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Intro to Anesthesiology > Principles of Inhalational Anesthesia > Flashcards

Principles of Inhalational Anesthesia Flashcards

(47 cards)

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1
Q

Hypotension during anesthesia is most likely due to which class of anesthetic drug?

A

Inhalants

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2
Q

History

A
  • 1800s: chloroform, ether, NO
  • 1951: halothane
  • 1958: methoxyflurane
  • 1981: isoflurane
  • 1992: desoflurane
  • 1994: sevoflurane
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3
Q

What 2 inhalant drugs are no longer available in the US?

A

Halothane and methoxyflurane

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4
Q

Inhalants

A
  • powerful drugs w/ low margin of safety
  • induce/maintain anesthesia
  • non-flammable
  • vaporizer, machine, breathing system
  • aversive to humnas and animals
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5
Q

How inhalant anesthetic is delivered

A

Delivered from an anesthesia machine to the patient’s lungs

- inhaled anesthetic drugs enter the blood and produces general anesthesia in the CNS

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6
Q

How to we measure inhalational anesthesia?

A

Measure levels in the lungs

- not feasible to measure inhalant anesthetic levels in the spinal cord/brain

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7
Q

MAC

A

Minimum alveolar concentration

  • MAC = potency = ED50 = dose/response
  • low MAC = high potency
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8
Q

MAC definition

A

Inhalant percent in alveoli that prevents movement in response to noxious stimulation in 50% of animals tested

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9
Q

Median ED50 response to supramaximal stimulation is used to define _____

A

Various endpoints

- MAC awake and MAC bar

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10
Q

How is MAC used?

A

To determine where to dial vaporizer setting

- surgical anesthesia in unpremedicated patients is approximately 1.3 times MAC

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11
Q

MAC is determined in the ______ of any other drugs

A

Absence

- drugs modify the MAC value, usually making it less than what it really is

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12
Q

***MAC of common agents

A
  • isoflurane: 1.3-1.6%
  • sevoflurane: 2.4-2.6%
  • desflurane: 7.2-10.3%
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13
Q

_____ MAC = higher potency

A

Lower

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14
Q

Factors decreasing MAC

A
  • hypothermia
  • pre-meds!!
  • pregnancy
  • old age
  • hypotension/hypoxemia
  • CNS depressants
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15
Q

Factors increasing MAC

A
  • hyperthermia

- CNS stimulants

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16
Q

Unchanged MAC

A
  • gender
  • duration of anesthesia
  • potassium abnormalities
  • thyroid disorders (either hypo or hyper)
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17
Q

What is a vapor

A

Gaseous phase of volatile liquid

  • volatile liquid: liquid at ambient temp
  • evaporates readily
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18
Q

Name one familiar volatile liquid

A

Water, gasoline

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19
Q

Volatile liquids

Study These Flashcards
A

Evaporate in gas (oxygen) for delivery to patient

20
Q

Vaporization

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A

Process of changing from liquid to gas

21
Q

Vapor pressure

Study These Flashcards
A

Atm pressure when liquid and gas are in equilibrium

22
Q

Vapor pressures of modern inhalants are ______ to be used safely without vaporizer

23
Q

Vapor pressures (Pv/Pb)

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A
  • sevo: 22% (170 mmHg)
  • iso: 31% (240 mmHg)
  • des: 88% (669 mmHg)
24
Q

If you measured the amount of liquid vaporizing at the liquid/gas interface on an open bottle, it is a _______

Study These Flashcards
A

Lethal amount

25
Vapor pressure vs MAC
- vapor pressure tells how much inhalant can be evaporated | - MAC tells how much vapor can be safely given to the patient
26
Absorption of inhalants
Diffuse down a partial pressure gradient from machine through breathing system to lungs
27
Absorption
Diffuse across alveolar membranes to blood | - similar to oxygen, but all is dissolved in plasma
28
Distribution of inhalants
Travel to spinal cord/brain to produce anesthesia | - amount of inhalant needed to produce unconsciousness is <50% of that needed to produce immobility
29
Factors affecting absorption and distribution
- machine: vaporizer setting, O2 flow rate (time constant) - breathing system: volume, ventilation status - patient: gas exchange, CO, volume of distribution - anesthetic agent: inspired conc, solubility!
30
Solubility
How well inhalant dissolves in blood | - measured as blood:gas coefficient
31
Distribution - solubility
Low B:G = insoluble in blood! - insoluble = rapid effect! - rapid unconsciousness and rapid recovery
32
You want an anesthetic agent that has _______
Low BG:solubilty
33
B:G Solubility
- desflurane: 0.42 (<1 min) - sevoflurane: 0.6 (1-2 min) - isoflurane: 1.4 (1-3 min)
34
The more soluble the inhalant is in blood, the ______ it takes to produce anesthesia and to recover from anesthesia
Longer!
35
Metabolism
- early inhalants: too much - modern inhalants: minimal - iso: 0.1% - sevo: 3-5%
36
More soluble =
More metabolism in the liver
37
Elimination
Dissolved in blood, inhalant travels back to lungs | - exhaled: breathing system (scavenge system or re-breathed)
38
Anesthesia
GABA agonists, voltage-gated ion channels + restricted syntaxin 1A moblity - bind to proteins, receptors - unconsciousness, relaxation, blunted autonomic responses
39
Inhalant safety margins
Therapeutic index --> low! | - dose-dependent depression of CV and respiratory function
40
Cardiovascular effects
Vasodilation = decreased blood pressure - heart rate increases, offsets decrease in blood pressure unless higher concentrations are used - dose-dependent decrease in contractility
41
Respiratory effects
Dose-dependent depression - hypoventilation > increased CO levels - apnea at high doses - animals can still have a high respiratory rate and be hypoventilated (function of CO2 being retained)
42
CNS effects
- increase cerebral blood flow - increase intracranial pressure - iso may be better than other agents
43
Renal effects
- dose-dependent - decrease renal blood flow - decrease GFR - decrease urine output - sevoflurane
44
Hepatic effects
Decreased hepatic blood flow at high concentrations
45
Nitrous oxide
Only anesthetic associated with adverse health effects! - not potent enough to produce anesthesia in animals (MAC > 200%) - large volumes used - can displace oxygen (hypoxia) - distends gaseous spaces inside the animal
46
Wash-in exponential function
y = yoo (1-e^-kt) - yoo: limiting value of y - e: base of natural logarithms - k: constant defining rate of build-up, reciprocal of the time constant - used for tissue blood flow, exchange of gases in lungs and other enclosed spaces
47
Time constant
t = 1/k - calculated as volume divided by inflow rate - increase inflow rate to make the time constant go faster (reduces time constant) - if you want something to happen faster with the anesthetic machine, then increase the vapor setting flow rate

Intro to Anesthesiology (10 decks)

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