inhalant anesthetics

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Anonymous
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11108
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inhalant anesthetics
Updated:
2010-03-18 14:03:02
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anesthesia
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Inhalant Anesthetics
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  1. Advantages?
    • Control of airway
    • High FiO2 (fraction of inspired oxygen)
    • Rapid changes in depth
    • Rapid recovery - ventilation
    • Compatible with renal or hepatic disease
    • Cost effective
  2. 3 Basic Steps
    • 1. Breathed in
    • 2. Absorbed - alveoli into blood
    • 3. Absorbed - blood to brain
  3. Uptake and Distribution
    Goal of Inhalant Anesthesia?
    To acheive an adequate partial pressure in the brain to cause the desired level of CNS depression.
  4. Alveolar Partial Pressure
    What are the components?
    • What is added
    • -Inspired Concentration
    • -Alveolar Ventilation
    • What is taken away
    • -Solubility of anesthetic in blood
    • -Cardiac output
  5. Alveolar Partial Pressure
    Inspired Concentration?
    • Directly proportional
    • Higher % concentration - faster to desired partial pressure
  6. Alveolar Partial Pressure
    What does it depend on? How is it set?
    • Function of vaporizer setting and fresh gas flow into the circle system
    • % concentration at induction is higher than at maintenance
    • -The higher the inspired partial pressure, the more rapidly the alveolar partial pressure approaches the inspired partial pressure
  7. Alveolar Partial Pressure
    Alveolar Ventilation - how does it affect it?
    • Better ventilation = faster increase in alveolar %
    • Depends on tidal volume and RR
    • Rapid shallow breathing = slow rise of alveolar pressure (dead space rebreathing)
    • Spontaneous vs. Mechanical (much more efficient)
  8. Alveolar Partial Pressure
    Time Constant - what is it?
    • Time required for flow through a container to equal the capacity of the container
    • Lung = FRC/Valveolar
    • Anesthetic circuit = circuit capacity/FGF
    • (FRC = functional residual capcity, FGF = fresh gas flow)
  9. Alveolar Partial Pressure
    Factors that offset the rise of PA?
    • Anesthetic Solubility
    • Cardiac Output
  10. Alveolar Partial Pressure
    Anesthetic Solubility
    • Capacity of blood to take up anesthetic
    • High solubility = more drug moves into blood from alveoli
    • Drug dissolved in blood = not available as a diffusible gas
    • High Sol = more molecules required to saturate blood before PP can increase
    • Speed of onset is closely related to anesthetics solubility
  11. Alveolar Partial Pressure
    Blood:Gas Partition Coefficient
    • Index of solubility of anesthetic agent in the blood
    • Measured at equilibrium
    • -PP are equal
    • -Concentrations are not equal
    • High number = high solubility
    • Methoxyflurane>Halothane>Isoflurane>Sevoflurane
  12. Alveolar Partial Pressure
    Cardiac Output
    • Blood flow through lungs removes inhalant from alveoli, lowers PA, delays onset
    • High CO = large distribution throughout body
    • = less than 8% total to brain
    • Low CO = Heart-Lung-Brain perfusion preserved
    • = very high % of total CO goes to brain
    • = large amount of anesthetic to brain
  13. Clinical Onset of Anesthesia
    • Rapid in
    • -sick/debilitated animals
    • -CV compromise
    • Slow in
    • -stressed/excited animals
  14. Alveolar Partial Pressure
    When does it increase/when does anesthesia occur faster?
    • Good alveolar ventilation
    • Low CO
    • Low anesthetic solubility
  15. Adequate Plane of Anesthesia?
    • PA high enough to guarantee sufficient PBr
    • MAC = minimum alveolar concentration
    • % at which 50% of animals will not move in response to surgical stimulus
    • more than 1 MAC to guarantee surgical anesthesia in 100% of patients
    • 1.2-1.3 x MAC required for most animals
  16. Alveolar Partial Pressure vs. Alveolar Concentration
    • Gas phase - partial pressure = % conc x atmospheric pressure
    • Use alveolar concentration in MAC determination for convenience
    • -vaporizer setting delivers a percent concentration
    • -do not need to know barometric pressure
  17. Relative MAC of anesthetics?
    halothane<isoflurane<sevoflurane

    comparison of potency (halothane has lower MAC and is more potent)
  18. Factors that Decrease MAC
    • hypothermia
    • pregnancy
    • old age
    • hypothyroidism
    • hypoxemia (<40mmHg)
    • hypercarbia (>95mmHg)
    • concomitant anesthetic drugs (inj, inhal, sed, analgesics)
  19. Factors that INCREASE MAC
    • hyperthermia
    • hyperthyroidism
    • hypernatremia
    • CNS stimulants
    • -increase catecholamines
    • -ephedrine, amphetamines
  20. What does anesthetic recovery depend on?
    • Movement of drug from brain back into blood
    • Movement from blood to alveolus
    • Removal from alveolus into breathing system and out scavenger

    (same factors influence anesthetic uptake)
  21. Physiologic Effects of Inhalant Anesthetics
    Which ones, margin of safety, what happens with OD?
    • Isoflurane, Sevoflurane, Desflurane - potent!
    • Low margin of safety (TI 2-4)
    • OD
    • -severe ventilatory depression
    • -severe cardiovascular depression
    • -severe CNS depression
    • -Death
  22. CNS - Mechanism?
    • unknown!
    • collection of many end points that are site specific
  23. CNS Effects
    EEG (Electro encephalogram) , etc
    • EEG wave changes
    • -awake: low amp, high freq
    • -inhalation anesthesia: high amp, low freq
    • -ISO, Sevo, Des 2 MAC or > = isoelectric pattern
    • All agents depress seizure activity
    • No analgesia
  24. What happens to cerebral blood vessels?
    • VASODILATION
    • increased CBF
    • decreased CMR (metabolic rate)
    • increased ICP
    • hyperventilation (PaCO2 < 30mmHg) reduces CBF with iso, sevo, des
  25. Cardiovascular effects
    Contractility
    • Dose dependent
    • Contractility
    • -not myocardial depressants around MAC in health
    • -may decrease contractility in pt w/heart dz
    • -may decrease contractility at conc > 1xMAC
  26. Cardiovascular effects
    Systemic Vascular Resistance
    SV
    HR
    Arterial BP
    • Dose dependent vasodilation (decreased resistance)
    • Decreased SV
    • Dose dependent increase in HR (suppressed w/opioids)
    • usually remains CONSTANT
    • Arterial BP decreases (less dramatic w/Des)
  27. CV Effects: Arrythmogenesis
    • Sensitization of myocardium to catecholamine induced arrhythmias
    • -stress/excitement can increase endogenous catecholamine levels
    • Iso, Sevo, Des - least effect
  28. Factors Influencing CV Effects
    • 1. Mode of Ventilation and PaCO2
    • 2. Noxious Stimulation
    • 3. Duration of Anesthesia
    • 4. Concurrent drug administration
  29. CV Effects: Mode of Ventilation
    • Spontaneous vs. Mechanical
    • Mechanical or IPPV (Intermittent positive-pressure ventilation)
    • -increased intrathoracic pressure
    • -decreased venous return
    • PaCO2 if increased
    • -direct depressant on heart and peripheral vessels - dilation
    • -indirect via SNS increasing arterial BP
  30. CV Effects: Noxious Stimulation
    • Sympathetic stimulation
    • Increase arterial BP
    • Increase HR and CO
    • Decreased or prevented by inhaled anesthetics
    • 1.5 to 2 x MAC
  31. CV Effects: Duration of Anesthesia
    • After several hours: CO and HR may increase
    • Cause unknown - dose? body position?
    • More important in research than clinically
  32. CV Effects: Concurrent Drug Administration
    • Sedatives and Injectables
    • -decreases anesthetic requirements (MAC)
    • -decrease effects from inhalant
    • -may accentuate CV depression
    • Positive Inotropes or Vasopressors
    • -do not affect anesthetic requirements
    • -counteract unwanted CV depression
  33. Respiratory Effects
    • Decrease minute ventilation (dose-dep) (Apneic index)
    • Decrease sensitivity to PaCO2
    • -depress resp. control center in medulla and peripheral chemoreceptors
    • Severely depress hypoxic drive
    • Depress hypoxic pulmonary vasocontstriction
  34. More Respiratory Effects
    • Decreased lung volume (FRC)
    • -atelectasis is common
    • Bronchodilation
    • -increased dead space ventilation
    • Airway irritation d/t pungency
    • -Des>Iso
    • -prevents mask induction w/Des
  35. Hepatic Effects
    • Depression of hepatic function and hepatocellular damage
    • -mild and transient or permanent
    • Reduce intrinsic hepatic clearance of drugs
    • Reduced blood flow
    • Iso, Sevo, Des - maintain adequate O2 supply so liver injury less likely
  36. Renal Effects
    • Dose-Depending decrease
    • -renal blood flow
    • -glomerular filtration
    • -urine output
    • Primarily pre-renal corresponding to decreased BP
    • Reversed after anesthesia
  37. Renal Effects - Sevoflurane
    • Breakdown of Sevo causes increased serum fluoride concentration
    • -potentially nephrotoxic
    • -horses? no effect
    • Sevo degradation in CO2 absorbents produces Compound A (nephrotoxic to rats)
    • -no evidence in dogs, cats, horses
    • -avoid low O2 flow rates w/prolonged anesthesia
  38. Biotransformation
    • Varying degrees of metabolism in liver
    • relative % metabolized by drug
    • Des<Iso<Sevo
    • Recovery mainly dependent on elimination through lung
  39. Skeletal Muscle Effects
    • Malignant Hyperthermia
    • -potentially life threatening genetic myopathy
    • -swine and humans
    • -other species too
    • -any agent can cause this
  40. Skeletal Muscle Effects
    Malignant Hyperthermia: Signs
    • Rapid rise in body temp (treat quick, may cause death)
    • Increased RR, dyspnea, apnea
    • Tachycardia
    • Metabolic acidosis
    • Muscle rigidity
    • Increase in end tidal CO2
  41. Skeletal Muscle Effects
    Malignant Hyperthermia: Treatment
    • Dantrolene IV (muscle relaxant, antipyretic)
    • Stop inhalant anesthesia
    • 100% O2
    • Body cooling
  42. Summary: Highly Soluble Inhalant
    • Slow rise in alveolar partial pressure
    • Slow to reach MAC
    • Slow to achieve adequate brain concentration
    • Slow to overdose
    • Slow to recover
    • More metabolism of drug
    • More hangover
  43. Summary: Low Solubility Inhalant
    • Rapid rise in alveolar partial pressure
    • Fast to reach MAC
    • Fast to achieve adequate brain concentration
    • Fast to overdose
    • Fast to recover
    • Less drug metabolism
    • Less hangover
  44. Relative MAC of drugs
    halothane<isoflurane<sevoflurane<desflurane

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