--- module: 061-02 language: en chapter: 61 title: "Pain Medicine, Analgesia, Anaesthesia, Sedation, and Procedural Safety" module_title: "Mechanism-based analgesia, sedation rescue, and anaesthetic physiological safety" source_sha256: dc246066115f251cfe7859a0258aabd26d450c4c39ea3b0a2f77cbc73a1bc1de --- # Mechanism-based analgesia and anaesthetic safety ## Goals and pain mechanisms ### Relieve suffering, preserve breathing and thinking ### Pain score is one measurement, not the endpoint #### Assess function, sleep, cough, mobilisation ### Escalating pain despite analgesia #### Compartment syndrome, ischaemia, perforation ### Analgesia enables repeated examination ### Nociceptive: somatic and visceral transduction ### Neuropathic: somatosensory lesion or disease #### Allodynia, hyperalgesia, burning, numbness ### Nociplastic: network amplification ### Mechanisms overlap and evolve #### Persistent input sensitises spinal circuits #### Fear, sleep loss, depression alter descending control ## Multimodal analgesia and the pain plan ### Different targets so no toxicity dominates ### Select treatments by mechanism ### Opioid-sparing only if replacement is safe #### Stacked sedatives worsen falls and ventilation ### Each addition needs benefit and review point ### One-page plan: regular, as-required, maximum dose ### Reconcile hospital and home medicines at discharge ### Follow bowels, delirium, retention, kidney function ### Distinguish regular from rescue dosing ## Paracetamol and non-steroidal drugs ### Paracetamol has a ceiling effect #### Hidden duplication is a major overdose route ### Adjust for low weight, alcohol, liver disease ### NSAIDs remove gastric, renal, platelet protection #### Harm amplified by dehydration, heart failure, age ### Topical use lowers exposure, absorption not zero ## Opioids and ventilation ### Effect depends on receptor, metabolites, clearance ### Intravenous boluses titrated in monitored pain ### Long-acting accumulation outlasts reassessment ### Incomplete cross-tolerance: reduce and retitrate ### Morphine metabolites accumulate in kidney failure ### Respiratory rate alone misses impairment #### Shallow breaths with rising carbon dioxide #### Oxygen preserves saturation ### Stimulate, open airway, oxygen, ventilate ### Naloxone to adequate breathing, not withdrawal #### Infusion when the opioid lasts longer ### Long-term therapy judged by function #### Constipation persists despite tolerance #### Abrupt discontinuation: withdrawal, suicide risk ## Neuropathic agents and local anaesthetics ### Neuropathic medicines work slowly and partially ### Tricyclics: anticholinergic, cardiac, overdose risk ### Gabapentinoids: sedation, respiratory synergy ### Success may mean better sleep and movement ### Local anaesthetics block sodium channels ### Vasoconstrictor reduces systemic absorption ### Total dose sums every infiltration and block ### Toxicity: circumoral numbness, tinnitus, seizure #### Then conduction delay, dysrhythmia, collapse ### Stop, help, oxygenate, lipid emulsion ### Fractionated injection with aspiration ### Protect the numb limb ## Pre-anaesthetic assessment ### Predict difficulty in oxygenation and ventilation ### No single score excludes a difficult airway ### Prepare backup devices and front-of-neck rescue ### First priority is oxygen, not intubation ### Establish medicines, allergies, last intake ### Indiscriminate stopping: withdrawal, thrombosis ### Decide continue, withhold, or bridge #### Record who restarts after operation ## Induction and general anaesthesia ### Preoxygenation replaces nitrogen with oxygen #### Shorter in pregnancy, obesity, children ### Induction removes airway tone and sympathetic support ### Positive pressure reduces venous return #### Arrest after successful intubation ### No single agent provides all components ### Paralysis conceals awareness ### Confirm tube by waveform carbon dioxide ### Distinguish crises by timing, pressures, skin ## Malignant hyperthermia ### Skeletal muscle calcium regulation crisis ### Triggered by volatiles or depolarising relaxant ### Early carbon dioxide rise, rigidity, hyperkalaemia #### Temperature rise can be late ### Stop trigger, oxygen, dantrolene, cooling ### Record for later testing and safe anaesthesia ## Neuraxial and peripheral blocks ### Sympathetic block causes vasodilation ### High block: bradycardia and shock ### Spinal haematoma risk from anticoagulant timing #### New back pain or weakness is an emergency ### Peripheral block can conceal surgical injury #### Check distribution, circulation, compartment signs ### Start oral analgesia before block regresses ## Sedation and recovery ### Continuum: response and stimulation vary ### Rescue at least one level deeper ### Oxygen delays desaturation, masks hypoventilation ### Propofol: apnoea, vasodilation, no reversal ### Repeat dosing before peak effect deepens sedation ### Recovery is a distinct high-risk phase #### Stimulation ends, absorption continues ### Written restrictions and return instructions