Anesthesiology: Analgesia and Pain

Analgesics and Pain Management

Terminology

• Analgesia: absence of pain in response to stimulation that would normally be painful
• Pain: unpleasant sensory/emotional experience associated with, or resembling that associated with, actual/potential tissue damage
• Acute pain: sudden, sharp, or intense, serving as a warning sign of disease or threat to the body
• Chronic pain: persisting or recurring for 3 months or more
• Neuropathic pain: caused by pathology of the somatosensory nervous system
• Referred pain: pain perceived other than at the site of stimulus/origin

Mechanism of Pain

• 1st order neurons: originating from dorsal root ganglia (DRG), nociceptors, and transmitting pain signals through A delta and C fibers
• Glutamate, substance P, calcitonin gene-related peptide, and prostaglandins involved in pain transmission
• 2nd order neurons: spinothalamic and spinoreticular tracts
• 3rd order neurons: thalamus to cortex
• Descending modulating pathways: from midbrain and medulla, influencing pain perception

Physiological Response to Pain

• Increased heart rate, blood pressure, peripheral vascular resistance, and myocardial oxygen demand
• Decreased vital capacity, cough inhibition, and increased risk of atelectasis, pneumonia, and hypoxemia
• Decreased bowel motility, gastric emptying, and increased risk of GI infections and ileus
• Decreased muscle activity, immobility, and increased risk of DVT, poor recovery, and wasting
• Decreased urinary output, and increased risk of UTI, urinary retention
• Catabolic hormone release, hyperglycemia, and impaired wound healing
• CNS sensitization, increasing risk of developing chronic pain, anxiety, and fear

Measuring Pain

• Not explicitly discussed in the provided text

Peri-operative Pain Management

• Aims: patient comfort, satisfaction, adequate breathing, and decreased risk of lung infections
• Pre-emptive analgesia: acting at nociception, DRG, and CNS to block central sensitization and decrease pain response intra-operatively and post-operatively
• Post-operative pain management: using strong analgesics, NSAIDs, and patient-controlled anesthesia

NSAIDs

• Cyclooxygenase inhibitors, inhibiting prostaglandin and thromboxane production
• Non-selective NSAIDs: binding to COX 1 and 2, including aspirin, ibuprofen, diclofenac, indomethacin, and ketorolac
• Selective NSAIDs: binding to COX 2 only, including celecoxib and parecoxib
• Common side effects: gastric irritation, peptic ulceration, bronchospasm, platelet dysfunction, renal dysfunction, hepatotoxicity, and asthma exacerbation

Opioids

• Opioid receptors: in the brain, spinal cord, and submucosal/mesenteric plexus, modulating pain by hyperpolarization and inhibiting neurotransmitter release
• Endogenous endorphins: beta-endorphins, dynorphins, and enkephalins
• Common side effects: nausea, vomiting, pruritus, constipation, urinary retention, reduced consciousness, muscle rigidity, and miosis
• Dependence, tolerance, and overdose risks

Organ System Effects

• CNS: sedation, euphoria, mental clouding, and altered mental state
• CVS: hypotension, cutaneous vasodilation, and bradycardia
• Respiratory: respiratory drive suppression, cough reflex inhibition, and increased intracranial pressures
• GI: nausea, vomiting, decreased intestinal peristalsis, and constipation
• Smooth muscle: biliary colic, ureteral and bladder sphincter hypertonicity, and increased uterine tone
• Ocular: miosis

Specific Opioids and Analgesics

• Codeine: 30-60mg PO 4-6 hourly
• Tramadol: 50-100mg PO and IV 4-6 hourly, inhibiting 5-HT and NE reuptake
• Pethidine: 50-100mg PO and IV 4-6 hourly
• Morphine: 0.1-0.2mg/kg 4-6 hourly IV and IM, with tachycardia and dry mouth side effects
• Fentanyl: 1-2μg/kg IV
• Remifentanil: 1-2μg/kg IV, short-acting
• Sufentanil and alfentanil: used in co-induction, inhibiting the “intubation response”
• Paracetamol: weak COX 1 and 2 inhibitor, analgesic and antipyretic, with dose-dependent hepatotoxicity
• Ketamine: NMDA receptor antagonist, opiate receptor mixed activity, dissociative amnesia, and analgesic effects
• Amitriptyline (TCA), carbamezapine, gabapentine, and alpha 2 agonists (clonidine, dexmetomidine) also discussed