MINIMAL ALVEOLAR CONCENTRATION (MAC)

MINIMAL ALVEOLAR CONCENTRATION (MAC)


Introduction:
  • Most important method for measuring potency of inhalational anesthetic agent.
  • MAC is lowest concentration of anesthetic in pulmonary alveoli producing immobility for any painful stimulus (surgical incision) in 50% individuals.
  • Higher MAC, less potent anesthetic.
Substance variations:
  • Minimum MAC – Methoxyflurane (0.16%).
  • Hence, most potent inhalational agent.
  • Maximum MAC – Nitrous oxide (105)
  • Hence, least potent inhalational agent.
  • Order of potency in decreasing order (MAC in increasing order): 
  • Methoxyflurane (0.16%) > Trilene (0.2%) > Halothane (0.74%) > Cholorform (0.8%) > Isoflurane (1.15%) > Enflurane (1.68%) > Ether (1.92%) > Sevoflurane (2.0%) > Desflurane (6.0%) > Cyclopropane (9.2%) > Nitrous oxide (104%).

Factor affecting MAC

Age:

  • Young age increases & old age decreases MAC.

Alcohol:

  • Chronic intoxication increases & acute intoxication decreases MAC.

Temperature:

  • Both hypothermia & hyperthermia decrease MAC.

Electrolyte:

  • Hypercalcemia, hypermagnesemia & hyponatremia decrease MAC.
  • Hypernatremia increases MAC.

Anemia:

  • Decrease MAC.

Gases in blood:

  • Hypoxia (p02 < 40) &hypercarbia (pCO2 > 95) decrease MAC.
  • Pregnancy: Decreases MAC.

Drugs:

  • Decreasing MAC: Local anesthetics (except cocaine) Opioids, Ketamine, Barbiturates, Benzodiazepines, Verapamil, Lithium, Sympatholytics (Methyldopa, reserpine, Clonidine, Dexmedetomidine), Chronic amphetamine use.
  • Increasing MAC: Acute amphetamine intake, Cocaine, ephedrine.

Meyer-Overton hypothesis:

  • States, “Because inhalational agents act through lipid-rich CNS cells, anesthetic potency increases, MAC decreases with lipid solubility”.
  • Suggests that anesthetic target site must be hydrophobic (lipophilic) in nature.

Explanation:

  • Inhalational anesthetics are soluble in fat.
  • Relative potency as anesthetic depends on both fat & water affinity.
  • i.e., fat (oil): water partition coefficient (measurement of lipid solubility).

Exceptions to Meyer-Overton rule:

1. Isomers:

  • Structural isomers anesthetics with same oil: gas partition coefficient have different potency.
  • Eg: Enflurane & isoflurane – Structural isomers.
  • Potency of isoflurane much higher than enflurane.
2. Convulsant gases.
3. Nonanesthetics (non-immobilised).
4. Transitional compounds.
5. Cut of effect.

Other potency measurement methods:
Oil: gas partition coefficient:

  • Also measures potency of inhalational anesthetic using lipid solubility property.
  • Higher lipid solubility = Higher oil:gas partition coefficient = More potency.
  • Direct relationship between MAC & lipid solubility in terms of oil:gas partition coefficient.
  • Methoxyflurane has maximum oil:gas partition coefficient –> Hence, most lipid soluble & most potent.
  • N20 has minimum oil: gas partition coefficient –> Hence, least lipid soluble & least potent.

Exam Important

  • MAC is lowest concentration of anesthetic in pulmonary alveoli producing immobility for any painful stimulus (surgical incision) in 50% individuals.
  • Minimal alveolar concentration (MAC) is the most important method for measuring potency of inhalational anesthetic agent.
  • Higher MAC, less potent anesthetic.
  • Minimum MAC is with Methoxyflurane (0.16%), hence most potent inhalational agent.
  • Maximum MAC is with Nitrous oxide (105), hence least potent inhalational agent.
  • Order of potency in decreasing order (MAC in increasing order):  
  • Methoxyflurane (0.16%) > Trilene (0.2%) > Halothane (0.74%) > Cholorform (0.8%) > Isoflurane (1.15%) > Enflurane (1.68%) > Ether (1.92%) > Sevoflurane (2.0%) > Desflurane (6.0%) > Cyclopropane (9.2%) > Nitrous oxide (104%).
  • Hypercalcemia, hypermagnesemia & hyponatremia decrease MAC.
  • Hypothermia decreases MAC.
  • Anemia & pregnancy decreases MAC.
  • Meyer-Overton hypothesis suggests that anesthetic target site must be hydrophobic (lipophilic) in nature.
  • Exceptions to Meyer-Overton rule includes nonanesthetics (non-immobilised).
  • Oil: gas partition coefficient also measures potency of inhalational anesthetic using lipid solubility property.
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