Peri-operative Pain Managment Flashcards
Definition of pain vs nociception?
Pain: “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage”
Nociception: “The detection, transduction and transmission of a noxious stimulus, associated with tissue damage and/or inflammation”
Pain is the detection of nociception that occurs in the brain and is determined by:
- memory
- emotion
- interpretation
- genetic influence
Pain classification?
Acute:
- recent onset
- short duration less than 6 weeks
- associated with trauma, surgery or acute illness
- resolves with healing
Chronic:
- persists beyond the usual course of an acute illness or injury (>3 months)
- outlasts the potential for healing
- can be AIDS related or due to terminal cancer
Mechanism of pain?
Harmful stimuli activate free nerve endings (nociceptors)
Depolarisation if threshold reached, and propogation of impulse from nociceptors to dorsal horn of spinal cord via 1st order neurons
Sharp well-localised pain from A-delta fibres, followed by dull, persistent pain from C fibres
Release of inflammatory mediators lowers firing threshold of nociceptors in area resulting in primary hyperalgesia (increased sensitivity to pain)
From dorsal horn via 2nd order neuron, impulse crosses midline and ascents via anterior and lateral spinothalamic tracts to thalamus
In thalamus, axons synapse with 3rd order neurons
Axonal projections sent to sensory cortex (and limbic system, cerebellum, peri-aqueductal grey matter and reticular formation)
Pain modulation occurs from efferent neurons from brainstem to outer dorsal horn where release of NT’s (incl. GABA) dampen the nociceptive transmission
Classes of drugs used in the management of peri-operative pain, which examples of each?
Simple analgesics (e.g. paracetamol) NSAIDS (e.g. diclofenac, ibuprofen) Opiates (e.g. codeine, tramadol, morphine, fentanyl) Local anesthetics (e.g. lignocaine, bupivicaine) Hypnotic agents (e.g. N20, Ketamine) Secondary analgesics (e.g. TCA, SSRI's, clonidine, dexmedetomidine)
Characteristics of Paracetamol?
Mild analgesic properties
Very efficacious
Can be used alone for minor surgery or in conjunction with other stronger agents
No anti-inflammatory properties
Anti-pyretic properties
Safe drug, that only has side effects in OD
Dosing of Paracetamol?
Oral/rectal form:
Adults = 1 g, 6hourly
Children = 15 mg/kg, 6 hourly
Onset of action is 30 minutes and duration of action is 2-5 hours
IV form (“Perfalgen”:
Dose same as for oral and rectal
Onset of action is 5-10 minutes
1g IV = 10mg IM morphine
NSAIDS MoA?
Inhibit COX (cyclo-oxygenase) - COX 1 and/or COX 2 Thereby inhibiting prostaglandin and thromboxane production from membrane phospholipids
Important side effects of NSAIDS?
Gastric irritation Bronchospasm Renal dysfunction (nephrotoxic) Platelet dysfunction (inhibiting thromboxane = reduced platelet aggregation) Hepatotoxicity MI
Modes of administering NSAIDS?
Orally
Rectally
IV
IM
Commonly used NSAIDS?
Aspirin = NOT used for intra- and post-op care
Non-selective NSAIDS:
- Ibuprofen (“Brufen”) = oral use, lowest incidence of S/E
- Diclofenac (“Voltaren”) = oral/rectal
- Indomethacin (“Indocid”) = oral/rectal
- Ketorolac (“Toradol”) = oral/IM/IV, indicated for moderate to severe acute pain requiring analgesia at opiate level (opiate sparing)
Onset of action for above given orally/rectally is 30-60 minutes (onset for IM 10m and IV 5m) and duration 6-8 hours)
COX-2 selective (contra-indicated in high risk patients):
- Celecoxib (“Celebrix”) = oral, indicated for acute pain of moderate to severe nature
- Parecoxib (“Rayzon”) = IV/IM
MoA of opiates?
Reduce neural cell excitability
Selective for pain and do not interfere with other sensory modalities
MAINSTAY OF PERI-OPERATIVE ANALGESIA FOR SEVERE PAIN
Common effects of opiates?
Analgesia Drowsiness Mood change N and V Bradycardia Respiratory depression Miosis Pruritis/itching Inhibition of gut motility with constipation Urinary retention
Patients can develop tolerance and addiction
Modes of administering opiates?
Oral Transdermally IM IV Epidural space Intrathecal space
Commonly used opiates?
Codeine (oral use only) Tramadol (oral or IV) Pethidine (IV or IM) Morphine (IV or IM) Fentanyl (IV) Sufentanil and Alfentanil (IV infusion) Remifentanil (IV infusion)
Fentanyl vs Morphine?
Fentanyl is more potent than morphine and 600 x more lipid soluble
Fentanyl seldom causes delayed respiratory depression when given epidurally or spinally
Fentanyl less likely to cause Histamine release
Fentanyl is used at induction with the IV agent - “co-induction” (aim is to reduce the dose of the induction agent with the synergistic effect of opiates)
Dosing:
Morphine 0,1-0,2 mg/kg 4-6 hourly
Fentanyl 1-2 mcg/kg
