MAC: factors affecting

Advanced, Basic, Basic Sciences, Clinical Subspecialties, Pharmacology

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.

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2018

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