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Anesthesia Pharm > Week 2 - MAC, Uptake, & Distribution of Inhalation Agents > Flashcards

Week 2 - MAC, Uptake, & Distribution of Inhalation Agents Flashcards

1
Q

What is Minimum Alveolar Concentration (MAC)?

A

Fraction of expired agent or alveolar concentration at which 50% of patients move upon noxious stimulation (incision)

*Alveolar concentration is the closest we can get to measuring the brain concentration

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2
Q

What are the general MAC Values?

A

MAC Awake = 1/3 to 1/4 MAC

MAC Intubation = ~1.2 MAC (>> MAC because it is more stimulating than incision)

MAC Incision = MAC (1)

MAC BAR (Blunt Autonomic Responses) = 30-40% greater than MAC

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3
Q

What factors decrease MAC?

A

Hypothermia
Severe HoTN
Advance age
Opioids, Ketamine
Chronic Admin of Amphetamine
Reserpine, alpha-methyldopa
Cholinesterase Inhibitors
IV local anesthetics
Pregnancy
Hypoxemia (PaO2 <40)
Anemia
Alpha2 agonists

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4
Q

What factors increase MAC?

A

Hyperthermia

Hyperthyroidism

Alcoholism

Acute admin of dextroamphetamine

Young Age

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5
Q

What factors have no effect on MAC?

A

Duration of anesthesia

Sex

Metabolic acid-base status

Hypercapnia and hypocapnia

Isovolemic anemia

Hypertension

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6
Q

What is the MAC and MAC Awake for Nitrous Oxide?

A

MAC = 105 Vol%

MAC Awake = 68 Vol%

MAC Awake % of MAC = 64%

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7
Q

What is the MAC, MAC Awake, and MAC BAR for Isoflurane?

A

MAC = 1.15 Vol %

MAC Awake = 0.49 Vol%

MAC Awake % of MAc = 42%

MAC BAR = 1.5

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8
Q

What is the MAC, MAC Awake, MAC Intubation, and MAC BAR for Desflurane?

A

MAC = 6 Vol%

MAC Awake = 2.5 Vol%

MAC Awake % of MAC = 37%

MAC Intubation = 7.7

MAC BAR = 7.8

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9
Q

What is the MAC, MAC Awake, MAC Intubation, and MAC BAR for Sevoflurane?

A

MAC = 1.6 Vol%

MAC Awake = 0.62 Vol%

MAC Awake % of MAC = 37%

MAC Intubation = 3 MAC

BAR = 3.2

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10
Q

What is the MAC and MAC Awake for Halothane?

A

MAC = 0.75 Vol%

MAC Awake = 0.41 Vol%

MAC Awake % of MAC = 55%

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11
Q

What is the MAC and MAC Awake for Propofol?

A

MAC = 14 mcg/mL

MAC Awake = 2.69 mcg/mL

MAC Awake % of MAC = 19%

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12
Q

How does the solubility affect anesthetic action?

A

Low Blood:Gas solubility means fast in/fast out

  • N2O, Des, Sevo have low solubility
  • Low Solubility means more molecules for anesthesia (goes to brain faster)

High solubility means slow on/slow off

  • Hal and Iso have high solubility
  • High solubility means more anesthetic in blood, muscle, fat, and less for the brain

*Once equilibration is reached, all anesthetic left goes to the brain

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13
Q

How long does it take for the vessel rich group coefficients, muslce:blood, and fat:blood to become saturated/reach equilibrium?

A
  • Alveolar = 1-5 min
  • Vessel Rich Group (Brain, Heart, Spleen, Liver, Kidney, Endocrine) = 5-20 min — 10% body weight, 75% CO, 70mL blood/100mL tissue/min (N2O 50% equilibrated in 2 min)
  • Muscle:Blood = starts 20min to 2-3 hours — 50% body weight, 19% CO, 3mL blood/100mL tissue (N2O 50% equilibrated in 20-25 min)
  • Fat:Blood = starts 2-3 hours till ___ (vaporizer off) — 20% body weight, 6% CO, total perfusion 400mL/min (N2O 50% in 75 min)
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14
Q

What are the components of the Uptake formula? (4)

A
  • Cardiac Output (pushes blood into lungs to soak up anesthetic molecules)
  • Blood:Gas solubility (depends on anesthetic dissolved in blood and tissue)
  • Alveolar to Venous partial pressure gradient
  • Barometric Pressure (not dependent on the agent itself, just the solubility of that agent)
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15
Q

How is uptake related to each component of the formula? (directly or inversely)

A
  • Uptake from lungs is DIRECTLY proportional to CO (decreased CO = less blood through lungs = less uptake)
  • Uptake from lungs is DIRECTLY proportional to alveolar ventilation (more alveolar ventilation = more anesthetic in alveoli = greater uptake)
  • Uptake from lungs is INVERSELY proportional to venous partial pressure (high venous partial pressure = less gradient = less uptake)

— No CO = No Uptake — No Tissue Gradient = No Uptake —

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16
Q

What does the Ostwald coefficient (λ), B:? coefficient tell you?

A

mLs of that gas that will dissolve in 11mL of blood at 37*C

-When the partial pressure of agent in the blood and gas are in equilibrium, the coefficient tells you how many mLs of agent are dissolved for each mL that is causing anesthesia

17
Q

What is Henry’s Law?

A

The amount of gas that dissolves in a liquid is directly proportional to the partial pressure of the gas over the liquid

18
Q

Describe the Alveolar and Venous partial pressure gradient related to uptake

A

P-Alveolar = Fi agent
-High P-A = High Delivered Concentration = Greater Uptake

P-Venous = Fe agent
-High P-V = High Tissue Concentration = Less Uptake

*The greater the difference between P-A and P-V = the greater uptake

19
Q

How do you calculate the partial pressures of inhalation agents?

A

Agent is delivered in Volume%

Barometric Pressure x Vol% gas = Partial Pressure

Ex: Des = 6% – 760mmHg x 0.06 = 46 mmHg Des

20
Q

Relationship between F-Alveolar/F-Inspired ratio and uptake

A

Inverse relationship between uptake and rise of F-A/F-I

  • if 2/3 of inhaled anesthetic is taken up (Highly soluble agent) then the F-A/F-I is 33%
  • if 1/4 of anesthetic is taken up (low solubility) the F-A/F-I is 75%

*F-A/F-I compares what’s in the alveoli to what you are giving

21
Q

Describe the shape and the “knees” of the Uptake and Distribution Graph

A
  • Initial rapid rise because alveolus has NO agent
  • 1st knee = VRG taking up anesthetic
  • 2nd knee = VRG is saturated and muscle group starts uptake (around 10-15 min)
  • 3rd knee = muscle saturate and fat group starts uptake (around 2-4 hr depending on agent)
  • Slope of line decreases as each compartment is saturated because F-A/F-I is rising more slowly as rate of uptake is decreasing
22
Q

F-A/F-I ratio related to solubility

A

Insoluble agents (Low B:G) reach higher F-A/F-I — reach higher and faster

Soluble agents (High B:G) lower F-A/F-I — reach lower and slower

23
Q

How does a higher concentration agent delivered (Fi) affect F-A/F-I ratio and CO?

A

Cause F-A/F-I ratio to be higher (increase rate of rise)

Higher concentration will depress CO

-Less uptake by blood –> less by tissues –> more in alveoli –> Fi rises faster

24
Q

How does greater minute ventilation affect F-A/F-I ratio?

A

it increases the ratio more rapidly because more agent to lungs and tissues

  • increased ventilation affects soluble agents more
  • insoluble agents rise so fast so there is no difference
25
Q

How does CO affect the F-A/F-I ratio?

A

Rise of ratio is inversely proportional to CO

  • High CO = more uptake = slower rise
  • Low CO = less uptake = faster rise

(Greater effect on more soluble agents)

26
Q

What happens during induction if you have high fresh gas flow, high agent concentration, and low CO? Will pt be anesthetized?

A

Decreased uptake

  • low CO = less uptake
  • high concentration = depress CO even more

Less uptake of anesthetic agent means the patient will not be properly anesthetized

27
Q

If CO and minute ventilation INCREASE what happens to the F-A/F-I ratio?

A

Rises Faster

28
Q

If CO and minute ventilation DECREASE what happens to the F-A/F-I ratio?

A

Rises Slower

29
Q

If CO increases and minute ventilation stays the same what happens to the F-A/F-I ratio?

A

Result in a reduced ratio secondary to more uptake (Slower rise)

30
Q

If CO stays the same and minute ventilation increases what happens to the F-A/F-I ratio?

A

Rises Faster

31
Q

What’s the effect of Age on F-A/F-I ratio (infants vs adults)?

A

Infants have a higher minute ventilation

VRG is 18% body weight in kids vs 6-10% in adults – increased proportion of CO to VRG (faster induction in kids vs slower in adults)

Agents have lower B:G in neonates (even less uptake and higher Fa/Fi; blood returning to lungs has higher concentration agent; decreased uptake and increased rate of rise of Fa/Fi)

32
Q

How do shunts and dead space affect uptake?

A

Dead space has minimal effect – no uptake (higher soluble agents have increased effect)

ASD/VSD minimal effect – blood already saturated R to L

Intrapulmonary Shunts (R mainstem ETT)
-over ventilated = more uptake --- under ventilated = less uptake
\*evens out (less effect with soluble agents)
33
Q

How does solubility affect the emergence curve?

A

More soluble agents wash out more slowly, resulting in a slower emergence (flatter curve)

Insoluble agents “wash-out” faster and result in a faster emergence (steeper curve)

34
Q

What happens to the F-A/F-I ratio during emergence? What is the order of the “wash-out”?

A

Vaporizer is turned off Ratio goes to zero (can’t go below 0)

  1. Lungs washed out of anesthesia
  2. VRG releases “washes out” anesthesia
  3. Muscle group dumps anesthesia into blood
  4. Fat slowly washes out anesthesia

*ALL based on the concentration gradient

35
Q

Match the inhalation agent to the correct curve

A

A: Halothane

B: Enflurane

C: Isoflurane

D: Sevoflurane

E: Desflurane

F: Nitrous Oxide

36
Q

Fill in the Blanks

Anesthesia Pharm (18 decks)

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