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Dec 3, 2010

Classification of Anesthetics

There are two bases for classification of general anesthetics;
  • Physical
  • Route of Administration
Inhalant Anesthetics
Anesthetics which are inhaled, reach the alveoli, exchanged with arterial blood and distributed to different body parts especially Brain are called as inhalant anesthetics.
There are 3 important phases of anesthesia;
  • Induction
  • Maintenance
  • Recovery
Factors Affecting Anesthesia
  • Blood Gas Solubility
  • Potency
  • Gas Lipid Solubility
Blood Gas Solubility
  • More the solubility of the inhalant anesthetic, it will take more time to saturate and more time to depress the Brain and vice versa.
  • So, more the solubility of an inhalant anesthetic, it will take more time for induction and recovery of Anesthesia.
Low solubility inhalant anesthetic
  • Low the solubility of inhalant anesthetic, minor change in concentration will lead to greater change in depth of anesthesia.
  • Low solubility inhalant anesthetics include Nitrous oxide, Ether, Halothene, Fluorine.
Potency
  • It is the ability of the drug to depress the CNS. It is measured by MAC value.
  • MAC (Minimum Alveolar Concentration)
  • Minimum concentration in alveoli which can induce anesthesia in 50% of the patients
  • Lower the MAC value, more potent is the anesthetic agent.
  • MAC value of drugs is in following order;
  • Methoxy Fluorine < Halothene < Isofluorine < Ether < Nitrous Oxide
Gas Lipid Solubility
  • If a drug is more lipids soluble, it will take more time for induction and recovery. Its value is in the following order;
  • Nitrous Oxide > Isofluorine > Halothene > Ether > Methoxy Fluorine
Examples of Inhalant Anesthetics
Volatile
Chloroform, Ether mixture, Halothene, Isofluorine, N-fluorine, Methoxy fluorine, vinyl ether
Gaseous
Cyclopropaine, Nitrous Oxide
Chloroform
  • In 1847, it was used as general anesthetic. It is still in use.
  • In 1846, ether was used as general anesthetic.
  • If it is used externally, it is rubifacient.
  • If it is used by oral route, it has anti-spasmatic (Muscle relaxant), anti-zymatic (stop production of gas) and carminative (removal of gases) effects.
  • If inhaled, it induces anesthesia. It induces rapid induction. It gives good analgesia and muscle relaxation.
  • It can also cause irritation in alveoli. So, Bronchial and salivary secretions are increased and there is possibility of pneumonia. So, along with this drug, Head should be down and atropine can also be used.
  • It also induces hepato-toxicity, myocardial sensitization to epinephrine. So, there are chances of Cardiac Arrhythmia.
  • It is non-explosive and non-inflammable.
Ether
  • It increases bronchial and salivary secretions when inhaled due to irritation.
  • Good analgesic and relaxant
  • Induction is slow.
  • It is inflammable and explosive.
  • Low toxicity problem. So, mixture of chloroform and ether is best combination as an anesthetic.
  • ACE (alcohol, chloroform and ether mixture) in the ratio of 1:2:3 is also available.
CycloPropaine
  • Inflammable, explosive
  • Good analgesic and relaxant
  • Can cause myocardial sensitization
Nitrous Oxide (Laughing Gas)
  • It induces light anesthesia (pleasant induction).
  • Used in humans
  • Low toxicity
  • Rapid induction and recovery
Injectable Anesthetics
These are of two types;
Depressants
  • Barbiturates (thiopental, pentobarbitone, secobarbitone, hexobarbitone)
  • Chloral-hydrate
  • Etomidate
  • Midazolam
Dissociative
  • Phencyclodine
  • Ketamine
  • Tiletamine
  • These induce superficial sleep. There is good analgesia, poor relaxation and poor hyporeflexia.
  • Myotonia, reflexes and amnesia is also present.
  • Eyelids remain open and there are chances of corneal ulceration. So, use some ointment to close eyes.
  • Use some tranquilizers or sedatives
  • Ketamine and Xylazine combination is best.
  • Ketamine is used in veterinary practice.
Examples of Injectable Anesthetics
Thiobarbitone
  • It induces rapid and smooth induction and recovery.
  • Its termination of effect is due to Re-distribution (It is lipid soluble. So, highly perfused organs like brain receive more concentration and then drug moves to periphery).
  • Give less dose of this drug.
Pentobarbitone
  • It is metabolized
Propofol
  • It induces smooth & rapid induction and recovery.
  • Its termination of effect is due to metabolism because drug moves to all body parts.
Chloral-Hydrate
  • It induces slow induction but prolonged recovery.
  • Termination is due to metabolism.
  • Hypnotic in nature due to metabolite called Trichloro Ethanol.
  • Perivascular leakage also leads to necrosis.
  • Still used in equines.
  • Pre-anesthetic agents are used with these drugs.

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