold, warmth)
3. Touch
4. Deep pressure
5. Motor function

Assessing the Block:

1. Patient unable to lift his legs from the bed.
   → Block is at least up to the mid-lumbar region.
2. Better to test for the loss of temperature sensation using a swab soaked in either ether or alcohol.

Local Anaesthetic Solution injected into the subarachnoid or Epidural space blocks conduction of impulses along all nerves with which it comes in contact.

Effects:

1. Cardiovascular System
2. Respiratory System
3. Other Organs

Effects on Cardiovascular System

• Vasodilation of resistance and capacitance vessels → Relative hypovolaemia and tachycardia, with a resultant drop in blood pressure.
• Exacerbated by blockade of the sympathetic nerve supply to the adrenal glands, preventing the release of catecholamines.
• If blockade is as high as T2, sympathetic supply to the heart (T2-5) is also interrupted and may lead to bradycardia.
• The overall result may be inadequate perfusion of vital organs.

Respiratory System

• Usually unaffected unless blockade is high enough to affect intercostal muscle nerve supply (thoracic nerve roots) leading to reliance on diaphragmatic breathing alone.
• High Spinal
• Decrease functional residual capacity (FRC)
• Paralysis of abdominal muscles.
• Intercostal muscle paralysis interferes with coughing and clearing secretions.
• Apnea is due to hypoperfusion of the respiratory center.
• This is likely to cause distress to the patient, as they may feel unable to breathe adequately.

Gastrointestinal System

• Blockade of sympathetic outflow (T5 – L1) to the GI
• Predominance of parasympathetic
• Active peristalsis and relaxed sphincters, and a small, contracted gut, which enhances surgical access.
• Splenic enlargement (2-3 fold) occurs.

Endocrine System

• Nerve supply to the adrenals is blocked leading to a reduction in the release of catecholamines.

Genitourinary Tract

• Urinary retention is a common problem with epidural anaesthesia.
• A severe drop in blood pressure may affect glomerular filtration in the kidney if sympathetic blockade extends high enough to cause significant vasodilatation.

It is essential to monitor:

1. ECG trace
2. Heart rate
3. Arterial blood pressure
4. Respiratory pattern
5. Arterial SpO2 (Pulse oximeter)
6. Level of consciousness

Resuscitation Equipment & Medications to be made available in Block Area

Equipment

1. Oxygen supply, nasal airway, and O2 masks.
2. Oral airways of different sizes, laryngeal masks, and endotracheal tubes
3. Laryngoscopes
4. Bag-mask ventilation device
5. Suction machine & catheters
6. Various size intravenous cannulas
7. Equipment to monitor blood pressure, heart rate, pulse oximeter, capograph
8. Defibrillator

Resuscitation Drugs

1. Atropine.
2. Epinephrine
3. Suxamethonium
4. Ephedrine
5. Phenylephrine
6. Glycopyrrolate
7. Diazepam/Midazolam
8. Intralipid 20%