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 purposeful 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 consistent 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%
• Halothane = 0.75%
• Enflurane = 1.63%
• Isoflurane = 1.17%
• Desflurane = 6.6%
• Sevoflurane = 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
• Natural red hair women, due to
  * Increased pheomelanin concentration
  * 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 [EEW1:p26]
• Postpartum
  * Returns to normal in 24-72 hours
• Lithium
• Lidocaine
• Cardiopulmonary bypass
• Hyponatremia
• Drug-induced decrease in CNS catecholamine level
• Severe hypoxia (pO2 < 38mmHg )
• Severe hypotension (BP <40 mmHg )
• Decreased catecholamine stores

Factors that does NOT affect MAC

• Gender
• Duration of anaesthetics
  * However, MAC for isoflurane 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 = 1.5 MAC
  * MAC required to block autonomic (sympathetic) response to a surgical stimulus in 50% of patients
• MAC awake = MAC that prevents appropriate response to command in 50% of subjects
  * [EEW1:p39]
• MAC-hr
  = The integral of MAC by time for exposure of an individual patient to an inhalational anaesthetic agent

MAC awake

Ratio (MAC awake / MAC) varies between anaesthetic agents
--> i.e. MAC awake expressed as a fraction of MAC

[EEW1:p27]

• Halothane = 0.55 MAC
• Isoflurane = 0.38 MAC
• Desflurane = 0.34 MAC
• Sevoflurane = 0.34 MAC
• Nitrous oxide = 0.64 MAC
• Propofol = 0.19 MAC (or 2.69 microgram/L)