2. • Concept was developed by Dr. Edmund Eger in
1965.
• As a tool to assess the potency of inhalational
agents and to titrate its dosage.
• Could replace subjective tools for analysis of
depth of anaesthesia including Gudels stages
of Anaesthesia or Woodbridge Concept of
Nothira.
3. • Definition:
Minimum alveolar concentration of inhaled
anaesthetic at sea level required to suppress
movement to a surgical stimulus in 50% of the
patients.
• Also known as ED50 for immobility, MAC-
movement or median alveolar concentration.
4. • MAC uses the measurement of end tidal
anaesthetic as a measure of the level of
anaesthetic within the alveoli and in turn at
the level of the central nervous system.
• Its is an easy and reproducible technique
making it a standard measurement.
5. Inhalational agents and immobility
• Volatile anesthetics depress spinal motor-
neuron excitability.
• Immobility is also mediated by other aspects
of anesthesia such as amnesia and hypnosis in
the subcortical and cortical regions of the
brain.
6. Meyer-Overton relationship
• All fat-soluble agents would function as
anesthetics due to their ability to cross the
lipid bilayer of neurons.
• MAC x Lamda = a constant,1.82 atmospheres
• Where Lamda equals the olive oil/gas
partition coefficient for the volatile anesthetic.
7. Factors affecting MAC
• Physiological factors
AGE: Peaks at 6 months of age
Decreases by approx 6% per decade- for all
agents.
• MACage = MAC40 × 10^ [-0.00269(age - 40)]
• where MACage is the MAC at a given age and
MAC40 is the MAC value at age 40.
8. MAC decreases with
• Anemia
• Hypercarbia
• Hypoxia
• Decrease in body temperature- 4-5% with every
decrease in degree centigrade.
• The anesthetic requirement continues to decrease in a
linear fashion, with complete elimination of the need
for anesthetic at 20 degrees C.
• Exception – Nitrous oxide- MAC doesn’t change with
change in temperature.
9. MAC and pregnancy
• MAC requirements are decreased during
pregnancy by as much as 30%.
• The increased potency of volatile anesthetics
for this population also extends into the early
postpartum period.
10. Pharmacological factors
• MAC is decreased by
sedative hypnotic agents
barbiturates
benzodiazepines
induction agents
ketamine
lithium
verapamil
11. Pathologic factors
MAC is decreased in
decreased consciousness as in trauma,
cerebrovascular insult.
• Dementia.
• Cognitive dysfunction.
• PaO2 <30mm Hg
• Acute metabolic acidosis.
• Acute hemorrghagic hypotension.
• Hyponatremia.
12.
13. Clinical significance
• The most commonly used anesthetics follow the
order of nitrous oxide
> desflurane > sevoflurane > isoflurane >
halothane, for highest to lowest MAC- This has
direct corelation with Meyer Overton rule.
• Additive effect of inhalational agents - 0.5 MAC of
nitrous oxide (54%) combined with 0.5 MAC with
sevoflurane (1%), will have the clinical effect of 1
MAC of any volatile anesthetic.
14. • 1 MAC – ED50
• 1.1 MAC- ED68
• 1.2 MAC – 95%
• SD-10% for MAC normal distribution curve.
15. Other types of MAC
• MAC awake - The anesthetic concentration needed
to suppress a voluntary response to verbal
command (i.e., eye-opening) in 50% of patients.
• MAC awake = MACimmobility x 0.33
• MACamnesia : concentration required to suppress
recollection or explicit memory of a noxious
stimulus.
• MACamnesia = MACimmobility x 0.25
16. • MAC for endotracheal intubation (MACEI) is defined as
the end-tidal concentration of inhaled anesthetic
agents at which 50% of patients can be intubated
smoothly.
• MACEI of sevoflurane = 1.3 x MAC in children.
• = 3x MAC in adults.
• Very high concentration of volatile agents which might
result in hemodynamic instability.
• So, muscle relaxants and opioids are used to facilitate
intubation as a part of balanced anesthesia.
17. • MAC bar- estimate of the MAC of volatile
anesthetic that blocks autonomic responses to
surgical incision in 50% of patients.
• MAC-BAR was determined by measuring the
level of catecholamine in venous blood and
has been calculated to be roughly 1.5 MAC.
