Pain and analgaesia

Pain

'an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.'

• Physiological pain:
  • Nociceptive
  • Inflammatory
• Pathological/maladaptive pain:
  • Neuropathic

Pathophysiology

• Transduction - noxious agent stimulates free nerve endings (nociceptors)
• Transmission - signal travels via nerve pathways - A and C fibres, in the spinothalamic and thalamocortical tracts
• Modulation - repression or enhancement of signal occurs in the dorsal horn of the spinal cord
• Perception - pain signal reaches cerebral cortex

Strategies to reduce pain

• Education
  • Reduce anxiety by discussing likely post-operative pain levels and course beforehand
  • Set realistic expectations
  • Encourage early rehabilitation
• Prevention
  • Various strategies to minimise tissue disruption
  • Pre-emptive analgaesia
  • Post-operative regional blocks/LA
• Multimodal analgaesia
  • LA
  • Regional anaesthesia - epidural, regional block, spinal

Approach to seeing a post-op patient in pain

• Review physiological status and operative details
• Review medication chart
• Discuss with ward nurses
• Review and discuss with the patient
  • Exclude a new problem, including ischaemia, bleeding, anastomotic leak and compartment syndrome
  • Airway - ensure no sedation or high-risk features
  • Breathing - RR and sats
  • Circulation
  • Disability - check LoC
  • Use SOCRATES or similar to characterise pain
• Consider possibilities:
  • Inadequate medication or dosage?
  • Incorrect technique used?
  • Unrecognised complication?
  • Other supplemental techniques are not used appropriately?

Management options:

• Analgaesic medications
  • Oral
  • SC/IM
  • IV
    • PCA
    • Bolus
• Regional anaesthetic techniques
  • Spinal/epidural
  • Peripheral nerve block
  • LA

Paracetamol

• Analgaesic and anti-pyretic
• Exact mechanism of action not fully understood
• Metabolised by the liver
• May need to adjust dose in hepatic impairment or underweight patients
• No significant benefit to IV over PO
• 1g q6h is the usual dose
• In patients <50kg, give 15mg/kg/day

NSAIDs

• Inhibit prostaglandin synthesis by inhibiting the enzyme cyclo-oxygenase
• Contraindications
  • Allergy including asthma
  • Previous CABG and cardiovascular disease in general
  • Pregnancy especially third trimester
  • GI bleeding/gastritis
  • Previous bariatric surgery
  • CKD/AKI
  • Diseases that impair platelet activity
  • Orthopaedic injuries

Opioids

• Derived from opium alkaloids
• Act on mu-opioid receptors
• Adverse effects:
  • Nausea/vomiting
  • Constipation
  • Pruritis
  • Bladder retention
  • Drowsiness and respiratory depression

Co-analgaesics

• Anticonvulsants - gabapentin
• Antidepressants - amitriptyline
• Ketamine