Chapter 30 Intrathecal Opioid Injections for Postoperative Pain Flashcards
KEY POINTS 1. The pharmacologic properties of IT opioids reflect the extent of the hydro- versus lipophilicity of the specific opioid: lipophilic opioids (fentanyl and sufentanil) have a shorter onset and duration of action, whereas hydrophilic duration of action (and certain side effects such as delayed respiratory depression). 2. Like opioids administered by other routes, IT opioids may result in widely recognized opioid-related side effects such as nausea, vomiting, pruritus, sedation, and
Common uses
of IT opioids for postoperative analgesia
obstetric and gynecologic surgery, orthopedic joint and spine procedures,
thoracic and vascular procedures, cardiac bypass, pediatric surgery, urologic procedures, and abdominal procedures.
Fibers that play a major role in the transmission of pain
Nociceptive information is transmitted by multiple afferent
neurons with small-diameter unmyelinated and thinly
myelinated fibers (C-fibers and Ad-fibers, respectively)
Central terminals of small unmyelinated fibers are located in
Rexed’s laminae I, II, and III
What provides the anatomic basis for selective analgesia by opioids injected into the cerebrospinal fluid (CSF)
Opioid receptors exist in
Rexed’s laminae I, II, and V in the dorsal horn of the spinal cord.
Spinal cord analgesia is likely mediated by what receptors?
mu- and k-receptors.
substance P
substance P is released into the CSF by electrical stimulation. This release is inhibited by the administration of morphine
into the CSF and possibly mediated by gammaaminobutyric
acid (GABA) presynaptically and glycine postsynaptically
Lipophilicity
Lipophilicity (versus hydrophilicity) is the key property affecting the speed of onset and duration of action. Highly lipid-soluble drugs such as fentanyl and
sufentanil have a faster onset but shorter duration of action when used intrathecally
Highly lipid-soluble drugs have shorter duration of action when used intrathecally
Shortly after injection,
CSF levels are barely detectible as the drug is quickly distributed to the spinal cord. This may result in a more
segmental spread of analgesia and a lower concentration reaching the brain, decreasing the risk of delayed respiratory
depression (e.g., 12 to 24 hr after injection)
Hydrophilic opioids
Hydrophilic opioids, such as morphine, have a slower onset and longer duration of action, and remain detectable
in the CSF long after injection. Delayed respiratory depression may be more likely with morphine than other
lipophilic drugs, as morphine remains in the CSF long enough to circulate rostrally to the brainstem and respiratory
centers
Only opioid has strong enough local anesthetic
properties to be used as a sole agent for surgery.
meperidine. IT injection of meperidine produces spinal anesthesia that
is qualitatively similar to that achieved with conventional local anesthetics.
Why can meperidine be used as the sole agent in
spinal anesthesia?
It is likely the combined action of its local anesthetic properties and its opioid receptor binding
meperidine vs. fentanyl
The onset of action for meperidine is similar to that of fentanyl despite being significantly less
lipid soluble; however, its duration is longer than fentanyl
meperidine vs. morphine
Meperidine has a shorter duration of action than
morphine, as meperidine dissipates from the CSF four times faster than morphine.
IT opioids dose vs. IV or epidural
Equianalgesic doses
of IT opioids are typically a small fraction of those used for intravenous or epidural use
The duration of analgesia for a hydrophilic opioid compared to IV or epidural
The duration of analgesia for a hydrophilic opioid
such as morphine is greater compared to intravenous or
epidural administration
A single IT injection of morphine 0.04 to 0.5 mg will provide analgesia up to
15 to 24 hr of analgesia.
hemodynamic changes of opioids when
applied intrathecally
Unlike neuraxially administered local anesthetics, which
may result in vasodilation and hypotension, opioids do not per se cause adverse hemodynamic changes when applied intrathecally and may not significantly attenuate
the neuroendocrine stress response even when administered
in extremely large doses (4.0 mg)
effect of opioids on motor blockade or sensory
opioids do not cause motor blockade or sensory loss,
potentially allowing earlier ambulation
benefit of IT opioids when used with local anesthetics
IT opioids do provide a sparing effect for local anesthetics, allowing lower doses to be used intrathecally or epidurally while still maintaining adequate analgesia
SIDE EFFECTS OF INTRATHECAL
OPIOIDS
most feared complications are respiratory depression
and arrest
Respiratory depression typically occurs when
within minutes to hours for the lipophilic opioids (fentanyl, sufentanil) with early respiratory depression (minutes) not being reported with a hydrophilic opioid such as morphine
Common Side Effects of Intrathecal Opioids
Mild respiratory depression Pruritus Sedation Nausea Vomiting Urinary retention
Uncommon Side Effects of Intrathecal Opioids
Respiratory arrest Generalized muscle rigidity Nystagmus Epileptic seizure Myoclonus Hyperalgesia Neurotoxicity Water retention
The risk of respiratory depression increases with
the addition of systemic opioids or sedatives, increasing age, lack of opioid tolerance (i.e., opioid-naive state), obesity, and sleep apnea
