2020A09 Outline the effects of opioids injected into the spinal intrathecal space
using both fentanyl and morphine to illustrate your answer.

 

List:

·      Intro: aims

·      Kinetics: drug, dose, distribution

·      Analgesia

·      Resp depression

·      Other side effects

 

Intro:

Aims

·   Surgical anaesthesia (synergistic with LA; elevates and prolongs block)

·   Post-op analgesia (e.g. intrathecal morphine for Whipple’s procedure)

Mechanism of opioid action

·   Activate Gi protein-coupled receptor

·   ↓Guanylyl cyclase activity -> ↓cAMP

·   Inhibit VDCC

·   ↑K+ efflux

·   Hyperpolarisation, ↓action potential

 

Pharmaceutics:

Preservative-free

·   Must use preservative-free morphine

·   No remifentanil (glycine additive is an inhibitory neurotransmitter)

 

Pharmacokinetics:

Drugs and doses

·   Tiny dose because:

o Near effect site

o Low volume of CSF

o Relatively slow reabsorption

·   Morphine:

o 0.5mg (range 0.2-1mg)

o IT:IV potency 300:1

·   Fentanyl:

o 15mcg (range 10-25mg)

o IT:IV potency 20:1

·   Other:

o Diacetylmorphine

o Pethidine (can be a sole intrathecal agent)

o Sufentanil (local anaesthetic effect at high dose)

If highly lipid soluble (fentanyl)

·   Rapid onset (5 mins?)

·   Rapid offset (1 hour?)

·   ↓ Effect from central spread via CSF due to faster diffusion out of it

·   ↑ Effect from systemic absorption

If poorly lipid soluble (morphine)

·   Slower onset (4-6 hours)

·   Slower offset (12-24 hours

·   ↑Effect from central spread via CSF due to slower diffusion out of it

·   ↓Effect from systemic absorption

 

Analgesia:

Spinal cord segments (++++)

·   Diffuse across pia mater into dorsal horn (fentanyl fast, morphine slow)

·   Height dose

·   Pre-synaptic inhibition of 1° afferent nociceptors especially layer 2 -> ↓Release of excitatory neurotransmitters (glutamate, ACh, substance P)

·   Some post-synaptic inhibition

Brain (++)

·   Circulate via CSF bulk flow due to arterial pulsation
(morphine yes, fentanyl minimal)

·   PAG and RVMM: inhibition of OFF cell -> disinhibition of ON cell -> ↑descending modulation (NAd > 5HT) -> pre-synaptic inhibition

·   ? Some post-synaptic inhibition

Systemic (+)

·   Absorption via epidural venous plexus

·   Very low peak plasma concentration

·   Effect on brainstem, spinal cord, GIT, etc

 

Respiratory depression:

Receptors

·   M, D

Location

·   Respiratory centre in the medulla

Highly lipid soluble (fentanyl)

·   Peak at 30 mins due to systemic absorption

Poorly lipid soluble (morphine)

·   Peak at 6-12 hours due to central spread of CSF

 

Other side effects of neuraxial opioid:

·         Nausea, vomiting (chemoreceptor trigger zone mu opioid receptor)

·         Urinary retention (spinal cord)

·         Itch (unclear cause, reversed by naloxone not antihistamines)

·         Foetal bradycardia if in labour (fentanyl > morphine)

 

 

Feedback welcome at ketaminenightmares@gmail.com