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InhaledAnesthetics Flashcards

1
Q

Anesthetic MAC Values

A

Halothane: 0.75 %
Isoflurane: 1.17 %
Seveoflurane: 1.8 %
Desflurane: 6.6 %
N2O: 104 %

Expressed as a partial pressure percentage of atmospheric pressure (760 mmHg)

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2
Q

MAC Defenition

A

The inhalational anesthetic alveolar concentration at which 50% of the population will not move to a painful or noxious stimulus (surgical incision)

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3
Q

MAC- BAR

A
  • MAC needed to Block Autonomic Response to painful stimulus
  • About 1.2 - 1.5 MAC
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4
Q

MAC Awake

A

MAC at which patient opens their eyes
* 0.4 0.5 MAC to lose consciousness
* 0.15 MAC to regain consciousness

Awareness and recall thought to be prevented at 0.4 - 0.5 MAC

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5
Q

MAC to Prevent Movement in 95% of Patients

A

1.2 - 1.3 MAC will prevent movement in 95% of surgical patients

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6
Q

Vapor Pressures of Inhaled Anesthetics

A

Sevoflurane: 157
Isoflurane: 240
Halothane: 244
Desflurane: 669
N2O: 38,770

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7
Q

Factors Causing No Changhe in MAC

A
  • Duration of anesthesia
  • Anesthetic metabolism
  • Hyperkalemia
  • Hyper/ hypocarbia
  • Gender
  • Thyroid function (no direct effect on MAC)
  • Metabolic alkalosis
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8
Q

Factors Causing MAC to Increase

A
  • Hyperthermia
  • Drugs that increase CNS catecholamine levels (MAO inhibitors, cocaine, ephedrine, levodopa)
  • Excess pheomelanin production (red hair)
  • Hypernatremia
  • Chronic ethanol abuse
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9
Q

Factors Causing MAC to Decrease

A

Hypothermia
Preoperative medications
Older age (decrease 6% per decade after 40)
Pregnancy
Alpha agonists
Acute alcohol ingestion
Hyponatremia
Induced hypotension (MAP < 50)
Drugs (Lidocaine, lithium, ketamine, opioids, benzos)
Severe anemia

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10
Q

Blood: Gas Partition Coefficients of IAs

A

Desflurane: 0.42
N2O: 0.46
Sevoflurane: 0.65
Isoflurane: 1.46

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11
Q

Blood: Gas Partition Coefficient Definition

A

Represents the relative solubility of an IA in the blood vs. in the alveolar gas when the partial pressures are in equilibrium

Higher solubility = slower induction/ emergence (isoflurane)
Lower solubility = rapid induction and emergence (desflurane)

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12
Q

Oil: Gas Partition Coefficient Definition

A

Parallels anesthetic requirements
Is a measure of potency of inhaled anesthetics
Higher oil: gas partition coefficient = higher potency (lower MAC)
Lower oil: gas partition coefficient = lower potency (hgher MAC)

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13
Q

Oil: Gas Partition Coefficient Values

A

Isoflurane: 91
Sevoflurane: 47
Desflurane: 19
N2O: 1.4

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14
Q

Effect of CO on rate of rise of FA/ FI

A
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15
Q

PA = Pa = Pbrain

What does this mean?

A

When the partial pressures of inhaled anesthetics have reached equilibrium, the partial pressure of the anesthetic in the alveoli is equal to that of the arterial blood, which is also equal to that of the brain

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16
Q

Goal of Inhalational Anesthetics

A

To establish a specific partial pressure (concentration) in the CNS (brain and spinal cord)
This is accomplished by establishing a specific partial pressure of the inhalational agent in the alveoli - which over time will equilibrate with that of the brain

17
Q

How to Measure the Partial Pressure (concentration) of an Inhaled Anesthetic in the Brain

A
  • At equilibrium, the partial pressure (cooncentration) in the alveoli is equal to the partial pressure (concentration) of the gas in the brain
  • Therefore, we can measure what is in the brain by measuring what is in the lungs
18
Q

What is FI

A
  • Fraction of inspired gas
  • It is what is controlled by the provider - the concentration of the gas that is exiting the anesthesia machine
18
Q

What is FI

A
  • Fraction of inspired gas
  • It is what is controlled by the provider - the concentration of the gas that is exiting the anesthesia machine
19
Q

Factors That Contribute to FI

A
  1. Fresh gas flow and rate
  2. Breathing system volume
  3. Absorption of gas into machine circuit
20
Q

Factors That Will Increase FI

A
  1. High fresh gas flows
  2. Small breathing circuit volume
  3. Less absorption into machine circuit
21
Q

What is FA

A
  1. The partial pressure (concentration) of anesthetic that is present in the lungs
  2. Direct reflection of the partial pressure (concentration) of anesthetic in the brain
22
Q

Factors That Determine FA

A
  1. The rate of anesthetic delivery from machine to alveoli
    (includes ventilation concentration )
  2. The rate of anesthetic transfer from the alveoli to the blood- uptake
    (includes second gas effect)
23
Q

Uptake

A

The amount of anesthetic gas that is being taken up into the blood, which opposes the build up of concentration in the alveoli
Uptake = [(λ) x (Q) x (PA-Pv)]/Barometric Pressure
λ = Solubility
Q = Cardiac Output
PA-Pv = alveolar- venous partial pressure difference

Uptake is directly proportional to solubility, cardiac output, and alveolar- venous partial pressure difference (PA-Pv)
More uptake = slower onset of anesthetic (slower build up of anesthetic in the alveoli)

24
Factors Affecting Anesthetic Uptake/ Distribution
1. Solubility More soluble = slower onset Less soluble = faster onset 2. Cardiac output High CO = more uptake (slower onset) Low CO = less uptake (faster onset) 3. Pa - Pv difference High Pa-Pv difference = more uptake (slower onset) Low Pa-Pv difference = less uptake (faster onset) ## Footnote Cardiac output has less effect on the low solubility agents as they are less dependent upon uptake (Desflurane)
25
Tissue Compartments and Cardiac Output Values
**VRG** 10% of body mass 75% of CO **Muscle** 50% of body mass 19% of CO **Fat** 20% of body mass 6% of CO **Vessel Poor Group** 20% of body mass < 1% of CO
26
Tissue Compartment Perfusion
**VRG** 55-100 mL/min per 100g of tissue **Muscle** 3 mL/min per 100g of tissue **Fat** 1 mL/ min per 100g of tissue
27
Rate of Rise of FA/FI
How quickly the alveolar concentration reaches the concentration that is being delivered to the alveoli * Low solubility agents (desflurane) have a faster rate of rise of FA/FI because there is less uptake (less anesthetic lost to the other body tissues) * High solubility agents (isoflurane) have a slower rate of rise of FA/FI because there is more uptake (more anesthetic is lost to other body tissues)
28
Factors **Increasing** Rate of Rise of FA/FI
1. **Increase wash in** High FGF High alveolar ventilation Low FRC Low time constant Low dead space **2. Decrease Uptake** Low solubilty agent Low CO Low Pa- Pv difference
29
Factors **Decreasing** the Rate of Rise of FA/FI
**1. Decreased wash in** Low FGF Low alveolar ventilation High FRC High time constant High dead space **2. Increased uptake** High CO High solubility High Pa-Pv difference

Inhaled Anesthetics (1 decks)

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