Minimal alveolar concentration (MAC)

[Ref: SH4:p33]

Definition of MAC

MAC of an AA is the concentration at equilibrium at 1 atmospheric pressure that prevents skeletal muscle movement in response to a supramaximal painful stimulus (surgical skin incision) in 50% of patients
* ie. the ED50 of AA

A measure of anaesthetic potency

Consistency

MAC is relatively connsistent between individuals
* Varying 10 to 15% among individuals

Site of action

Immobility produced by AA is mediated principally by effects on spinal cord
* Also some minor cerebral effect

NB:

• Decerebration does not change MAC

Comparativity

Different AA at similar MAC
--> Equivalent depression of the spinal cord
* But different effects on other organs

Dose-response curve

• MAC is only ONE point on the dose-response curve
• The dose-response curves for different AA are NOT parallel
• All dose-response curves are steep
  * 1 MAC prevents movement in 50%
  * 1.3 MAC prevents movement in at least 95%

Additive

MAC values for AA are additive

For example,

0.5 MAC of N2O plus 0.5 MAC iso would have the same effect at brain as does a 1 MAC of either AA alone.

 

MAC values for different AAs

[SH4:p33, table 1-8]

• N2O = 104%
• Hal = 0.75%
• En = 1.63%
• Iso = 1.17%
• Des = 6.6%
• Sevo = 1.80%
• Xenon = 63-71%

Conditions at which MAC is measured

(except for N2O)

• 30 to 55 years old
• 37 degrees
• PB = 760mmHg

For N2O

• 21 to 55 years old
• Hyperbaric chamber used

 

Factors influencing MAC

Main factors

• Age
• Body temperature

Factors that INCREASE MAC

• Alcoholism [BH1:p50]
  * Different from SH4
• Hyperthermia
• Increased pheomelanin concentration
  * i.e. natural red hair women
  * Due to mutation of melanocortin-1 receptor gene
• Cyclosporine
• Hypernatremia
• Drug-induced increase in CNS catecholamine level
• Increased catecholamine stores

Factors that DECREASE MAC

• Increasing age
  --> Decrease in MAC of 6% per decade after 30 y.o.
  * Also decreased in <6-8 months old
• Hypothermia
• Preoperative medication
  * e.g. Opioids
• Acute alcohol ingestion
• Alpha-2 agonists
  * e.g. clonidine
• Pregnancy
  * (???) Due to progesterone
• Postpartum
  * Returns to normal in 24-72 hours
• Lithium
• Lidocaine
• Cardiopulmonary bypass
• Hyponatremia
• Drug-induced decrease in CNS catecholamine level
• PaO2 < 38mmHg
• BP <40 mmHg
• Decreased catecholamine stores

Factors that does NOT affect MAC

• Gender
• Duration
  * However, MAC for iso might decrease with duration
• Chronic alcohol abuse
  * Different from BH1
• Anaesthetic metabolism
• Thyroid dysfunction
• K+ derangement
• BP >40mmHg
• PaO2 >38mmHg
• PaCO2 15-95mmHg

========

 

Different kind of MAC

MAC bar = 0.5 MAC

MAC awake = 0.5 MAC

MAC with N2O = decrease by 60-70%