Xvi / 15B07: Anatomy relevant to lumbar puncture

15B07: Exam Report

Draw and label a cross section of the lumbar epidural space (50% of marks). Describe the pharmacology of bupivacaine (50% of marks).

35% of candidates passed this question.

It was expected answers would include a diagram of a cross section and label the lumbar epidural space and the key landmarks namely dura, subarachnoid space, epidural space.

Most candidates were able to give a schematic representation even if not being able to draw.

Some candidates confused the subdural space with the epidural space.

Pharmacology of bupivacaine needed to cover both pharmacokinetics and pharmacodynamics.

Several candidates addressed only one of these components and so missed the opportunity to score marks.

Xvi / 15B07: Anatomy relevant to lumbar puncture

Definition

Lumbar puncture is a diagnostic/therapeutic procedure whereby CSF is removed from the spinal canal.

Surface anatomy

  • L1/L2 = termination of spinal cord
  • L4/L5 for needle insertion
  • L4 = top of ASIS
  • Midline → between spinous processes

Lumbar vertebrae

  • 5 lumbar vertebrae
    • Structural support of the body
    • Protection of spinal cord
  • Largest vertebral bodies
  • Spinal cord bound by:
    • Anterior – vertebral body
    • Lateral – pedicles & T-process
    • Posterior – lamina + spinal process
Lumbar vertebrae​

Spinal ligaments

  • Ant. + post. longitudinal lig.
  • Ligamentum flavum
  • Interspinous lig.
  • Supraspinous lig.
Spinal ligaments

Spinal canal

  • Spinal cord
  • Meninges
  • Fatty tissue
  • Venous plexus

Meninges

  • Pia → closely adheres to the spinal cord
  • Arachnoid mater
  • Dura mater
  • Spinal cord from foramen magnum → L1
  • CSF is in subarachnoid space

Cross section: Skin → CSF

Skin → subcut. tissue → suprasinous lig. → interspinous lig. → lig. flavum → (Epidural space) → Dura (CSF)

Bupivacaine

Bupivacaine

Chemical

Amide local anaesthetic

Use

Local anaesthesia

Presentation

Clear colourless solution

 

Racemic (S- & R- enantiomers)

 

Concentrations of 0.25% and 0.5%

+/- Adrenaline 1:200,000 and preservative sodium metabisulphite

Hyperbaric solution of 0.5% contains 80mg/ml glucose (specific gravity 1.026)

Dose

1-2mg/kg up to a maximum of 150mg

Toxic Dose

Max dose 3mg/kg

Route

Peripheral n blocks, epidural, spinal

Onset

Slower cf lignocaine

DoA

T1/2b = 0.3-0.6hrs

MoA

Unionised diffuses across neuronal cell memb

Becomes ionized within cell

Binds and OPENS Na+ ch → blocks them

↓depol of membrane →Cardiac myocyte, Motor & sensory n fibre APs blocked

Bupi is x4 more potent cf lignocaine & prolonged DoA, but slower onset

PD

CNS

  • Reversible neural blockade
  • Levobupi produces less motor block but longer sensory block following epidural administration

CVS

  • Reduces SVR
  • Negative inotropy
  • Cardiotoxic: binds myocardial proteins & blocks Na ch to ↓rate of increase Ph0 cardiac AP
  • Levobupi has less cardiotoxicity

PK

A

Dose & concentration dependant

IC > Caudal > Epidural > Brachial Plx > Subcut

+Adr = delayed absorption

Highly lipid soluble, take up into fat is rapid

Drug has direct vasodilatory effect

D

95% PPB to a1 glycoprotein

Vd 1L/kg

M

Low extraction ratio drug => less influence by HBF cf lignocaine

Primarily liver metabolised via conjugation w glucuronic acid

Main metabolic is 2,6 pipecoloxylidine (catalysed by P450 3A4)

E

Clearance 7ml/min/kg

T1/2b = 0.3-0.6hrs

Metabolites in urine.  16% unchanged

Adverse Effects

Drug interactions: propranolol

Cardiotoxicity

Systemic toxicity of LAs → neurotoxicity, cardiotoxicity, death