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  • Potent (halogenated) agents currently in use: isoflurane, sevoflurane, desflurane; older agents: halothane, enflurane, methoxyflurane
  • Nitrous oxide chemically unrelated
  • Blood gas partition coefficient is a measure of solubility of the inhalation agents. The less soluble the agent, the more rapid the onset and offset:
    • Liposolubility: halothane > isoflurane > sevoflurane > desflurane

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Anesthetic Effect of Various MAC Values
MAC valueEffect
1 MACAlveolar concentration of anesthetic agent necessary to prevent movement in response to surgical incision in 50% of patients
1.5 MAC90% will not move
0.3 MACMAC awake: patients will become conscious if no other agent used
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Factors Affecting MAC
AgeTemperatureSubstancesOther
Increased MACYoung (maximum at age 6 months)Hyperthermia
  • Acute amphetamines
  • Chronic alcohol
Decreased MACOldHypothermia
  • Acute alcohol
  • Concurrent opioids
  • Pregnancy
  • Regional anesthesia
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See following table.

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Properties of Volatile Anesthetics
DesfluraneSevofluraneIsofluraneNitrous oxide
MAC (%)6.01.71.2105
B:G partition coefficient0.450.651.40.47
PungentYesNoYesNo
SVRDecreaseDecreaseDecreaseIncrease
Heart rateIncrease secondary to sympathetic stimulationNo changeIncrease secondary to baroreceptor reflex from decreased MAPModerate increase secondary to sympathetic stimulation
MAPTransient increase with rapid rise in concentration, and then decreaseDecreaseDecreaseVariable
ContractilityDecreaseDecreaseDecreaseDecrease
MetabolismMinimalModerateMinimalNo
Other properties
  • Agent that is most likely to form carbon monoxide when in contact with dry CO2 absorbent
  • Breakdown product compound A is nephrotoxic in animals (unclear clinical significance)
  • Avoid flows <2 L/min
  • Expands air spaces (bowel, lung blebs)
  • Increases pulmonary vascular resistance
Clinical considerations
  • Faster emergence than sevoflurane and isoflurane
  • Good for inhalation induction
  • Inexpensive
  • May increase nausea
  • Second gas effect speeds induction and emergence
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  • All halogenated agents are vasodilators; hypotension can result with high inhaled concentrations
  • Desflurane has a sympathomimetic effect, especially during rapid increase in concentration, with tachycardia and possibly hypertension. This usually resolves after a few minutes
  • Halothane also depresses myocardium (useful in tetralogy of Fallot)
  • N2O has a mild sympathomimetic effect, usually not clinically significant

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  • All inhalation agents interfere with somatosensory-evoked potential monitoring: See Chapter 105
    • Decrease in amplitude
    • Increase in latency
    • Maintenance of general anesthesia at a half MAC or less is usually acceptable (+IV propofol infusion)
    • Avoid rapid changes in concentration

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Effect of Intracardiac Shunts on Speed of Inhalation Induction
ShuntSpeed of inhalation induction
L → RFaster
R → LSlower
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Respiratory Effects
IncreaseDecrease
BronchodilationVentilatory response to hypercarbia
Respiratory rateVentilatory response to hypoxia
Hypoxic pulmonary vasoconstriction
Tidal volume
Minute ventilation
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