Factors Affecting Minimum Alveolar Concentration

The correct answer is C
Explanation

The minimum alveolar concentration (MAC) is the concentration of anesthetic at 1 atmosphere of pressure that produces immobility in 50% of patients exposed to a noxious stimulus.

Hence, it is inversely related to potency.

The rationale for this measure of anaesthetic potency is:
Alveolar concentration can be easily measured
Near equilibrium, alveolar and brain tensions are virtually equal
The high cerebral blood flow produces rapid equilibration

Factors which support the use of this measure are:
MAC is invariant with a variety of noxious stimuli
Individual variability is small
Sex, height, weight & anaesthetic duration do not alter MAC
Doses of anaesthetics in MAC's are additive

Factors Which Affect MAC

Increase MAC
Hyperthermia
Hypernatraemia
Sympathoadrenal stimulation
Chronic alcohol abuse
? Chronic opioid abuse
Increases in ambient pressure
Hypercapnia
Decreasing age
Thyrotoxicosis

Decrease MAC
Nitrous oxide
Hypothyroid/myxoedema
Hypocapnia
Hypothermia-decrease is roughly linear
Hyponatraemia
Increasing age - (MAC Hal <> 60 years ~ 0.64 %)
Hypoxaemia
Hypotension
Anaemia
Pregnancy
CNS depressant drugs - opioids,
benzodiazepines, major tranquilizers, TCA's
Other drugs - lithium, lidocaine, magnesium, methyl dopa, clonidine
Acute alcohol abuse

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