Local Anesthetics & Pain Management: Pharmacology, Uses, and Adverse Effects

Local Anesthetics

Medications that block sodium channels to prevent nerve impulse generation.

Ester-type Anesthetics

Inactivated by blood esterases.

Amide-type Anesthetics

Inactivated by liver enzymes.

Adverse Effects of Local Anesthetics

CNS: seizures, coma, excitation → depression; Cardiovascular: dysrhythmias, bradycardia, cardiac arrest; Allergic reactions (more common with esters); Labor & Delivery: suppress uterine contractions, neonatal bradycardia; Methemoglobinemia: rare (prilocaine).

Ester

Type of local anesthetic metabolized by blood esterases, with a higher allergic risk.

Amide

Type of local anesthetic metabolized by liver enzymes, with a rare allergic risk.

Mechanism of Action

Blocks sodium influx, preventing nerve impulse propagation.

Selectivity

Local anesthetics work locally and do not cause unconsciousness.

Onset Factors

Size: smaller → faster onset; Lipid solubility: higher → faster onset; Ionization: nonionized at pH → faster onset.

Termination of Anesthesia

Depends on regional blood flow.

Use with Vasoconstrictors

Epinephrine reduces blood flow, prolongs anesthesia, and lowers systemic absorption.

Pharmacokinetics of Ester Anesthetics

Metabolized in the blood.

Pharmacokinetics of Amide Anesthetics

Metabolized in the liver.

CNS Adverse Effects

Seizures and coma.

CV Adverse Effects

Dysrhythmias and cardiac arrest.

Labor/Delivery Adverse Effects

Prolonged labor and neonatal bradycardia.

Methemoglobinemia

A rare condition associated mainly with prilocaine.

Chloroprocaine

An ester local anesthetic with rapid onset, brief duration, and low toxicity.

Lidocaine

An amide local anesthetic that is the most common and can treat dysrhythmias.

Cocaine

An ester local anesthetic known for CNS stimulation/euphoria and being a potent vasoconstrictor.

Topical Use of Local Anesthetics

Used for pain/itch relief in burns, insect bites, sunburn, and minor procedures.

Injection Types

Infiltration: directly in tissue for minor surgery; Nerve block: near nerve for dental and limb surgery; Spinal: in subarachnoid space; Epidural: in epidural space for labor and post-op pain.

General Anesthetics

Medications that induce unconsciousness and insensitivity to pain.

Balanced Anesthesia

A combination of inhaled anesthetic, opioid, and neuromuscular blocker for smooth induction and fewer side effects.

MAC (Minimum Alveolar Concentration)

A measure of anesthetic potency; low MAC indicates high potency.

Adverse Effects of General Anesthetics

Respiratory and cardiac depression; Malignant hyperthermia is rare but life-threatening.

Opioids

Analgesics that provide pain relief without loss of consciousness.

Opioid Receptors

Mu, Kappa, Delta.

Opioid Mechanism

Mimic endogenous opioids, mainly at mu receptors for analgesia.

Serious Adverse Effects of Opioids

Respiratory depression, constipation, urinary retention, orthostatic hypotension, miosis, emesis, birth defects, and increased intracranial pressure.

Tolerance

Analgesia, euphoria, sedation, respiratory depression.

Physical dependence

Withdrawal if abrupt cessation.

CNS depressants

Increase sedation and respiratory depression.

Anticholinergics

Increase constipation and urinary retention.

Overdose Triad

Coma, respiratory depression, pinpoint pupils.

Strong agonists

Example: Morphine; High analgesia, abuse potential.

Moderate/strong

Example: Codeine; Less analgesia, lower abuse potential.

Agonist-antagonist

Example: Pentazocine; Lower abuse, precipitates withdrawal in dependent patients.

Pure antagonist

Example: Naloxone; Reverses effects, treats overdose.

Patient-controlled analgesia (PCA)

Steady pain control, avoids over/under sedation.

Migraine

Neurovascular disorder; artery dilation and inflammation.

Abortive drugs

Stop attack.

Prophylactic drugs

Reduce frequency/severity.

Nonspecific analgesics

Example: acetaminophen, NSAIDs.

Triptans

5-HT1B/1D agonists; vasoconstriction.

Contraindications for triptans

Heart disease, uncontrolled hypertension.

Interactions for triptans

Other triptans, SSRIs/SNRIs; serotonin syndrome.

Ergot alkaloids

Second-line, risk of ergotism, dependence, contraindicated in pregnancy.

Beta-blockers

Example: propranolol.

Anticonvulsants

Example: divalproex.

Antidepressants

Example: amitriptyline.

CGRP drugs

Monoclonal antibodies and small-molecule antagonists.

Estrogen supplements

For menstrual migraines.

COX-1

Gastric protection, platelet aggregation.

COX-2

Inflammation, pain, fever.

Aspirin

First-gen NSAID; irreversible COX inhibition, antiplatelet effect ~8 days.

Ibuprofen/Naproxen

COX-2 selective, less GI risk, increased cardiovascular risk.

Celecoxib

Second-gen NSAID.

Acetaminophen

Non-NSAID; pain/fever relief, no anti-inflammatory/GI effects.

NSAIDs adverse effects

GI ulcer, bleeding, renal impairment.

Acetaminophen adverse effects

Hepatotoxicity (overdose), SJS/TEN risk.

Aspirin + other NSAIDs

Reduces antiplatelet effect.

Acetaminophen + alcohol

Increases liver toxicity.

Acetaminophen + warfarin

Increases bleeding.

Nociceptive pain

Transduction, transmission, perception, modulation.

Acute pain

Protective, sudden, short.

Chronic pain

>3-6 months, impacts quality of life.

Multimodal analgesia

Combines drugs with different mechanisms for better pain control.

Barriers to pain management

Misconceptions, cultural beliefs, lack of protocols/access.

Person-centered care

Integrates patient preferences, values, individual circumstances.

Interprofessional collaboration

Work with healthcare team to set pain goals.

Cultural sensitivity

Respect cultural influences on pain expression/management.

Ester vs amide anesthetics

Differences in metabolism and allergy.

Epinephrine with local anesthetic

Used to prolong effects.

Topical anesthetic toxicity reduction

Name four methods.

Most serious opioid effect

One that doesn't develop tolerance.

Aspirin vs other NSAIDs

Difference in COX inhibition.

Acetaminophen vs NSAIDs

Difference in anti-inflammatory/GI effects.

Risks of combining triptans

With other triptans/ergot drugs.

Acute vs chronic pain

Contrast between the two.