Neuraxial Opioids Flashcards

0
Q

Describe neuraxial analgesia

A
  • 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
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1
Q

Describe receptor distribution

A
  • 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
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2
Q

What is the MOA of neuraxial opioids?

A

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
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3
Q

What determines how fast agents diffuse across the dura with an epidural dose?

How much of the dose actually enters the CSF?

A

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

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4
Q

How fast does fentanyl reach its CSF peak concentration with and epidural?

A

20 min

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5
Q

How fast does sufentanil reach its CSF peak concentration with an epidural?

A

6 min

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6
Q

How fast does morphine reach its CSF peak concentration with an epidural?

A

1-4 hrs

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7
Q

How are epidural opioids absorbed into vascular system?

A
  • 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
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8
Q

How fast does fentanyl reach its peak blood concentration through vascular absorption with an epidural?

A

5-10 min

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9
Q

How fast does sufentanil reach its peak blood concentration through vascular absorption with an epidural?

A

< 5 min

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10
Q

How fast does morphine reach its peak blood concentration through vascular absorption with an epidural?

A

10-15 min

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11
Q

How can vascular absorption be decreased?

A

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
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12
Q

What is the duration of morphine epidural?

A

4-24 hrs

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13
Q

What is the duration of fentanyl epidural?

A

1-3 hrs

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14
Q

How do opioids move once in the CSF?

A
  • 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
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15
Q

What accelerates movement of opioid cephalad?

A

Coughing or straining

  • *not body position
  • consider timing of emergence when placing…extubated? Done coughing?
16
Q

What affects side effects of neuraxial opioids?

A
  • 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
17
Q

What are the four classic side effects of neuraxial opioids?

A
  • severe pruritus (most common)
  • nausea and vomiting
  • urinary retention
  • ventilatory depression
18
Q

What is pruritus likely due to?

A

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
19
Q

How can the pruritus be treated?

A

narcan 0.25 mcg/kg/hr

  • more effective than antihistamines which cause sedation
  • enough narcan to reverse itching but not reverse analgesia
20
Q

What is urinary retention caused by?

A

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
21
Q

When is urinary retention more common?

A
  • in young males
  • with neuraxial opioids than IV or IM
  • not related to dose size or systemic absorption
22
Q

How is urinary retention reversed?

A

Narcan

*usually won’t use with young children having urology procedures

23
Q

What is the most serious side effect?

A

Ventilation depression

*1% incidence (same as with IV or IM)

24
Q

What is the onset of ventilatory depression?

A

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
25
Q

What are some risk factors for ventilatory depression?

A
  • 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)
26
Q

What increases the risk of delayed ventilatory depression?

A
  • 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

27
Q

What decreases risk of ventilatory depression?

A

-increased stimulation from progesterone in OB patients

28
Q

What is the most reliable sign of ventilatory depression?

A

Decreased level of consciousness (sleep) possibly caused by hypercarbia

  • hypercarbia is a sympathetic stimulation
  • tachycardia, HTN, dysrhythmias
29
Q

What is the treatment for ventilatory depression?

A
  • narcan (infusion may help)

- supplemental oxygen (helps with oxygenation not ventilation)

30
Q

What is sedation related to with neuraxial opioids?

A

Dose related

  • most common with sufentanil
  • *be alert for ventilatory depression
31
Q

What may happen with patients with herpes virus when given epidural morphine?

A

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

32
Q

What may happens with neonates?

A

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
33
Q

What are some other misc. side effects of neuraxial opioids?

A
  • 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)