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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