Chapter 3 - MAC

Question 1

What is MAC?

Answer 1

MAC is the minimum alveolar concentration of anesthetic at (1 ATM) that ablishes movement in response to a noxious stimulant in 50 % of subjects

Question 1

Anethesia potency relates to

Answer 1

MAC value

Question 2

What is MAC-Bar?

Answer 2

• MAC- Bar is the minimal concentration at which adrenergic/autonomic responses are blocked.

Question 3

MAC awake

Answer 3

The end tidal anesthetic concentration that suppresses appropriate response to command in 50% of subjects

Concentration that almost assures you supression of remembrance

Question 4

Why 50 percent for MAC?

Answer 4

o 50% response is the easiest response to determine with the least number of experimental subjects.
* Variability higher up (closer to 100%) is greater vs variability that is closer to 50% is smaller so less numbers are required to acquire to define this point.

Question 5

What part of CNS mediates this capacity of inhaled anesthetic to produce immobility

Answer 5

Spinal Cord

Question 6

Propofol works best on the ___ with minimal effect on _____

Answer 6

brain

minimal effect on the spinal cord

Question 7

How do we determine MAC in animals?

Answer 7

Concentration of anesthetic in alveolus aka MAC vs the movement.

Question 8

Why alveolar concentration?

Answer 8

• It will most accurately reflect spinal cord concentration.
• Have to wait until alveoli and spinal cord equalize→ Direct measure of partial pressure of anesthetic at whatever site of action.

Length to equilibrate: 10-25 min

Question 9

N2O- ______ of desflurane and sevoflurane by roughly 1% at this age (30-60 yo)

Answer 9

**reduces MAC

Differes at different ages

Question 10

What physiological factors do not change MAC?

Answer 10

Gender, duration of anesthesia, body mass(fat,thin,dwarf or tall)

Question 11

Question 12

PHYSIOLOGICAL FACTORS that decrease MAC:

Answer 12

• Increased Age
• Decreased body temperature* Decreased CNS sodium
• Pregnancy.

Question 13

Disease process that contribute to decreased CNS sodium (therby decreasing MAC)

Answer 13

• Due to dehydration, SIADH, cerebral salt wasting syndrome.
• Transurethral resection of prostate- irrigation fluid that is used to look at prostate can be absorbed by venous sinusoids leading to Na drop.
• Intracranial operation- making the brain more “slack” by infusing mannitol.
• Osmotic diuretic → withdrew water from the brain parenchyma into the blood volume →increasing Na concentration in the plasma.
• ↑Na= ↑MAC

Question 14

With Sevo and age, you have a progressive decrease in MAC with SEVO
T/F

Answer 14

T

Question 15

• Steeper decrease for Sevo in 60% of N2O.
  T/F

Answer 15

T
* 60% of N2O becomes a greater fraction of MAC as age increases.
* Same concentration of N2O adds much more to the effect of anesthetic in the older pt vs younger pt.

Question 16

Acute alcoholic require more MAC T/F

Answer 16

F - it is chronic alcoholics

Question 17

DEPRESSANT DRUGS that decrease MAC:

Answer 17

1. Opioids (synergistic effect)
2. Benzodiazepines (dose dependent).
3. Barbiturates
4. Propofol
5. Local anesthetics
6. Ei Lidocaine decrease mac
7. Nitrous oxide.

Question 18

Fentanyl does not synergistically decrease MAC
T/F

Answer 18

F
* Acute responses at lower concentrations which are similar to responses at much higher concentrations suggesting synergistic interaction between opioids and Sevo MAC.

Question 19

Would muscle relaxants have any affect on MAC?

Answer 19

• Muscle relaxants- prevents transmission of motor end- plate potential at post synaptic junction by inhibiting/blocking Ach receptors→ produces blockade of impulses.

Controvesial some studies say yes some no

Muscle relaxants should not have any effect on the brain as they DO NOT cross BBB due to them being ionized/polar.

Question 20

Other drugs that can/might decrease MAC:

Answer 20

• Alpha 2 sympathetic agonists (Precedex, Clonidine)
• Some beta- blockers
• CCB
• Adenosine and ATP
• False neurotransmitters.
  D- Medetomidine has an effect, not L- Medetomidine.

Question 21

Why is MAC awake important?

Answer 21

• Amnesia.
• Determinant of anesthetic recovery.
• Not complete assurance of safety.

Question 22

Potential for regurgitation at less than MAC awake.

Answer 22

• 25% of MAC→ increased potential for regurgitation (relaxation of upper esophageal sphincter).
• 50% → greater risk.
• Have to get quickly below MAC awake. Quicker the better!
• MAC awake is affected by some of the same factors as MAC itself: age.
• MAC awake can vary among anesthetics.

Question 23

Effect of Isoflurane vs N2O on MAC

Answer 23

• N2O has higher MAC awake (above 60%)
• Isoflurane (20-30%)
  o This would help with recovery time (choose N2O for shorter recovery time).
  o Potent inhaled anesthetic→ allows recovery time to be quicker.
   Why nitrous so important allows quicker recovery!
  o MAC of N2O is 105%, MAC awake of N2O is 70% -→ narrow window between MAC awake and maximal amount you can deliver→ need to supplement.
   Contribution of N2O is less of the factor in terms of awakening than what you use to supplement it with…like propofol, iso, sevo, etc.
   Use of N2O helps with rapidity of awakening.
  o Downside of N2O is that MAC awake is high meaning it is not a very good amnestic agent. HAVE to give anesthetic to supplement as we do not want them to remember.
   N2O at 70% on its own is not sufficient to suppress awareness.