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