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Anesthesia > Inhalant Anesthetics > Flashcards

Inhalant Anesthetics Flashcards

1
Q

purpose of inhalant anesthetics

A
  • produce general anesthesia by:
    • rendering patient unconscious
    • providing some degree of muscle relaxation
  • do not have any inherent analgesic properties
  • suitable for wide variety of species
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2
Q

main difference with inhalant anesthetics

A

administered and in large part removed from the body via the lungs

do not rely on hepatic metabolism and renal elimination

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3
Q

how inhalants are administered

A

liquid anesthetics are first vaporized and then administered in an enriched concentration of oxygen via a breathing circuit to the patient

requires specialty equipment (heavy, bulky)

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4
Q

equipment needed for inhalants

A
  • vaporizer
  • source of oxygen
  • anesthetic machine
  • breathing circuits
  • scavenging equipment
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5
Q

scavenging equipment is essential to:

A

help minimize occupational hazards involving waste gases

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6
Q

potency of inhalants

A

is an expression of the relationship between the administered dose of an inhalant and the anesthetic effect that is obtained

MAC is the most commonly used expression of potency of inhalants

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7
Q

MAC

A

minimum alveolar concentration of anesthetic which prevents gross, purposeful movement in 50% of patients exposed to a noxious stimulus

similar to ED50

determined in young healthy animals without use of additional CNS depressant drugs

mirrors the brain partial pressure of the inhalant

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8
Q

MAC of isoflurane

A

Dog: 1.14-1.5%

Cat: 1.28-1.6%

Horse 1.3-1.6%

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9
Q

MAC of sevoflurane

A

Dog: 2.1-2.4%

Cat: 2.6-3.1%

Horse: 2.3-2.8%

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10
Q

MAC of desflurane

A

Dog: 7.2-10.3%

Cat: 9.8-10.3%

Horse: 7.0-8.0%

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11
Q

general guideline for MAC during surgery

A

anesthesia is begun at 2-3 times MAC for the particular agent and anesthesia can be maintained at 1.5-2 times agent’s MAC value

will depend on individual patient, other drugs used and type of surgery

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12
Q

factors that can increase MAC

A
  • hyperthermia
  • hypernatremia
  • drugs that cause CNS stimulation (ephedrine)
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13
Q

variables that decrease MAC

A
  • drugs such as premedications and induction agents
  • use of local anesthetics
  • mean BP below 50 mmHg
  • hyponatremia
  • hypothermia
  • substantial alterations in respiratory gases
  • severe anemia
  • age of patient
  • pregnancy
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14
Q

factors that do not alter MAC

A
  • gender
  • normal respiratory gas concentrations
  • duration of anesthesia
  • metabolic acidosis or alkalosis
  • moderate anemia
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15
Q

vapors

A

administered as a vapor mixed in a gas (usually oxygen) but at ambient temperature and pressure exist as liquids

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16
Q

saturated vapor pressure of a liquid

A

escaping gas molecules from a liquid exert pressure on the sides of the container

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17
Q

highest anesthetic concentration that can be achieved is determined by:

A

saturated vapor pressure of the agent

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18
Q

equation for maximum percentage of inhalant that can be achieved

A

anesthetic vapor pressure

————————————– x 100

atmospheric pressure

atmopsheric pressure at sea level = 760 mmHg

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19
Q

delivered concentration of anesthetic

A

percent setting that is on the vaporizer dial

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20
Q

inspired or inspiratory concentration (FI) of an anesthetic

A

concentration of inhalant that the patient inspires

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21
Q

What should Fi be if patient is apneic?

A

inspired concentration of anesthetic is 0

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22
Q

What is the Fi if the patient is on a non-rebreather?

A

the inspired concentration of anesthetic should be equal to that set on the vaporizer

23
Q

What is the FI if the patient is on a circle rebreathing system?

A

the inspired concentration of anesthetic will be less than that set on the vaporizer as the concentration in the circle breathing system is diluted out by expired gases

24
Q

expired anesthetic concentration

A

the concentration of inhalant anesthetic that is contained in the expiratory gases of the patient

reflection of the alveolar concentration of inhalant

25
Q

alveolar concentration (Fa) of anesthetic

A

concentration of anesthetic that is in the arterial blood and delivered to the brain

the anesthetic concentration in the brain and spinal cord produce unconsiousness and immobility during inhalant general anesthesia

26
Q

factors that affect getting the inhalant administered from the vaporizer to the brain:

A
  • inspired anesthetic concentration
  • solubility of the anesthetic in the blood
  • patient’s CO
  • ventilation
  • the alveolar to venous partial pressure difference of anesthetic
27
Q

factors that affect inspired concentration of inhalant delivered to patient:

A
  • rate at which fresh gas is flowing into the system
  • volume of breathing circuit
  • if inhalant is absorbed by the anesthetic machine or breathing circuit (rubber)
    • not very common anymore with newer inhalants
28
Q

higher the carrier gas flow rate is through the vaporizer =

A

the more rapid the circuit concentration will approach the concentration exiting the vaporizer

therefore, rate at which the circuit in a circle rebreathing system and inspired anesthetic concentration can be increased is directly proportional to rate of fresh gas flow going through vaporizer and entering breathing circuit

29
Q

effect of the volume of the breathing circuit

A

a circuit with a larger volume will take longer to equilibrate and reach the desired concentration of anesthetic being delivered to patient

30
Q

time constant

A

amount of time it takes to fill a ‘container’ with desired substance

time constant (T) (min) = volume (L) / flow (L/min)

3 time constants to reach 95% of desired concentration

container = lungs and anesthetic machine

31
Q

factors that affect concentration of inhalant in alveoli

A
  • solubility of anesthetic in the blood
  • patient’s CO
  • alveolar ventilation
  • alveolar to venous PP difference of anesthetic

Fa reflects amount of anesthetic in brain

32
Q

anesthetic solubility

A
  • inherent property of gas and is influenced by ambient temperature
  • major factor in uptake and distribution of anesthetic agents
  • expressed as partition coefficient
33
Q

partition coefficient of an anesthetic gas

A

measure of its solubility in a specific solvent at a specific temperature

defined as the ratio of gas concentrations in the two phases at equilibrium

34
Q

blood:gas partition coefficient

A

blood and inspired gas

provides information about speed of onset, recovery and change in anesthetic depth

the higher the blood:gas partition coefficient, the more inhalant agent will favor being in the blood (greater solubility)

35
Q

an inhalant with a higher blood:gas partition coefficient:

A

will have greater uptake by the tissues and will have a lower Fa/Fi ratio

longer onset of action

36
Q

greater the cardiac output

=

A

the greater amount of blood carrying inhalant away from the alveoli and to the tissues

this will decrease the alveolar (Fa) anesthetic concentration and slow down the rate at which the Fa reaches the inspired anesthetic concentration (Fi)

37
Q

a decrease in cardiac output =

A

causes less blood to flow through the lungs and therefore less anesthetic is removed from them rendering the onset of general anesthesia more quickly

anesthetic overdose

38
Q

delivery of agent to alveoli depends on the:

A

inspired anesthetic concentration

and

alveolar ventilation

39
Q

higher the alveolar ventilation

=

A

the faster the alveolar anesthetic concentration will reach the inspired anesthetic concentration

anesthetics that decrease ventilation will decrease rate of rise of alveolar concentration of inhalant anesthetics –>support ventilation

40
Q

(Pa-Pv)

A

partial pressure difference between alveolar and venous blood

venous blood will retain some inhalant when it returns to the lungs for re-oxygenation

highly perfused tissues (brain…) equilibrate rapidly with Pa of anesthetic compared to less well perfused tissues

41
Q

role of metabolism in inhalant removal

A

minimal

but toxic metabolites can still be produced!

42
Q

inhalants and cardiovascular system

A

inhalants have a large impact on CV

all reduce CO in a dose dependent fasion

type, dose, and concurrent drug administration during anesthesia all affect CV system

detrimental due to effects on oxygen delivery (perfusion)

43
Q

oxygen delivery

A

product of oxygen content of blood and CO

44
Q

equations:

CO (Q) =

BP =

A

SV x HR

Q x systemic vascular resistance (SVR)

45
Q

inhalants have a positive or negative inotropic effect?

A

negative

results in a decreased stroke volume

46
Q

inhalants cause an increase or decrease in peripheral vascular resistance?

A

decrease

which in combo with reduction in CO will causes a decrease in arterial BP

47
Q

what factors enhance CV compromise due to inhalants?

A
  • mechanical ventilation
  • changes in PaCO2
  • surgical stimulation
  • length of exposure to the inhalant
  • other drugs administered with inhalant
48
Q

ventilation

A

arterial concentration of CO2

tidal volume and respiratory frequency play an important part

inhalants cause a dose related decrease in ventilation

49
Q

an increase in PaCO2 will __________________ in a conscious animal?

what do inhalants do to this response?

A

stimulate respiration

blunt the response

“safety” mechanism

50
Q

mechanical ventilation’s effect on the “safety mechanism”

A

as inhalant concentration increases in the brain, ventilation becomes reduced or absent, and thus inhalant uptake will be reduce

mechanical ventilation causes the safety mechanism to be lost

= respiratory arrest, then cardiac arrest can occur

51
Q

use of a balanced anesthetic technique

A

will allow for a suitable plane of anesthesia with decreased cardiorespiratory compromise as long as the drugs themselves do not further exacerbate unwanted side effects

MAC sparing drugs: benzos, sedatives (Ace and alpha2s), opioids, and local anesthetics

52
Q

nitrous oxide use

A

can be used to supplement inhalants

gas at room temp, not a volatile anesthetic agent

MAC in animals is ~188% (can’t be used alone)

mild analgesic and anesthetic effects, few CV side effects and minimal respiratory depression

53
Q

detrimental consequences of nitrous oxide

A

if administered at greater than 70% of total gas flow, severe hypoxemia can occur

very insoluble gas which readily diffuses out of blood and into closed gas spaces-will cause rapid expansion of closed gas spaces (pneumothroax, sinuses) causing volume and pressure in cavity to increase

can cause bone marrow supppression and anemia

fairly high abuse potential which can be fatal

Anesthesia (14 decks)

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