MAC: factors affecting

Factors affecting MAC:

1. 1. Significant hypoxia (below 40 mm Hg) decreases MAC.
2. 2. Hypothermia decreases MAC.
3. 3. Hyperthermia increases MAC.
4. 4. Hyperthyroidism slightly raises MAC (about 15%)
5. 5. Increasing age decreases MAC
   • Neonatal Halothane MAC: 1.1
   • 70 y.o. MAC: 0.63
6. 6. Circadian rhythms alter MAC + 10%
7. 7. Opioids reduce MAC, but there is a ceiling effect beyond which additional opioid does not lower MAC.
8. 8. Diazepam (and presumably other benzodiazepines) reduces MAC.
9. 9. Scopolamine slightly lowers MAC.
10. 10. Other volatile agents reduce MAC:
    • MACs are additive: 50% N₂O + .5MAC Halothane = 1 MAC of anesthetic
11. 11. Drugs which reduce central catecholamines lower MAC:
    • Reserpine
    • Alpha methyl dopa (Aldomet)
    • chronic amphetamine use
    • clonidine
12. 12. Drugs which raise central catecholamines raise MAC
    • acute amphetamine intoxication
    • ephedrine
    • MAO inhibitors
13. 13. Relaxants lower MAC slightly (but don’t push your luck!)
14. 14. Pregnancy decreases MAC
15. 15. Recently, it has been demonstrated that hair color effects MAC with red-haired people requiring about 20% more than others.