MAC: factors affecting

Factors affecting MAC

1. Significant hypoxia (below 40 mm Hg) decreases MAC.
2. Hypothermia decreases MAC.
3. Hyperthermia increases MAC.
4. Hyperthyroidism slightly raises MAC (about 15%)
5. Increasing age decreases MAC:

   • Neonatal Halothane MAC: 1.1
   • 70 y.o. MAC: 0.63

6. Circadian rhythms alter MAC + 10%
7. Opioids reduce MAC, but there is a ceiling effect beyond which additional opioid does not lower MAC.
8. Diazepam (and presumably other benzodiazepines) reduces MAC.
9. Scopolamine slightly lowers MAC.
10. Other volatile agents reduce MAC:

    • MACs are additive: 50% N2O + .5MAC Halothane = 1 MAC of anesthetic

11. Drugs which reduce central catecholamines lower MAC:

    • Reserpine
    • Alpha methyl dopa (Aldomet)
    • chronic amphetamine use
    • clonidine

12. Drugs which raise central catecholamines raise MAC:

    • acute amphetamine intoxication
    • ephedrine
    • MAO inhibitors

13. Relaxants lower MAC slightly (but don’t push your luck!)
14. Pregnancy decreases MAC
15. Recently, it has been demonstrated that hair color effects MAC with red-haired people requiring about 20% more than others.