From lecture by Dr Chris Taggart, 2016:
Review of somatic v visceral v neuropathic pain.
– directly stimulate pain receptors
– directly stimulate reward centers of brain, especially if delivered quickly to CNS
– tolerance quickly develops for anesthetic and euphoric effects
– tolerance slowly develops for respiratory center effects (resp depression)
this means that quick escalation of doses is a respiratory threat without getting much more pain control
– physical dependence: withdrawal symptoms when stopped (piloerection, diaphoresis, etc.)
– addiction: separate from tolerance and dependence; different molecular and brain mechanisms
addiction develops more slowly than tolerance and dependence, but also lasts longer
Addiction prevalence: we really don’t know.
– best study of opioids in chronic back pain showed that around 23% developed addiction
– 11 studies looking at chronic back pain – prevalence range from 3-66%
Natural: morphine, codeine
Semisynthetic: hydromorphone, oxycodone, hydrocodone, oxymorphone
Synthetic: fentanyl, methadone, tapentadol
Other (mixed): buprenorphine, nalbuphine
Morphine: can be administered in any way possible: PO, PR, IV, IM, SQ, epidural, intrathecal, intracerebroventricular
– renal failure will impact tolerance to morphine
– renal failure increases risk of adverse reactions
– IV/IM/SC: onset in 15-30 mins, duration 2-4 h
– PO: onset 30-60 mins, duration 4-6 h
Opioid equivalents: 15mg PO morphine = 10mg oxycodone = 50 mcg fentanyl
Reduce dose by 1/3 to 1/2 when changing opiates (e.g., morphine–>hydromorphine)
In severe acute pain, no need to decrease the opiate dose
Breakthrough dose should be 50-100% scheduled dose
TAGGART’S RULES ON OPIOID MANAGEMENT:
- Start with IV/SC meds for acute, severe pain, preferably with PCA.
- Start lower than you think you need and increase every few hours.
- Chance to PO once: you have their pain improved, you have an idea what their 24h need is, they can take PO
Non-opioid analgesics: APAP, NSAIDs, topical lidocaine, topical NSAIDs.
Antidepressants: SNRIs, TCAs
Antiepileptics: gabapentin, pregabalin, carbamazepine
NSAIDs especially useful in bone mets, OB-related pain, inflammatory-related pain. Avoid in acute injury like fractures (in theory).
Acetaminophen = 3 morphine equivalents; unknown MOA.
– increased NE and 5-HT activity causes inhibition of nerves
– adjuvant for neuropathic, regional, central pain
– especially useful in comorbid depression/anxiety, neuropathic pain, central pain syndromes
– good adjuvant with neruopathic and centralized pain
– use low doses, eg, 10mg nightly
– especially useful in nerve injury, phantom pain, burns, etc.
– consider for fibromyalgia and burn injuries
– consider if muscular trauma (MVAs), fibromyalgia (cyclobenzaprine)