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1.13 Inhaled General Anesthetics (pharmacology and indications, volatile anesthetics)

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Lets start by the two golden rules of Inhaled Gases: MAC and Blood/Gas Partition Coefficient.

  1. MAC (Minimum Alveolar Concentration)
    1. Simple, the minimum alveolar concentration upon which 50% of the patients do not respond to a painful stimulus.
    2. The lower the MAC, the lower the amount of gas needed, the more potent the gas.
  2. Blood/Gas Partition Coefficient
    1. Simply, does the gas want to dissolve in the blood or remain as a gas? If it dissolves in the blood, it has higher Blood/Gas partition coefficient. Now the problem is that if it dissolves in the blood, its not going to affect the brain. So the lower the blood/gas particicle coefficient, the less it dissolves in the blood and the more it is available for the brain so more potent it is.

Notice, as both MAC and blood/gas partition coefficient decrease, potency increases.

Now, the stars of the show arrive:

  1. Nitrous Oxide (N2O)
    1. Pros: It has good analgesic effects (thats why it is also known as laughing gas) and acts very fast.
    2. Cons: Has a very high MAC (>100%). Always mixed with Oxygen during inhated anesthesia.

Next ones are the halogenated anesthetics:

  1. Sevoflurane: Non-irritating and commonly used. Also, used specifically in children as it is non-irrating.
  2. Isoflurane: used for maintainance of anesthesia
  3. Halothane:
    1. Pros: A very low MAC, so very high potency
    2. Cons: can cause Halothane Hepatitis and Halothane Heart (makes the heart more susceptible to arrhythmias).
  4. Desflurane: irritating and is terrible for the environment

The most dangerous side effect of using halogenated anesthetics if the person suffers from malignant hyperthermia. It is a mutation of the Ryanodine receptors in the muscles when they release all the calcium in the muscle cells causing muscle rigidity and sky-high fever.