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Anesthesia > Inhalational anesthesia > Flashcards

Inhalational anesthesia Flashcards

1
Q

What are used for inhalational anesthesia?

A

-commonly volatile liquids or compressed gases

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2
Q

Inhalational anesthesia steps

A

1.A device on the anesthetic machine vaporizes the drug into a form which can be inhaled using anesthetic breathing system
>Vaporizing gas= oxygen
2. Entry into the body is via lungs and pulmonary circulation

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3
Q

Mechanism of inhalants

A

**largely unknown

1.Enhance inhibitory activity of GABA A receptors in brain and glycine receptors in the spinal cord

  1. May also inhibit excitatory effects at cholinergic (muscarinic and nicotinic) receptors, glutamate receptors, NMDA)

3.Depress various calcium channels

  1. May depress some Na and K channels
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4
Q

Differences in Inhalational compared to injectable

A
  1. Controllable- anesthetic depth can be rapidly changed compared to infusion (except when animal wakes suddenly)
  2. Provides oxygen and ability to ventilate lungs

3.No accumulation of drugs; recovery is rapid (little liver metabolism, main anesthesia long term)

  1. high safety margin
  2. can produce more cardiovascular depression (dose dependent)
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5
Q

Minimal alveolar concentration (MAC)

A

-the concentration in partial pressure of a vapour in the alveoli of the lungs that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus

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6
Q

Measurement of MAC

A

-Measured as partial pressure (PP)

-Used as a measure of potency (strength) of inhalational drug
>High mac=less potent

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7
Q

Partial pressure

A

-Follow Dalton’s Law= total pressure is the sum of its partial pressures
>each molecule exerts pressure on wall of contained and is independent of the molecules around it

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8
Q

Gas composition of dry air

A

-High in N2
-Moderate O2
-Very low CO2

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9
Q

Atmospheric pressure

A

760 mmHg at sea level (comes from adding up the partial pressures of all gases in the air= N2, O2, CO2)

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10
Q

Meaning of 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

Vapour pressure

A

-pressure exerted by the gas on the walls of the container

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12
Q

Atmospheric pressure

A

-pushing down trying to force the gas back into liquid form or prevent more liquid from converting to a gas

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13
Q

Equilibrium of inhalant concentrations

A

Equilibrium between alveolar concentration and brain concentration

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14
Q

MAC of isoflurane

A
  1. MAC of isoflurane in dog = 1.3%
    >when isoflurane accounts for 1.3% of total pressure within intra-alveolar space, you have reached 1 MAC volume of isoflurane
  2. At sea level:
    760 X 0.013= 9.88 mmHg= PP occupied at sea level by isoflurane at 1 MAC
  3. At altitude
    double the amount of isoflurane being released, even if keep dial on vapourizer the same because of decreased pressure at altitude
    380 (ampP at altitude) x 0.026= 9.88 mmHg
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15
Q

Anesthetic gas movement

A

-move down partial pressure gradients UNTIL equlibrium=maintenance

**recovery reverses the gradient, so drug leaves the body

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16
Q

define Uptake and distribution with inhalants

A

Uptake= absorption

Eliminations= excretion

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17
Q

Uptake and distribution of inhalants

A

Uptake via lungs

Distributed to the brain

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18
Q

Factors affecting uptake

A

-physical properties of drug
-inspired anesthetic agent concentration (achieved by altering vapourizer setting
-loss of agent (eg. via diffusion through anesthetic breathing system)
-alveolar ventilation rate
-CO

19
Q

Physical properties of the drug affecting uptake

A
  1. Blood:gas solubility
  2. Oil:gas solubility
  3. MAC as a function of potency
  4. Loss of agent
20
Q

Blood:gas solubility co-efficient

A

-Refers to how soluble an inhalant drug is in blood compared to its vapour state

**anesthetic effect is achieved only when the concentration of inhalant in the blood and brain is the same

21
Q

High solubility in blood

A

*high B:G partition coefficient

-more needed to dissolve in blood to reach equilibrium
-slower onset of action

22
Q

Low solubility in drug

A

**Low B:G partition coefficient

-less needed to dissolve in blood to reach equilibrium
-faster onset of action

23
Q

Ratio of concentration of anesthetic in alveolar gas to inspired gas

A

-Shows the ratio between the inspired (FI) and the alveolar (FA) concentration of inhaled anesthetics over time

**least soluble drugs approach equilibrium the fastest

24
Q

Oil:gas solubility co-efficient

A

-Measure of how much inhalant will be taken up by adipose tissue

25
Q

Low oil:gas solubility coefficient means?

A

-The less lipid soluble the drug
-The less inhalant that gets deposited into the fat
-The fastest its elimination and recovery time

26
Q

3 compartment model

A

Compartment 1: blood

Compartment 2: vessel rich (brain)

Compartment 3: vessel poor (fat)

27
Q

Relation between lipid solubility and potency

A

-more lipid soluble the anesthetic drug= more potent= MAC value is lower
**BUT it has slower uptake and elimination

28
Q

What characteristics affect uptake?

A

-lung ventilation
-lung perfusion

29
Q

Lung ventilation

A

A higher minute ventilation (resp rate X tidal volume) will increase uptake

**artificially ventilating the lungs will increase uptake

30
Q

Lung perfusion

A

-Low cardiac output increases the ability of the alveoli to reach a higher partial pressure = uptake is faster with low cardiac output states because drug moving slowly by alveoli

-Excited animals are hard to mask induce anesthesia using inhalant drugs

31
Q

Elimination and metabolism

A

80-90% of inhalant drug must be eliminated for full recovery through lungs
-turn off vapourizer and let oxygen displace any inhalant drug in breathing system
-lipid soluble drugs stay in fatty tissues longer and can produce hang over effect

**modern drugs have minimal metabolism

32
Q

Emergence delirium

A

-caused by rapidly eliminated inhalant drugs

Need to:
-have sedatives ready to give IV
-talk to patient
***risk of bites and scratches (be ready to restrain patient)

33
Q

Properties of ideal inhalation agent

A

-easily vaporized at/near room temp
-non flammable
-stable in storage
-does not react with materials of breathing system or vaporizer
-compatible with soda lime
-non toxic to tissues
-minimally metabolized
-environmentally friendly
-non-irritant to MM
-non pungent
-induction and recovery should be excitement free
-allows rapid control of anesthetic depth (low blood solubility)
-some analgesia and/or muscle relaxation
-few cardio respiratory effects
-no renal or hepatic toxicity
-inexpensive
-not requiring an expensive vaporizer

34
Q

Halogenated ethers

A

**drugs ending in ‘flurane’

-Based on ether with substitute of one or more hydrogen atoms with a halogen atom (F, Cl, Br, I)

-Reduced flammability and analgesia, minimal metabolism

35
Q

Characteristics of halogenated ethers

A

Less lipid soluble results in:

  1. less potent= higher MAC
  2. Rapid elimination= less hang over
  3. Easy to rapidly change depth of anesthesia
36
Q

Isoflurane

A

-MAC for dog= 1.28%, cat= 1.71%

-rapid uptake and elimination
-pungent smell= coughing and breath holding
-very little hepatic metabolism (0.2%)
-stable compound
-commonly used in Vet med

37
Q

Sevoflurane

A

-Fluorinated Ether
-MAC in dogs (2.4%), cats (3.0%)

-rapid uptake and elimination
-no pungent smell
-3x as expensive as isoflurane
5% metabolized by liver

38
Q

Metabolism of sevoflurane by liver

A

-can produce FI- ions which can be neprhotic
-rapid elimination through the lungs helps to reduce amount of FI- ions so clinically not a problem
-can be broken down the carbon monoxide and another nephrotoxic compound by older/cheaper types of CO2 absorber used in breathing systems

39
Q

Desflurane

A

-Fluorinated methyl ether

-very rapid uptake and elimination
-boiling point close to room temperature
-requires expensive vaporizer with heating unit
-unlikely to enter vet med

40
Q

Nitrous oxide

A

-not potent
-MAC dog (188%), cat (255%)
-clinically used to supplement inhalant at 60% inhaled
-delivered as a gas= contaminant= global warming
-action on NMDA and opioid receptors= analgesic
>used for analgesic properties in childbirth and dentistry

41
Q

Bone marrow depression with nitrous oxide

A

Occurs if inhaled for more than 24hrs or long term
>drug abusers; not for staff using proper waste scavenging

Basically the N2O inactivates vit B12 dependent methionine synthase

42
Q

Waste gas scavenging

A

used to avoid inhalation of trace amounts of inhalant drugs
>check equipment functions correctly (leak test)
>divert waste gas to outside building or use charcoal absorber
>avoid spills, fill vaporizers with keys and when few staff are around

43
Q

Pregnancies and inhalant use

A

-no known proof that inhalant drugs cause problems in trace amounts
>reported problems stem from older drugs in ORs and dental practices not using proper scavenging practice

-can also use masks that absorb volatile compounds but they are uncomfortable for long term

Anesthesia (24 decks)

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