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Intro to Anesthesiology > Principles of Inhalational Anesthesia > Flashcards

Principles of Inhalational Anesthesia Flashcards

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

A

Evaporate in gas (oxygen) for delivery to patient

20
Q

Vaporization

A

Process of changing from liquid to gas

21
Q

Vapor pressure

A

Atm pressure when liquid and gas are in equilibrium

22
Q

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

A

Too high

23
Q

Vapor pressures (Pv/Pb)

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 _______

A

Lethal amount

25
Q

Vapor pressure vs MAC

A
  • vapor pressure tells how much inhalant can be evaporated

- MAC tells how much vapor can be safely given to the patient

26
Q

Absorption of inhalants

A

Diffuse down a partial pressure gradient from machine through breathing system to lungs

27
Q

Absorption

A

Diffuse across alveolar membranes to blood

- similar to oxygen, but all is dissolved in plasma

28
Q

Distribution of inhalants

A

Travel to spinal cord/brain to produce anesthesia

- amount of inhalant needed to produce unconsciousness is <50% of that needed to produce immobility

29
Q

Factors affecting absorption and distribution

A
  • 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
Q

Solubility

A

How well inhalant dissolves in blood

- measured as blood:gas coefficient

31
Q

Distribution - solubility

A

Low B:G = insoluble in blood!

  • insoluble = rapid effect!
  • rapid unconsciousness and rapid recovery
32
Q

You want an anesthetic agent that has _______

A

Low BG:solubilty

33
Q

B:G Solubility

A
  • desflurane: 0.42 (<1 min)
  • sevoflurane: 0.6 (1-2 min)
  • isoflurane: 1.4 (1-3 min)
34
Q

The more soluble the inhalant is in blood, the ______ it takes to produce anesthesia and to recover from anesthesia

A

Longer!

35
Q

Metabolism

A
  • early inhalants: too much
  • modern inhalants: minimal
  • iso: 0.1%
  • sevo: 3-5%
36
Q

More soluble =

A

More metabolism in the liver

37
Q

Elimination

A

Dissolved in blood, inhalant travels back to lungs

- exhaled: breathing system (scavenge system or re-breathed)

38
Q

Anesthesia

A

GABA agonists, voltage-gated ion channels + restricted syntaxin 1A moblity

  • bind to proteins, receptors
  • unconsciousness, relaxation, blunted autonomic responses
39
Q

Inhalant safety margins

A

Therapeutic index –> low!

- dose-dependent depression of CV and respiratory function

40
Q

Cardiovascular effects

A

Vasodilation = decreased blood pressure

  • heart rate increases, offsets decrease in blood pressure unless higher concentrations are used
  • dose-dependent decrease in contractility
41
Q

Respiratory effects

A

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
Q

CNS effects

A
  • increase cerebral blood flow
  • increase intracranial pressure
  • iso may be better than other agents
43
Q

Renal effects

A
  • dose-dependent
  • decrease renal blood flow
  • decrease GFR
  • decrease urine output
  • sevoflurane
44
Q

Hepatic effects

A

Decreased hepatic blood flow at high concentrations

45
Q

Nitrous oxide

A

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
Q

Wash-in exponential function

A

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
Q

Time constant

A

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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