General Anesthetics I & II Flashcards Preview

Nervous System: Unit IV > General Anesthetics I & II > Flashcards

Flashcards in General Anesthetics I & II Deck (39)
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1

General purpose of general anesthetic

  • suppress pain/knowledge of pain during surgery
  • = analgesia + amnesia + LOC + suppressed sensory and autonomic reflexes
  • rapid and smooth onset w/rapid recovery upon termination of drug administration
  • wide margin of safe use

2

MOA of inhaled anesthetics

  • not completely understood
  • no single "receptor"
  • uncharged, nonpolar molecules
  1. drugs act via action on lipid component of nerve cell membrane
  2. drugs act @ protein component of membrane

3

Classes of inhaled general anesthestics 

  • inorganic gases
  • ethers (diethyl ether)
  • hydrocarbons
  • chlorinated hydrocarbons
  • fluorinated hydrocarbons
  • fluorinated ethers

4

Inorganic gases: examples

  • xenon
  • nitrous oxide
  • nitrogen

5

Hydrocarbons: examples

  • cyclopropane
  • ethylene

6

Chlorinated hydrocarbons: examples

  • chloroform
  • tricholoroethylene

7

Fluorinated hydrocarbons: examples

halothane

8

Fluorinated ethers: examples

  • enflurane
  • isoflurance
  • desflurane
  • sevoflurane

9

Classes of IV general anesthetics (+examples)

  • Barbituates
    • thiopental
  • Benzodiazepines
    • diazepam
  • Opioid analgesics
    • morphine
    • fentanyl
  • Glutamate receptor agent
    • ketamine
  • Misc. agents
    • propofol
    • etomidate

10

Lipid theory of general anesthetic action

  • volatile anesthetics partition into oil > water
    • higher oil:water partition coefficient ==> increased potency
  • minimul alveolar concentration of anesthetic is inversely proportional to potency
  • exert effects by partitioning lipid component of nerve cell membrane

11

Protein theory of general anesthetic mechanism

  • anesthetics act via interactiosn w/hydrophobic pockets of membrane proteins
    • hydrophobic domains of membrane proteins = "receptors"
  • interaction w/membrane proteins may lead to decreased membrane excitability
  • size cut-off for structurally-related compounds (i.e. molecules that are too large don't have anesthetic properties) indicate that they might need to fit into pockets of specific sizes @ membrane proteins
  • ESR evidence supports immobilization due to proteins in lipid membranes

12

Physicochemical properties of inhaled general anesthetics vs. potency

  • higher oil: water partition ==> more potent
  • size cut-off for structurally similar molecules
  • stereoselectivity of anesthetic action
  • Minimal alveolar concentration of anesthetic that produces insensitivity = inversely proportional to potency

13

Action of inhaled general anesthetics @ nervous system

  • depress neuronal excitability @ CNS
  • occurs via potentiation of GABAA receptor activity
  • ==> increased duration of inhibitiory postsynaptic potentials ==> inhibition @ CNS
  • conduction block is NOT believed to underlie anesthesia

14

Development of General Anesthetic Action

  • descending depression: progressive loss of fxn from higher (cognition/consciousness) to lower (respiratory) levels @ CNS
  • Stage I = analgesia
  • Stage II = excitement, delirium
  • Stage III = surgical anesthesia
  • Stage IV = medullary paralysis
    • respiratory failure, vasomotor collapse, circulatory failure 

15

Phases of Stage III anesthesia

  • Plane 1 = regular, metronomic respirations
  • Plane 2 = onset of muscular relaxation, fixed pupils
  • Plane 3 = good muscular relaxation, depressed excursion of intercostal muscles during respiration
  • Plane 4 = diaphragmatic breathing only, dilated pupils

16

Time course of surgical anesthesia

  • Induction: time between initiation of administration of anesthetic and attainment of surgical anesthesia
  • Maintenance: time during which surgical anesthesia is in effect (surgery carried out during this period)
  • Recovery: time following termination of administration of anesthetic until complete recovery of patient from anesthesia

17

Factors that impact the rate at which anesthetic reaches brain

(1) concentration of the anesthetic 
in inspired air

(2) alveolar ventilation rate

(3) pulmonary blood flow (cardiac output)

(4) blood:gas partition 
coefficient

(5) potency (oil:gas partition coefficient)

18

Factors that determine rate of recovery from anesthesia

  • same as rate of onset:
  • concentration
  • AVR
  • CO
  • blood:gas coefficient
  • potency

19

Factors that impact reaching steady-state of general anesthetic: uptake factors

  • lung factors
    • over-ventilation ==> increased rate of induction
  • uptake by blood from alveoli
    • solubility in blood/pulmonary blood flow
    • rate of approach to stage 3 is inversely proportional to PBF & solubility of gas 

20

Factors that impact reaching steady-state of general anesthetic: uptake into tissue factors

  • rate of uptake into tissues depends on:
  1. anesthetic gas solubility @ tissue
    • anesthetics ~ equally soluble in blood vs. lean tissues, but more soluble in fatty tissues ==>
    • fatty tissue = resevoir for anesthetic
  2. tissue blood flow
    • higher blood flow ==> faster delivery
  3. patrial pressures of anesthetic @ blood/tissues
    • rate of uptake is fast at first due to large difference in partial pressures
    • rate slows as anesthesia develops

21

Factors that impact reaching steady-state of general anesthetic: tissue distribution factors

  • vessel-rich tissue: highly vascular tissue, e.g. brain, kidney, liver, heart, endocrine
    • high uptake rate
  • muscle tissue: muscle/skin
    • 2-4 hours
    • slower b/c lower perfusion compared to vessel-rich group
  • fat tissue:
    • very slow uptake b/c high solubility and low perfusion
    • impacts recovery: longer duration anesthesia ==> longer recovery due to anesthetic loading @ fatty tissue

22

General physiologic principle dictating behavior of uptake/elimination of general anesthetics

  • @ steady state: concentration of anesthetic @ alveoli = concentration of anesthetic @ blood = concentration of anesthetic @ tissues
  • during uptake: anesthetic @ alveoli is increased ==> increased @ blood ==> increased @ tissues
  • during elimination: anesthetic @ alveoli is decreased (via expiration) ==> decreased @ blood ==> decreased @ tissues

23

Major process of general anesthetic elimination

  • anesthetic is primarily cleared by the lungs
  • metabolism @ liver is generally not important in terminating anesthetic action

24

Nitrous oxide (N2O): advantages, disadvantages

  • +
    • only true gaseous agent
    • excellent analgesic
    • rapid onset/recovery
    • increases cerebral blood flow less (consider in head injury)
  • -
    • low potency: can't be used alone
    • hypoxia can result after termination
    • contraindicated in pregnancy

25

Diethyl ether: advantages, disadvantages

  • not currently used in practice
  • +
    • "complete anesthetic"
    • good analgesic
  • -
    • flammable/explosive
    • excessive respiratory tract secretions ==> choking
  • slow induction/recovery

26

Chloroform: advantages, disadvantages

  • no longer in common use
  • -
    • ==> cardiac arrhythmias
    • ==> hepatotoxicity

27

Halothane: advantages, disadvantages

  • +
    • mod-high potency
    • induction/recovery not prolonged
    • non-explosive
    • non-irritant (reduced respiratory secretions)
  • -
    • poor analgesic
    • ==> respiratory/CV failure (arrhythmias)
    • ==> liver damage (fever, nauseau ==> hepatic failure)
    • ==> malignant hyperthermia (muscle rigidity ==> fever)

28

Tx for malignant hyperthermia

  • give dantrolene (muscle relaxant)
  • ice water immersion
  • use IV anesthetic if hx indicates risk for MH

29

Enflurane: advantages, disadvantages

  • +
    • excellent analgesic
    • induction/recovery moderately fast
    • good muscle relaxant
  • -
    • ==> seizures (@ induction/recovery)
    • some CV effects/hepatotoxicity (but less than halothane)

30

Isoflurane: advantages, disadvantages

  • similar to enflurane; most widely used
  • +
    • more potent than enflurane
    • little hepatic/renal toxicity
    • no seizures
    • rapid/smooth induction/recovery
    • minimal CV depression
    • good muscle relax
  • -
    • pungent ==> coughing