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ANESTHESIA > 16 – Inhalation Anesthesia Pharmacology > Flashcards

16 – Inhalation Anesthesia Pharmacology Flashcards

1
Q

What is inhalation anesthesia?

A
  • Commonly volatile LIQUIDS or compressed gases
  • Device on machine vaporizes drug into a form which can be INHALED
  • Vaporized gas=oxygen
  • Entry into body is via lungs and pulmonary circulation (uptake)
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2
Q

How do inhalants work?

A
  • *mechanism of action=UNKNOWN
  • Most enhance inhibitory activity at
    o GABA receptors in brain
    o Glycine receptors in SC
  • May inhibiting excitatory effects
    o Cholinergic
    o Glutamate
    o NMDA
  • May depress Ca channels
  • May inhibit some Na or K channels
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3
Q

Inhalation (IH) compared to injectable anesthesia

A
  • Controlled
    o Anesthetic depth can be RAPIDLY changed
  • Provides O2 and ability to ventilate lungs
  • Do NOT accumulate and recovery is rapid
  • High safety margin
  • Can produce MORE CV depression (dose dependant)
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4
Q

IH do NOT accumulate and recovery is rapid

A
  • Very little liver metabolism
    o Eliminated via lungs
  • Can maintain anesthesia as long as required
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5
Q

Minimum alveolar concentration (MAC)

A
  • Concentration of vapour in alveoli of lungs that is NEEDED to PREVENT movement (motor response) in 50% of subjects in response to surgical (pain) stimulation
  • Partial pressure (PP)
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6
Q

MAC as a measure of potency (strength) of IH drug

A
  • High MAC=LESS potent
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7
Q

Dalton’s law of partial pressure

A
  • TOTAL pressure is sum of its partial pressures
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8
Q

MAC and partial pressure: what is MAC in the equation?

A
  • Partial pressure of VOLATILE ANESTHETIC IN ALVEOLI
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9
Q

Partial pressures in lungs (3 ‘types’)

A
  • ‘partial pressure of gas in solution’
  • Vapor pressure
  • Atmospheric pressure
  • *if all other variable remain constant=reach equilibrium
  • *ALVEOLAR CONCENTRATION=BRAIN CONCENTRATION
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10
Q

‘partial pressure of gas in solution’

A
  • Reflects a ‘force’ of gas to escape out of solution
  • Fighting against the atmospheric pressure being exerted on it
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11
Q

Vapor pressure

A
  • Pressure exerted by the gas on wall of the container
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12
Q

Atmospheric pressure

A
  • Pushing down trying to force gas into liquid form OR prevent more liquid from converting to gas
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13
Q

Vaporizer dials in in concentration percent: sea level vs. altitude

A

*actual % output varies with atmospheric pressure
**PP remains the same

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14
Q

Anesthetic gas movement

A
  • Moves down partial pressure gradients
  • Until equilibrium occurs=MAINTENANCE
  • Recovery reverses the gradient, so drug leaves the body
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15
Q

Uptake of IH drugs is equivalent to

A
  • Absorption
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16
Q

Elimination of IH drugs is equivalent to

A
  • Excretion
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17
Q

Uptake of IH via the

A
  • Lungs
  • *drug is DISTRIBUTED to the BRAIN
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18
Q

What are the factors affecting uptake?

A
  • Physical properties of drug
  • Inspired anesthetic agent concentration (achieved by altering vaporizer setting)
  • Loss of agent (via diffuse through anesthetic breathing system)
  • Alveolar ventilation rate
  • Cardiac output
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19
Q

What are the 4 physical properties of drug affecting uptake?

A
  1. BLOOD:GAS solubility
  2. OIL:GAS solubility
  3. MAC as function of POTENCY
  4. Loss of agent
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20
Q

BLOOD:GAS solubility co-efficient refers to

A
  • How soluble an IH drug is in blood compared to its vapour state
21
Q

High B:G solubility coefficient

A

High B:G solubility coefficient
- More soluble in blood
o more needed to dissolve in blood to reach equilibrium
o slower onset of action
- *gas rapidly moves into blood, but concentration that reaches brain increases MORE SLOWLY (‘can fill the blood box more’)

22
Q

Low B:G solubility coefficient

A
  • less soluble in blood
    o less needed to dissolve in blood to reach equilibrium
    o faster onset of action
23
Q

When is anesthetic effect achieved?

A
  • ONLY when concentration of IH in blood/brain is the SAME as that in the gas in the ALVEOLI
24
Q

Ratio of concentration of anesthetic in alveolar gas (Fa) to inspired gas (Fi)

A
  • Least soluble drugs approach equilibrium (Fa/Fi) the FASTEST
  • Ex. NO or desflurane
25
OIL:GAS solubility coefficient is a measure of
- How much IH will be taken up by adipose tissue
26
Lower OIL:GAS solubility coefficient
- Less lipid soluble the drug - Less IH that gets deposited in fat - Faster it’s elimination and recovery time
27
Higher OIL:GAS solubility and recovery time
- Longer recovery
28
3 compartment model:
1. Blood 2. Vessel rich (brain) 3. Vessel poor (fat)
29
Distribution from blood to body tissues
- Rapid absorption in alveolar and vessel-rich, lower in muscle + fat - *when turn off anesthetic: levels drop in vessel-rich and alveolar and remains in muscle and fat for longer
30
How is lipid solubility and potency related?
- More lipid soluble=more potent=MAC value is LOWER - *but slower uptake and elimination
31
What are some patient characteristics that affect uptake?
1. Lung ventilation 2. Lung perfusion
32
Lung ventilation affect on uptake
- Higher minute ventilation (RRxTV) will INCREASE uptake - Artificially ventilating lungs will INCREASE uptake
33
Lung perfusion affect on uptake
- Low CO increases ability of alveoli to reach a higher partial pressure (‘more time to move across the gradient’) - *uptake will be FASTER with low CO states - Excited animals are hard to ‘mask induce’ anesthesia using IH drugs
34
Elimination and metabolism
- 80-90% of IH must be eliminated for full RECOVERY through lungs - Turn of vaporizer and let oxygen displace any IH drug in breathing system - Lipid soluble drugs can produce ‘hang-over’ effect - Rapidly eliminated IH drugs can cause ‘emergence delirium’ - Modern drugs have MINIMAL METABOLISM
35
Emergence delirium
- *inhalant causing behavioural abnormalities - Have sedative ready to give IV - Talk to patient - Risk of bites and scratches!
36
Properties of an ideal IH agent
- Easily vaporized near ROOM T - Non-flammable - Stable on storage - Does not react with materials of breathing system - Compatible with soda lime (what gets rid of CO2) - Non-toxic to tissues - Minimally metabolised - Environmentally friendly - Non-irritant to MM - Non-pungent - Induction and recovery should be excitement free - Allows rapid control of depth - Some analgesia - Some muscle relaxation - Few cardiorespiratory effects (hypotension + hypoventilation) - No renal or hepatic toxicity - Inexpensive - Not requiring an expensive vaporizer
37
Halogenated ethers: end with ‘flurane’
- Based on ETHER - Reduced flammability and analgesia - Minimal metabolism - *LESS lipid soluble= o Less potent=higher MAC o Rapid elimination=little ‘hang-over’ o Easy to rapidly change depth of anesthesia - *improved delivery and reliability of IH
38
Examples of halogenated ethers
- Isoflurane - Sevoflurane - Desflurane
39
Isoflurane
- Rapid uptake and elimination - Pungent smell (coughing and breath-holding) - Very little hepatic metabolism - Stable compound - Very common in VET MED
40
Isoflurane MAC in dog and cat
- Dog: 1.28% - Cat: 1.71%
41
Sevoflurane
- Rapid uptake and elimination - Not pungent smelling - 3x as expensive as Isoflurane - 5% metabolized in liver
42
Sevoflurane MAC in cat and dog
- Dog: 2.4% - Cat: 3.0%
43
Sevoflurane: 5% metabolized in liver
- Can produce Fl- ions=can be nephrotoxic o Rapid elimination through lungs helps to reduce Fl- ions=NOT a PROBLEM - Can be broken down to CO and other nephrotoxic compound by older/cheaper types of CO2 absorber used in breathing systems
44
Desflurane
- Very rapid uptake and elimination - Boiling point close to room temperature o Requires expensive vaporizer with heating unit - Unlikely to enter vet med
45
Nitrous oxide (N2O)
- Not potent - Used to supplement inhalant at 60% inhaled - Delivered as gas (contributes to global warming) - Action on NMDA and possibly opioid receptors=analgesic - Used for analgesic properties only: childbirth and dentistry - Bone marrow depression if inhaled for >24hrs or long term
46
Nitrous oxide MAC in dogs and cats
- Dog: 188% - Cat: 255%
47
Bone marrow depression with nitrous oxide inhaled for more than 24hrs
- Inactivates vitamin B12 dependent methionine synthase - Use OR scavenging
48
Waste gas scavenging
- AVOID inhalation of trace amounts of IH drug o Headaches, tiredness, nausea o *LEAK TEST - Divert gases to outside or a charcoal absorber - Don’t place exhaust close to intake vent - Don’t put scavenging hose into recirculating ventilation system - *avoid spills, fill vaporizers with keys and when staff are around
49
Pregnancy and ORs (operating rooms)
- No known proof that IH drugs can cause problems in trace amounts - Reported problems are from older drugs and ORs in dental practices NOT using proper scavenging - Follow basic rules to limit exposure - Use masks that absorb volatile compounds but uncomfortable for long periods

ANESTHESIA (27 decks)

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