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Flashcards in Inhalation Anesthetics Deck (63):
1

Two major measurable effects of inhaled anesthetics (for research purposes)

1. Immobility in response to surgical stim

2. amnesia to intraop events

2

Define MAC

minimum alveolar concentration of anesthetic required to inhibit movement on surgical incision in 50% of patients

3

mech of action of immobility for NO

activates descending noraderenergic pathways from periaquaductal gray matter in brainstem

- these pathways inhibit nociceptive input to DH of spinal cord

4

MOA of amnestic effects of inhaled anesthetics

involves amygdala, hippocampus, cortex (most likely)...

5

MOA of CNS depression of inhaled anesthetics

1. Enhance function of inhibitory ion channels (= hyperpolarization via Cl or K+ mediated GabaA and glycine receptors)

2. Block function of excitatory ion channels (prevents depolarization, prevents positive charge ions via NMDA or Na channels)

6

State of NO at room temp? other inhaled anesthetics?

NO = gas

inhaled anesthetics = liquid at room temp

7

What anesthetics cause nephrotoxicity after prolonged anesthesia?

methoxyflurane --> inorganic floride

Sevoflorane/Halothane --> Compound A

8

How can you prevent nephrotoxicity with Sevoflorane?

Limit low fresh gas flow (<2L/min) to less than 2 MAC hours of Sevoflorane

9

How do you avoid carbon dioxide absorbent reaction w/ inhaled anesthetics? Which anesthetic is usually the culprit?

Sevoflorane causes most exothermic rxns

1. maintain adequate hydration in CO2 absorbent
2. change regularly
3. turn fresh gas flow down/off on unattended machines
4. limited fresh gas flow during anesthesia
5. change absorbant when in doubt

10

How can you measure the RELATIVE potency of inhaled anesthetics?

MAC = minimum alveolar concentration of anesthetic required to prevent movement on surgical incision in 50% of patients

11

Describe the response curve of MAC re movement on surgical incision

SD of MAC is 10%

50% not move with 1 MAC
95% not move with 1.2 MAC
99% not move with 1.3 MAC

12

Which variables do NOT affect MAC

gender

duration of anesthesia

13

Factors that INCREASE MAC (5 min.)

1. Drugs (amphetamine, cocaine, ephedrine, chronic EtOH)

2. Age (highest 6 mo)

3. Electrolytes (hypernatremia)

4. Hyperthermia

5. Red Hair

14

Factors that Decrease MAC?

1. Drugs
2. Age (Elderly)
3. Electrolytes (Hyponatremia)
4. Anemia
5. Hypercarbia
6. Hypothermia
7. Hypoxia
8. Pregnancy

15

Drugs that DECREASE MAC?

1. IV anesthetics (propofol, etomidate, ketamine, dexmedetomidine)
2. Barbiturates/Benzos
3. Ethanol (acute use)
4. Local anesthetics
5. opioids
6. amphetamines (chronic use)
7. lithium
8. Verapamil

16

MAC of Isoflorane

1.15

17

MAC of Halothane

0.76

18

MAC of Desflurane

6

19

MAC of Sevoflorane

185

20

MAC of nitrous oxide

104

21

Blood gas coefficent of inhaled anesthetics (smallest to largest)

NO
Desflurane
Sevoflorane
Isoflorane
Halothane

22

Factors that effect transfer of anesthetic from machine to alveoli

Pi (inspired partial pressure)
alveolar ventilation
breathing system characteristics (absorb into plastic, etc)

23

Factors effecting transfer from alveoli into blood

BG partition coefficient
CO
alveolar-venous partial pressure difference

24

Factors effecting transfer anesthetic from blood to brain

Blood-brain partition coefficient
Cerebral blood flow
art-vein partial pressure difference

25

High Pi----> help or hurt increase in Palv

HELPS

Neccessary to have high PI initially to ofset uptake into blood

accellerates induction of anesthesia

26

How should you change Pi as time increases at induction

Need to decrease Pi to match decreasing uptake to maintain a constant/optimal Pbr

If PI maintained constant, PA would progressively increase

27

What is the second gas effect?

example?

ability of large volume gas uptake of one gas (first gas) to accelerate the rate of increase of the PA of a concurrent administered gas (second gas) = 2nd gas effect

Example: initial large vol uptake of NO accelerates uptake of companian gasses (volatile anesthetics and oxygen). PaO2 increases by about 10%

28

How does the second gas effect work?

increased flow of both gasses
or
concentration of the second gas in a smaller lung volume=concentrating effect

29

How does alveolar ventilation effect PA?

increased Va (hyperventilate) = more rapid rate of increase of PA and more rapid induction

hypoventilation = decreased rate and slower induction

30

How does hyperventilation and controlled ventilation increase induction speed? What should you do about it?

1. Increase Va (ventilation)

2. Decrease venous return by hypervent = slow CO = decreased uptake into blood (quicker rise of PA)

this can result in overdose--> LESSON- decrease PI when switching from spontaneous to controlled ventilation to adjust for this change

31

Blood-gas coefficient:
higher means?
lower means?

Higher = more soluble, large amount of inhaled anesthetic needs to be dissolved before blood/alvoli are in equilibrium = LONGER ONSET

Lower= less soluble, less amount needed for equilirium =QUICKER ONSET

32

Brain-blood coefficients are measured how?

in a time constant....complete equilibriation of any tissue with Pa = 3x time constants

33

Brain-blood coefficient of isoflurane? total equilibriation?

3-4 min

total time = ~10-15 min (x3 time coefficient)

34

Are des/sevoflurane and NO more or less soluble in brain than iso?

less soluble = 2 min time coefficient (6 min total equilibriation)

35

Which anesthetic interacts with methionine synthase?
What does it do and why do we care?

NO

methionine synthase regulates B12 and folate metabolism...decrease can increase homocysteine and increase risk of vascular/cardiac disease/sequela

36

Why is it important not to give NO to someone with a pneumo?

NO blood gas coefficient 34x greater than nitrogen (main component of normal ambient air)

Causes increased volume of distensible air space or pressure of non-distensible air space.

75% NO doubles pneumothorax volume in 10 min

Closed ptx is a contraindication to NO

37

How does CO affect induction speed?

Low CO = less uptake = faster induction (pts in shock = quicker induction)

Faster CO (fear) = more uptake = slower induction

38

How do shunts affect induction speed?

R->L cardiac shunt slows induction speed (dilutional effect of shunted blood with no anesthetic)

L->R cardiac shunt (AV fistula, etc) offsets dilutional component of R to L.

39

How does dead-space (wasted ventilation) effect induction speed?

It doesn't effect it.

Principle effect is production of difference between PA and Pa of anesthetic (similar to difference between observed end tidal PCO2 and PaCo2).

40

What is the importance of the PA-PV ratio?

reflects tissue uptake of inhaled anesthetics

tissues with more blood flow equilibriate rapidly with Pa

3x time constant = Pv = PA for anesthetic (PI and PA difference narrows)....decrease PI at this time to maintain a constant PA

41

How does anesthesia recovery differ from anesthesia induction?

1. absence of a concentration effect (PI cannot be less than zero)

2. variable tissue concentrations at start of recovery

3. metabolism effect on rate of decrease in PA

42

Does time of exposure to anesthetics matter more if it is a soluble or non-soluble anesthestic?

soluble= will take longer to unload high tissue concentrations (i.e. isoflurane)

43

Which anesthetics does metabolism matter for?

methoxyflurane
halothane

44

Define context sensitive half time

the dependency of elimination of inhaled anesthetics on 1. length of administration ("context")
2. solubility of inhaled anesthetics in blood/tissue

45

Define diffusion hypoxia

hypoxia that occurs after NO administration in patients given fresh gas (but no oxygen)

NO takes up so much of alveolar space at discontinuation that PAO2 dilutes to the point of PaO2 decreasing

GIVE PTS O2 AFTER NO

46

Effect of inhaled anesthesia on MAP

major changes that cause this?

DECREASE
sevo/des/isoflurane = (dec SVR > dec CO)
halothane = dec CO > dec SVR

47

How can you minimize MAP effects of inhaled anesthetics?

Co-administer NO (increase MAP)

48

HR effect of iso/des/sevoflurane?
Which anesthetics have greatest effect?

increase HR

Isoflurane (0.25 MAC) >> Desflurane (1 MAC) > Sevoflurane (1.5 MAC)

49

How can you attenuate HR effects of desflurane?

b blocker, opioids, time

NOT nitrous oxide

50

Does a rapid or slow increase in desflurane have greater circulatory effects?

Faster change = bigger change

slow the rate of increase, attenuate with opioids/b-blockers

similar effect not seen with other anesthetics

51

Inhaled anesthetics that effect cardiac contractility?

Halothane (induction alone in children, or + NO in adults)

sevoflurane (1 MAC in adults)

52

INhaled anesthetic that can predispose to dysrhythmia?
what increases this risk?

halothane

increased with catecholamines, hypercarbia

53

Which anesthetic increases QTc? What should you do to prevent issues?

Sevoflurane

Give B blocker

54

How do inhaled anesthetics effect ventilation?

Increase RR, decrease Vt = maintain minute ventilation (with more dead space)

inefficient ventilation = increase PaCO2

55

What happens to PaCO2 responsiveness?

Blunted, resulting in apnea

56

Effect of inhaled anesthetics on airway resistance

- bronchodilate (mostly when bronchoconstriction already present)...minimal effect clinically in normal airway

Not the case with NO

57

Whcih anesthetic has modestly increased airway resistance in smokers?

Desflurane

58

Which anesthetics would you pick for inhalation induction? Why?

Halothane or Sevoflurane = smell nice and aren't irritating to airway

59

Do inhaled anesthetics effect cerebral vascular responsiveness to PaCO2?

No

60

NO effect on cerebral blood flow and CMRO2?

- increased CBF, increase CMRO2 (small)

61

CBF/CMRO2 effect of halothane/isoflurane/sevoflurane/desflurane?

CBF- increase (if MAC > 0.6)
CMRO2- DECREASE

1 MAC CBF predominantes

62

ICP effect of inhaled anesthetic

increase with all anesthetic > 1 MAC

autoregulation impaired

63

Neuromuscular effect of inhaled anesthetics?

dose-related muscle relax ( Des > sevo/iso)

= enhance activity of NM blocking drugs (eliminating anesthetic = quicker recovery from NM block)