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

Learn about the definitions and concepts of analgesia, pain, and acute pain in the field of anesthesiology. Explore the importance of pain management and its impact on well-being.