Nagelhout Ch 56 Acute Pain: Physiology & Management

Pain Classification and Physiology

• Pain can be classified based on longevity (acute vs. chronic) and underlying pathophysiology (nociceptive or non-nociceptive)
• Nociceptive pain is associated with stimulation of specific nociceptors and can be either somatic or visceral
• Somatic pain has an identifiable locus and is well localized, sharp, and hurts at the point/area of stimulus
• Visceral pain is often associated with distension of organ capsules or obstruction of hollow viscus, and is diffuse, dull, cramping, squeezing, or vague in nature
• Non-nociceptive pain is caused by damage to peripheral or central neural structures, resulting in abnormal processing of painful stimuli
• Inflammatory pain results from sensitization of the nociceptive pathway from mediators released at the site of tissue inflammation without neural injury

Somatic Nociceptive Pain Pathway

• Transduction: transformation of noxious stimuli into an action potential
• Peripheral nociceptors are categorized according to morphology:
  • Myelinated Aδ elicit fast-sharp pain
  • Nonmyelinated C fibers elicit slow-dull, burning, throbbing, and aching pain
• Biochemical events take place, including:
  • Release of chemical mediators from inflammatory response and neurotransmitters from nociceptive nerve endings
  • Substance P release from peripheral afferent nociceptor C fibers, involved in slow, chronic pain
  • Glutamate release from Aδ and C primary afferent fibers, causing initial, fast, sharp pain
  • Bradykinin release during inflammatory process, directly stimulating peripheral nociceptors
  • Histamine release from mast cells, basophils, and platelets, acting on various histamine receptors
  • Serotonin release from platelets, acting on multiple receptors and nociceptors
  • Prostaglandins synthesized from COX-1 and COX-2, sensitizing peripheral nociceptors
  • Cytokines released in response to tissue injury, leading to increased PG production
  • Calcitonin gene-related peptide release from peripheral afferent nociceptor C fibers, producing local cutaneous vasodilation and plasma extravasation

Transmission and Modulation

• Transmission: action potential conducted from periphery to CNS
• Primary afferent neurons Aδ and C fibers cell bodies located in dorsal root ganglia of spinal cord
• 2 types of 2nd-order neurons:
  • Nociceptive neurons receive input from Aδ and C fibers
  • Wide-dynamic-range (WDR) neurons receive input from Aδ, C, and Aβ fibers
• Modulation: involves altering neural afferent activity along the pain pathway
• Suppression occurs through local inhibitory interneurons and descending efferent pathways
• Descending axons from cerebral cortex, hypothalamus, thalamus, periaqueductal gray area, nucleus raphe magnus, and locus coeruleus synapse with and suppress pain transmission to brainstem and spinal cord dorsal horn

Perception

• Occurs once the signal is recognized by various areas of the brain, including amygdala, somatosensory areas of cortex, hypothalamus, and anterior cingulate cortex

Physiologic Consequences

• Acute pain is responsive to pharmacotherapy and treatment of the precipitating cause
• Poorly controlled acute pain may lead to chronic pain states
• Contraindications for certain medications, such as recent MI, prolonged QTi, cardiac dysrhythmias, and unstable CHF

Pharmacological Management

• SNRIs (duloxetine, venlafaxine) lack affinity for cholinergic, histaminergic, and adrenergic receptors
• SSRIs primarily used for treatment of depression, with relatively weak analgesic effects
• Corticosteroids used as adjuncts for management of acute onset and chronic pain syndromes
• Methadone: a synthetic opioid with a racemic mixture of two enantiomers, D-isomer (S-methadone) and L-isomer (R-methadone)
• Intraoperative management: opioids may need to be increased by 30-100% compared to opioid-naïve patients
• Postoperative management: consider IV PCA if unable to do RA/CPNB

Special Populations

• Elderly: alterations in pain processing and perception, with decreased nociceptive processing and neurotransmitters in CNS
• Pediatric: nociceptive pathways are well developed in premature and full-term infants, but maturation of descending pathway precedes ascending neural pathway