Anesthesia and Analgesia Terminology

Anesthetics

• Anesthesia: loss of all sensation
• Analgesia: loss of pain sensation
• Local anesthetics: block conduction of nerve impulses in peripheral nerves or spinal cord
• General anesthetics: block cortical neuronal activity underlying consciousness and all sensation

General Anesthesia

• Used to prevent consciousness during major surgical procedures
• Produces loss of consciousness and amnesia (patient doesn't recall the surgical procedure)

Drug Properties

• Local anesthetics divided into two main groups: ester-type (e.g., cocaine, benzocaine) and amide-type (e.g., lidocaine, bupivacaine)
• Duration of action: short, medium, or long, determined by rate of diffusion and absorption away from site of administration
• Factors influencing duration: chemical properties, pH, and blood flow
• Epinephrine can be added to prolong an anesthetic's duration of action by producing vasoconstriction

Metabolism

• Ester-type local anesthetics metabolized to PABA derivatives
• Amide-type local anesthetics metabolized by CYP450 enzymes
• Metabolites excreted in urine in both cases

Allergic Reactions

• Reactions are fairly common, especially in those who receive repeated administration of topical anesthetics
• Ester-type anesthetics cause hypersensitivity reactions more frequently than amide-type anesthetics

Nerve Block Anesthesia

• Local anesthetic injected into or adjacent to a peripheral nerve or nerve plexus
• Examples: radial nerve block, infraorbital block, brachial plexus and cervical plexus blockade

Spinal Intrathecal Anesthesia

• Used to block somatosensory and motor fibers during procedures like surgery on lower limb or pelvic structures
• Anesthetic injected into subarachnoid intrathecal space below level at which spinal cord terminates
• Spread of anesthetic controlled by horizontal tilt of patient and specific gravity of local anesthetic solution
• Risks: HA associated with CSF leakage from LP, respiratory depression if anesthetic ascends too high, and small risk of meningitis and infection

Lidocaine

• Produces local anesthesia after topical or parenteral administration
• Most widely-used local anesthetic
• Available in many forms (e.g., topical solutions/ointment, oral sprays, viscous gels, and parenteral formulations)
• Part of EMLA (along with prilocaine), can anesthetize intact skin to a depth of 5 mm
• Used in kids for local anesthesia before venipuncture, IV placement, or circumcision

Induction Rate

• Three major factors determine induction rate of anesthesia: alveolar partial pressure of the anesthetic in inspired air, ventilation rate, and rate of increase in anesthetic partial pressure in blood
• Blood:gas partition coefficient influences induction rate

Pharmacologic Effects

• Four stages of anesthesia induction:
  • Stage 1: analgesia and conscious sedation
  • Stage 2: paradoxical excitation (not usually observed with modern anesthesia)
  • Stage 3: goal of surgical anesthesia, LOC, and inhibition of spinal reflexes
  • Stage 4: can cause cardiovascular collapse, depression of respiratory and vasomotor nuclei in brainstem

Thiopental and Propofol

• Both drugs have rapid onset of action, causing unconsciousness in about 20 seconds
• Duration of action is short (5-10 minutes) due to redistribution from brain to peripheral tissues
• Propofol is rapidly metabolized and eliminated, causing little hangover
• Thiopental accumulates in fats and muscles, causing slow elimination and some hangover
• Both drugs can depress cardiac and respiratory function

Fentanyl

• Strong opioid agonist used to treat moderate-to-severe pain
• Given IV or epidurally in combo with other drugs for surgical or obstetric analgesia and anesthesia
• Used to provide anesthesia during cardiac surgery (e.g., CABG) because it doesn't cause CV toxicity
• Doesn't produce amnesia or complete LOC, so usually combined with a benzo (e.g., midazolam) to produce amnesia and increased sedation

Ketamine

• Produces dissociative anesthesia when given IV
• Characterized by analgesia, reduced sensory perception, immobility, and amnesia
• Increases blood pressure but has little effect on respiration
• Major drawback: tendency to cause unpleasant effects during recovery like delirium, hallucinations, irrational behavior
• Often used in kids because they are less likely to experience these adverse effects (usually combined with a benzo)