Inhalation Anaesthetics

Exert effect at the Brain
1. Pressure of gas in Brain closely follows pressure in alveolus
  1. Equilibrium
    1. Brain
      1. Arterial Blood
        Alveolus
        End-expiration
Factors that determine Anaesthetic Depth:
1. Inspired inhalational agent concentration
  1. Aveolar ventilation
    1. Va = RR x Vt - Vd
      1. RR: resp. rate
        Vt: Tidal volume
        Vd: Dead space
    2. Blood:Gas solubility
      1. Tissue:Gas solubility
        Cardiac Output
        Opposite Effect in Injectable Anaesthetics
        High CO => Decreased absorption of inhalation agents from blood to brain => Anaesthetic induction time INCREASED
        Alveolar - venous anaesthetic Pp
        The greater the difference, the faster the anaesthetic is taken up
        V/Q mismatch
Advantages
1. O2 supplementation
  1. Protect airways
    1. Rapid Induction
      1. Rapid recovery
        Minimal Hepatic metabolism
        Rapid change of depth
Disadvantages
1. Cost
  1. Equipment
    1. Pollution
      1. Toxic compounds
Drugs
1. Diethyl Ether
  1. Explosive in Oxygen
    1. Inflammable in air
      1. Still used in developing countries
  2. Chloroform
    1. Toxic to Liver and Kidney
      1. No longer used
2. Halothane
  1. Halogenated hydrocarbon
  2. Liquid + Vapor
  3. Non-flammable
  4. Degraded by UV light
  5. Non-irritant
  6. Preservative: Thymol
    1. Clogs up Vaporizer
  7. 20% Hepatic metabolism
    1. Not used anymore
  8. CNS
    1. Increases CBF
      1. Increases ICP
        Doesn't alter CMRO2
        Cerebral metabolic rate: O2 demand
  9. CVS
    1. - ve Inotropic
      1. Decreases arterial BP
        Doesn't alter SVR
        Sensitizes Myocardium to Catecholamines
  10. Resp.
    1. Less respiratory depressant than isoflurane
  11. Liver
    1. Decreases Hepatic blood flow
    2. Fulminant hepatic failure
      1. Metabolites
  12. Kidney
    1. Decreases Renal blood flow and GRF
  13. Muscle
    1. Malignant Hyperthermia
  14. Cheap
3. Isoflurane
  1. Halogenated Ether
  2. Liquid + Vapor
  3. Not flammable
  4. Not degraded by UV light
  5. Irritant
    1. Mucous membranes
  6. No preservative
  7. < 1% Hepatic metabolism
  8. CNS
    1. Increase CBF
      1. Increase ICP (Less than Halothane)
        Decreased CMRO2
  9. CVS
    1. -ve Inotropic
    2. Decreases arterial BP
      1. Decreases SVR
  10. Resp.
    1. Depressant (More than Halothane)
  11. Liver
    1. Hepatic blood flow maintained
  12. Kidney
    1. May be reduced: RBF and GFR
  13. Muscle
    1. Can trigger malignant Hyperthermia
  14. Slightly more expensive than Halothane - Cheap
  15. May react with soda lime => CO
4. Sevoflurane
  1. Halogenated Ether
  2. Liquid + Vapor
  3. Non-flammable
  4. Not degraded by UV
  5. Pleasant odor
  6. No preservative
  7. 3% Hepatic metabolism
    1. OK
  8. CNS
    1. Increase in CBF
      1. Decreases ICP
        Decreases CMRO2
  9. CVS
    1. -ve Inotropic
    2. Decreased ABP
      1. Decreased SVR
  10. Resp.
    1. Depressant (between Iso & Halo)
  11. Liver
    1. Hepatic blood flow maintained
  12. Kidney
    1. Low toxicity
  13. More expensive than Isoflurane
  14. May react with soda lime => Compund A (Nephrotoxic in rat)
    1. Hypothetical
5. Desflurane
  1. Halogenated Ether
  2. Liquid + Vapor
  3. Non- flammable
  4. Not degraded by UV light
  5. Irritant
  6. No preservative
  7. Fast solubility
  8. 0.02% Hepatic metabolism
  9. CVS
    1. -ve Inotropic
    2. Decreases ABP
      1. Decreases SVR
  10. CNS
    1. Increases CBF
      1. Decreases ICP
        Decreases CMRO2
  11. Resp.
    1. Depressant (between Iso and Halo)
  12. Liver
    1. Hepatic blood flow maintained
  13. Kidney
    1. Toxicity not reported
  14. More expensive than Iso
  15. Reacats with soda lime => CO
    1. Requires specfic heated vaporizer
      1. Really expensive, Never used
6. N2O
  1. Inert organic Gas
  2. Non Irritant
  3. Non- flammable
  4. Storage: pressurized liquid => Blue cylinders
  5. Can NOT be used as Anaesthetic
    1. Oil: Gas solubility 1.4
      1. Very high MAC
  6. Lowers MAC of volatile agents
  7. Analgesic
  8. No Hepatic Metabolism
  9. 2nd Gas Effect
    1. Sparing effect of volatile agents
  10. Diffusion Hypoxia
  11. Distends gas filled spaces
    1. N2O diffuses into gas filled (air pockets) faster than N2 diffuses out
      1. Worsens pneumothroax,GDV, Vascular air emboli
    2. Don't use if enclosed gas filled space
  12. Minimal CVS and Resp. Effects
    1. Reduces the fraction of inspired air (FiO2)
    2. May increase CBF & ICP
  13. Pollution => Scavenging!
    1. Exposure may affect Bone Marrow function
Metabolism
1. Ideally NOT metabolized
  1. Opposite of Injectable Anaesthetics
  2. Toxic metabolites
    1. Liver damage

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Inhalation Anaesthetics

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	Created by melian.yates over 10 years ago