Neuraxial Opioids Flashcards
Describe neuraxial analgesia
- analgesia is dose related
- epidural dose is 5-10x Subarachnoid dose since it has to diffuse through dura mater
-specific for visceral (organ) pain rather than somatic (muscles, tendons, fascia, bone)
- doesn’t include sympathetic denervation, skeletal muscle weakness or loss of perception of pressure like with IV opioids or epidural or subarachnoid injection of local anesthetic
- good for laboring, doesn’t block motor
Describe receptor distribution
- all three (Mu, kappa, delta) found in high concentrations in the dorsal horn of the spinal cord
- mainly Mu receptors present in the substantia gelatinosa
What is the MOA of neuraxial opioids?
Opioids placed in the epidural space diffuse across the dura to affect the Mu receptors on the spinal cord AND are absorbed to produce the effects like IV opioids
- highly vascular space
- higher lipid soluble fentanyls cross dura rapidly and are absorbed more systemically (may have no advantage over IV admin)
- less lipid soluble morphine has slower onset and longer duration
What determines how fast agents diffuse across the dura with an epidural dose?
How much of the dose actually enters the CSF?
The more lipid soluble, the faster the diffusion into the CSF
*sufentanil > fentanyl > morphine
Only 3% of epidural dose enters CSF
*with fentanyl and sufentanil, probably a less % @ peak concentration d/t vascular absorption
How fast does fentanyl reach its CSF peak concentration with and epidural?
20 min
How fast does sufentanil reach its CSF peak concentration with an epidural?
6 min
How fast does morphine reach its CSF peak concentration with an epidural?
1-4 hrs
How are epidural opioids absorbed into vascular system?
- rapidly absorbed via the extensive venous plexus in the epidural space
- absorption produces blood levels similar to an IM injection
- less morphine goes to the blood
How fast does fentanyl reach its peak blood concentration through vascular absorption with an epidural?
5-10 min
How fast does sufentanil reach its peak blood concentration through vascular absorption with an epidural?
< 5 min
How fast does morphine reach its peak blood concentration through vascular absorption with an epidural?
10-15 min
How can vascular absorption be decreased?
Add epinephrine to the solution to cause vasoconstriction
- if injected into the epidural space, does not affect diffusion across the dura
- if injected into the subarachnoid space, analgesia is enhanced with morphine
- causes prolonged duration since it is not taken up into circulation
What is the duration of morphine epidural?
4-24 hrs
What is the duration of fentanyl epidural?
1-3 hrs
How do opioids move once in the CSF?
- lipid solubility affect degree of cephalad movement (towards head)
- higher lipid solubility limits movement as drug is taken up by spinal cord (travels higher up spinal cord)
- lower lipid soluble morphine longer time in CSF and movement cephalad
What accelerates movement of opioid cephalad?
Coughing or straining
- *not body position
- consider timing of emergence when placing…extubated? Done coughing?
What affects side effects of neuraxial opioids?
- either with opioids in CSF or systemic circulation
- dose dependent
- may be r/t opioid receptors but now always
- bigger the dose, more chance of side effects
What are the four classic side effects of neuraxial opioids?
- severe pruritus (most common)
- nausea and vomiting
- urinary retention
- ventilatory depression
What is pruritus likely due to?
Opioids in CSF moving up to interact with opioid receptors in the trigeminal nucleus
- more likely in OB pts d/t estrogen effect on opioid receptors
- itching may be confined to face, neck or upper thorax or could be generalized
How can the pruritus be treated?
narcan 0.25 mcg/kg/hr
- more effective than antihistamines which cause sedation
- enough narcan to reverse itching but not reverse analgesia
What is urinary retention caused by?
Interaction of opioid with opioid receptors of the sacral spinal cord
- inhibition of sacral parasympathetic outflow causing detrusor muscle relaxation, increasing bladder capacity, and increased sphincter tone not allowing excretion
- occurs within 15 min and last up to 16 hrs
When is urinary retention more common?
- in young males
- with neuraxial opioids than IV or IM
- not related to dose size or systemic absorption
How is urinary retention reversed?
Narcan
*usually won’t use with young children having urology procedures
What is the most serious side effect?
Ventilation depression
*1% incidence (same as with IV or IM)
What is the onset of ventilatory depression?
Unpredictable onset- may be within minutes or 6-12 hrs after neuraxial morphine
- early onset (within 2 hrs) with fentanyl and sufentanil d/t systemic absorption and cephalad movement
- delayed onset with morphine as opioid moves up to effect receptors in the ventral medulla where respiratory center is
What are some risk factors for ventilatory depression?
- larger doses
- geriatric
- received other opioids or sedative systematically
- prolonged or extensive surgery (increased risk regardless)
- presence of co morbidities (pulmonary or cardiac history)
- thoracic surgery (dura morph given more with thoracic surgery)
What increases the risk of delayed ventilatory depression?
- patient also receiving IV opioid or sedative
- patient coughing to move opioid cephalad in CSF
*dont need to give neuraxial opioids to same day patient, need to be monitored for sudden respiratory depression
What decreases risk of ventilatory depression?
-increased stimulation from progesterone in OB patients
What is the most reliable sign of ventilatory depression?
Decreased level of consciousness (sleep) possibly caused by hypercarbia
- hypercarbia is a sympathetic stimulation
- tachycardia, HTN, dysrhythmias
What is the treatment for ventilatory depression?
- narcan (infusion may help)
- supplemental oxygen (helps with oxygenation not ventilation)
What is sedation related to with neuraxial opioids?
Dose related
- most common with sufentanil
- *be alert for ventilatory depression
What may happen with patients with herpes virus when given epidural morphine?
Reactivation
- occurs 2-5 days post epidural admin
- same sensory innervation as primary infection (facial areas innervated by trigeminal nerve)
- d/t cephalad movement and interaction of trigeminal nucleus
*usually don’t use morphine with laboring patients
What may happens with neonates?
respiratory depression of neonate from admin to maternal patient
- r/t systemic absorption (fentanyl more lipid soluble to cross placenta)
- minimal opioid found in breast milk
What are some other misc. side effects of neuraxial opioids?
- miosis nystagmus, vertigo (balance)
- delayed gastric emptying (if coming to surgery from floor and been on opioids treat as “full stomach”)
- decreased body temp d/t inhibition of shivering (resets body temp)
- water retention d/t release of vasopressin (caution with CHF)
- *spinal cord injury r/t effects of toxic preservatives (must use preservative free)
