Analgesic Pharmacology and Pain Management

Pain

An unpleasant sensory and emotional experience associated with actual or potential tissue damage, serving as a personal, individual sensation with both physiological and psychological elements.

Pain Threshold

The level of stimuli required to produce the perception of pain.

Pain Tolerance

The subjective degree or duration of pain a person can endure, which varies significantly among individuals.

Acute Pain

Sudden, intense, sharp, and localized pain typically defined by duration, commonly accompanied by physiological sympathetic responses such as tachycardia, elevated blood pressure, pallor, and diaphoresis.

Persistent Pain (Chronic Pain)

Dull, aching, recurring pain lasting $3\text{--}6\text{ months}$ or longer that is persistent, long-lasting, and difficult to treat.

Superficial Pain

Pain originating from the skin or mucous membranes.

Deep Pain

Pain originating in tissues situated below the skin level.

Vascular Pain

Pain originating from perivascular or vascular tissues, such as migraine headaches.

Referred Pain

Pain perceived at a location in the body other than its actual site of origin.

Nociceptive/Neuropathic Pain

Pain described as burning, stabbing, shooting, or prickling resulting from injury or damage to peripheral nerve fibers of the CNS, PNS, or both.

Somatic Pain

Aching, dull, or throbbing pain localized in skeletal muscles, ligaments, and joints.

Visceral Pain

Dull, throbbing, or squeezing pain typically originating from internal organs or smooth muscles.

Phantom Pain

Burning, itching, tingling, stabbing, or searing sensations felt in a body part that has been surgically or traumatically removed.

Cancer Pain

Acute or chronic pain caused by tumor pressure on nerves/organs, hypoxia from blood supply blockage, metastases, pathological fractures, muscle spasms, or treatment side effects.

Central Pain

Pain resulting from tumors, trauma, inflammation, or disease processes (e.g., stroke, MS, diabetes, cancer) affecting Central Nervous System tissues.

Gate Theory of Pain Transmission

A four-step model of pain perception involving noxious stimulus insult, transmission from spinal cord to brain, perception, and descending modulation where neurotransmitters block impulses.

Transduction

The process where mechanical, chemical, or thermal stimuli convert into electrochemical energy, prompting injured cells to release chemicals like prostaglandins, bradykinin, serotonin, substance P, histamine, and potassium.

Transmission (Pain)

The movement of pain signals along A-delta and C-fibers through the ascending spinothalamic tract in the dorsal horn to the thalamus.

Modulation (Pain)

Neural activity along descending spinal pathways releasing endogenous neurotransmitters (enkephalins, endorphins, serotonin, norepinephrine, GABA) that bind to opioid receptors and close the gate to reduce pain transmission.

A-delta Fibers

Large, myelinated nerve fibers that respond to intense, potentially harmful stimulation and act to close the spinal gate to pain.

C-fibers

Small, unmyelinated nerve fibers that transmit poorly localized, dull, achy pain and act to open the spinal gate to pain.

Opioid-Induced Neurotoxicity (OIN)

A condition presenting with delirium, agitation, hallucinations, myoclonus, hyperalgesia, and worsening pain caused by an accumulation of opioid metabolites during prolonged or high-dose therapy.

Myoclonus

Involuntary, brief, rapid muscle movements often associated with opioid-induced neurotoxicity.

Hyperalgesia

A paradoxical worsening of pain characterized by increased sensitivity to painful stimuli or pain provoked by normally non-painful stimuli.

Morphine

A naturally occurring alkaloid strong opioid prototype that acts primarily as an agonist at Mu and Kappa opioid receptors in the CNS.

Fentanyl Citrate

A synthetic strong opioid analgesic $100\times$ stronger than morphine that binds strongly to Mu receptors with minimal effect on Kappa receptors.

Rigid Chest Syndrome

A rare, potentially fatal complication of fentanyl administration (typically at doses $> 5\text{ }\frac{\text{mcg}}{\text{kg}}$ or rapid IV push) causing severe respiratory muscle spasm, high airway pressures, and difficulty ventilating.

Hydromorphone (Dilaudid)

A strong, long-acting opioid agonist at Mu receptors in the CNS, where $2\text{ mg}$ IV provides an analgesic effect equivalent to $10\text{ mg}$ of IV morphine.

Oxycodone (Oxycontin)

A strong, long-acting oral opioid agonist at Mu receptors, where $15\text{--}20\text{ mg}$ oral is equivalent to $10\text{ mg}$ subcutaneous morphine.

Meperidine (Demerol)

A strong opioid agonist at Mu and Kappa receptors used for acute severe pain, restricted from long-term use due to toxic metabolite accumulation.

Buprenorphine

An opiate receptor agonist-antagonist with partial antagonist properties at Mu and Kappa receptors, used for acute/chronic pain, patients with opioid addiction history, and withdrawal suppression.

Cyclooxygenase (COX)

An enzyme found in all tissues responsible for synthesizing prostanoids, including prostaglandins and related compounds.

COX-1

A physiological COX isoform found in all tissues that protects gastric mucosa, maintains renal perfusion, and promotes platelet aggregation via TXA2.

COX-2

An inducible COX isoform expressed at sites of tissue injury that mediates inflammation, pain receptor sensitization, fever in the brain, and renal vasodilation.

Ibuprofen

A non-selective COX-1 and COX-2 inhibitor NSAID indicated for mild to moderate pain, dysmenorrhea, inflammatory conditions, and fever over $39\text{ }^\text{o}\text{C}$.

Ketorolac (Toradol)

A potent non-selective COX-1 and COX-2 inhibitor NSAID with high analgesic potency comparable to morphine, indicated for moderate to severe pain.

DRESS

Drug Reaction with Eosinophilia and Systemic Symptoms; a severe adverse reaction monitorable during Ketorolac therapy characterized by fever, rash, lymphadenopathy, and facial swelling.

Acetaminophen

A non-opioid analgesic and antipyretic with no anti-inflammatory properties that acts via selective inhibition of COX limited to the CNS.

Methoxyflurane (Penthrox)

A volatile halogenated hydrocarbon anesthetic gas that acts on GABA and neurotransmitter pathways to provide short-term acute pain relief.

Nitrous Oxide

A potent self-administered analgesic and weak CNS depressant gas where inhaling a $$20 ext