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Primary Pharmacology > Anaesthetic Gasses And Vapours > Flashcards

Anaesthetic Gasses And Vapours Flashcards

1
Q

Characteristics of voletile anaesthetic agents

A

Liquids with low boiling points (lower than water) and high saturated vapour pressure (higher than water) that evaporate easily

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2
Q

How are voletile anaesthetic distributed through the body at equilibrium

A

Same partial pressure all over (if not would be out of equilibrium and move down gradient) but varied concentrations dependant on partition coefficients

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3
Q

Phases of volatile anaesthetic distribution to brain

A

Delivery phase
Pulmonary phase
Circulatory phase

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4
Q

What is the delivery phase of volatile anaesthetic distribution

A

Introduction of agent into the inspired gas mixture

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5
Q

Why does the actual concentration of volatile inspired differ from the dialled concentration on the vapourisor

A

Dilution with existing gas
Uptake by co2 absorbers
Uptake by rubber or plastic components in the circuit

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6
Q

Where should concentration of volatile agents be measured in a circuit

A

As close to the patient as possible

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7
Q

What factors influence uptake of anaesthetic form inhaled gas to the blood

A

Inhaled concentration
Alveolar ventilation
Diffusion
Blood gas partition coefficient
Pp in the pulmonary artery
Pulmonary blood flow
Ventilation perfusion distribution
Concentration effect
Second gas effect

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8
Q

How can alveolar volatile levels be made to reach inspired levels faster
What acts to decrease the alveolar level

A

Increasing alveolar minute volume
Diffusion of the gas into the blood

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9
Q

What limits diffusion of voletile from alveoli to blood

A

Disease eg pulmonary fibrosis increasing thickness or emphysema reducing available area

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10
Q

What is the blood gas partition coefficient
What assumptions are made

A

The amount of agent that must be transferred to the blood to achieve equilibrium for a given tension
Assumes blood volume is known and there is no transfer on to other tissues.

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11
Q

What is the implication and effect of a low blood gas partition coefficient

A

Low blood solubility
Equilibrium reached after a small transfer of gas
Thus equilibrium reached rapidly

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12
Q

Which 4 anaesthetic agents have the lowest blood gas partition coefficient
What values

A

Xenon 0.12
Desflurane 0.42
N2O 0.44
Sevo 0.69

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13
Q

Which 3 anaesthetic agents have the highest blood gas partition coefficient
What values

A

Halothane 2.3
Enflurane 1.9
Isoflurane 1.43

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14
Q

Why does pp of volatile in pul artery effect uptake of volatile from alveoli

A

Increasing pp in artery decreases gradient so decreases uptake

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15
Q

What is the effect of pul blood flow on uptake of volatile agent

A

Increase flow clears concentrated blood thus increases rate of transfer

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16
Q

What is the concentration effect with respect to transfer of volatile agent from alveoli to blood

A

Uptake of volatile will leave a lower concentration for rest of the ventilatory cycle, thus lower tension, thus lower transfer for rest of that cycle (assuming lower proportion of other alveolar gases absorbed). Bigger effect proportionally when starting concentration is lower.

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17
Q

What is the second gas effect

A

If another alveolar gas is absorbed quicker than the volatile then the conc. increases thus faster uptake

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18
Q

What factors influence the circulatory phase of volatile delivery

A

Cardiac output
Cerebral blood flow
Distribution to other tissues

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19
Q

What is the formulae for uptake of volatile agent by tissues

A

Uptake = tissue blood flow x tissue:blood solubility x av tension difference

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20
Q

What is the effect of the differing distribution of blood flow to different organs on volatile distribution

A

Volatile follows blood thus large proportion of absorbed anaesthetic directed to the brain

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21
Q

What is the significance of cerebral auto regulation on distribution of volatile anaesthetic

A

In shock cerebral blood flow maintained thus even greater proportion of blood flow goes to brain thus equilibrium reached more rapidly
Hypoventilation during anaesthetic will increase co2 and further speed equilibrium
Hyperventilation (eg at induction) decreases co2 and thus would slow equilibrium

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22
Q

What is the effect of distribution of volatiles to other tissues on uptake to the brain

A

Initially decreases it as the agent is absorbed elsewhere
Then acts as a resevoir releasing it back into the blood and thus back to the brain after agent turned off - both acts as a damper to change and slows recovery

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23
Q

What is MAC

A

Minimum alveolar concentration
The concentration of anaesthetic agent that at equilibrium will prevent a reflex response to skin incision in 50% of subjects

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24
Q

What is the effect of altitude on MAC

A

Reduced pressure thus reduced concentration of anaesthetic for a given percentage

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25
What is an AD95 for an anaesthetic
Anaesthetic dose to prevent response to surgical stimuli in 95%
26
MAC percentages for common anaesthetic agents (% v/v)
Halothane 0.75 Isoflurane 1.15 Sevoflurane 2.0 Enflurane 1.68 Desflurane 6.35 Xenon 71 Nitrous oxide 105
27
Characteristics of ideal volatile agent
Liquid at room temp Low latent heat of vapourisation, low specific heat capacity Saturated vapour pressure high Stable Non flammable Inexpensive Environmentally safe Pleasant smell Low blood gas solubility High oil water solubility Low mac Analgesics effect No side effects Cardiovascularly stable Non irritant No increase in ICP Minimal metabolism Not effected by organ dysfunction
28
What are common halogens
Fluorine Bromine Chlorine Iodine
29
Which volatile contains bromine
Halothane
30
Which three volatiles have the same carbon skeleton What is it
Isoflurane Enflurane Desflurane C - O - C - C
31
Effects of volatile agents on CNS
Dose dependant suppression of cerebral activity Lowering oxygen consumption Increased cerebral blood flow and thus ICP
32
Which volatile increases epileptiform activity on eeg
Enflurane
33
Which volatile agents effect cerebral blood flow and ICP the most
Halothane more than all others Desflurane has slightly higher ICP effects compared to sevo and iso
34
Which volatile has analgesic properties
Nitrous only
35
Effect of volatiles on respiratory system
Dose dependant resorption depression Reduced tidal volumes and increased rate (except sevo which lowers rate) PaCO2 raises Response to hypercapnia and hypoxia blunted Bronchodialation (halothane and sevo) Irritation (Desflurane and iso)
36
Effect of main volatiles on resp depression
D > I > H,S
37
What are the mechanisms of action of volatiles on cardiovascular depression
Contractility decrease Occurs by interference with calcium movement. Usually increased heart rate (except halothane that decreases) Decreased SVR - especially iso and sevo
38
Which voletile causes most cardiovascular depression
Halothane
39
What is the effect of volatiles on muscle relaxation
Dose dependant depression if nm function potentiating muscle relaxants
40
Effect of volatiles on metabolic rate
Decreases causing decrease in o2 consumption and co2 production 2 MAC decreases BMR by 30%
41
Which halogen is associated with lowest toxicity Why
Fluorine Decreased metabolism
42
What is produced when dry soda lime contacts desflurane or isoflurane How to avoid
Carbon monoxide Low flows - keeps air moist so no reaction
43
Impact of volatiles on immune system
Reduce neutrophil killing function by interfering with calcium flux and superoxide genratjon
44
Colours of volatile agent bottles
Hal - red Iso - purple Sevo - yellow
45
Effect of sevoflurane and soda lime mixing Effect on body
> 65oC degrades to form compounds A-E, Lower temps forms only compound A and small amounts compound B Toxic renal, hepatic and brain effects
46
Effect of volatiles on salivary secretions
Increases
47
Elimination of volatiles
Des and iso - Most by lungs with 0.02 to 0.2 respectively hepatically metabolised halothane - 20% metabolised in liver and metabolites excreted in urine for weeks Sevo - 5% metabolised by cyp450
48
Boiling points of volatiles and SVP
Des - 22.8, 88.5 Iso - 48.5, 31.9 Halo - 50.2, 32.5 Sevo 58.6, 21.3
49
Which volatile vapourisor is different? Why?
Des Due to high svp needs heated pressurised vapourisor with microprocessor control unit that injects gas into the incoming mixture.
50
What is the implication of the low blood/gas coefficient of des?
Rapid onset and also rapid offset - very controllable.
51
Why is des bad for a gas induction
Very irritant Increases bronch and salivary secretions Conc above 6% can stimulate coughing and laryngospasm esp in children
52
What needs to be done to halothane vapourisors Why
Clean them Thymol preservative doesn’t vapourise so builds up
53
Which volatile maintains cardiac contractility more than others
Des
54
Cardiovascular effects of halothane
Decreased contractility and rate due to vagal stimulation Irritability resulting in pvcs Sensitisation to catecholamines thus risk of arrhythmia with adrenaline administration
55
Which agent is good for inhalational induction where airway obstruction may be a significant issue? Why
Halothane Non-irritant, Reduces secretions, causes bronchodilation,
56
Metabolites of halothane
Trifluroacetic acid Chloride Bromide
57
What is ‘steal’ in relation to volatiles
Dilation of healthy vessels diverts blood away from critically perfused non compliant vessels producing compromised areas
58
Effect of Sevo on resp function
Decreases tidal volumes and resp rate Decreases hypoxic drive and co2 sensitivity Reduces bronchial smooth muscle tone
59
Metabolite of sevoflurane
5% metabolised by cyp450 Produces fluoroisopropanol and inorganic fluoride ions These can be toxic (renal)
60
What are wet and dry Sevo presentations? Why
Can either come with higher or lower water content No convincing evidence either way but risk of breakdown of Sevo in contact with metal to an acid and water inhibits this.
61
Boiling point of nitrous oxide Saturated vapour pressure of nitrous oxide
-88oC 5300
62
How is nitrous oxide presented
French blue cylinders Pure liquid under pressure (44bar) Filling ratio 0.75
63
Why is nitrous an issue around fire
Not flammable itself be decomposes to oxygen and nitrogen above 450oC
64
Effect of nitrous on cvs
Increases vascular tone and increases blood pressure
65
Issues with nitrous oxide
Diffuses into air filled spaces Causing expansion Prolonged use interferes with Vit B12
66
How is nitrous oxide synthesised
Heating ammonium nitrate to 240
67
What happens to nitrous oxide cylinder pressure as it is used Why
Stays the same until all liquid nitrous oxide used up then falls rapidly The pressure is specific for the temperature (SVP) and is replenished from the liquid as gas is removed. NB slight fall is seen due to temperature drop
68
Effect of nitrous oxide on spinal impulses
Suppresses them
69
How does nitrous oxide effect cvs
Suppressed myocardial contractility by direct effect but this is neutralised by increased sympathetic tone Generally increased vascular tone giving rise in pvr and SVR
70
Effect of nitrous oxide on skeletal muscles
Increased activity Does not potentiate nmb
71
How does nitrous oxide effect vitamins
Interacts with b12 converting movovalent cobalt to bivalent. This prevents it functioning as a coenzyme for methionine synthase, thus increase in homocysteine and decrease in methionine. This impairs dna synthesis and thus can cause megaloblastic anaemia
72
Time frame for nitrous to cause vitamin effects
6-24 Hrs but can be faster in seriously ill patients.
73
What is the coshh regulation limit on nitrous oxide exposure
Less than 100ppm over 8hrs
74
Why does nitrous oxide causes air space expansion
More soluble in blood than nitrogen (34 fold) Diffuses into air filed spaces quicker than nitrogen diffuses out increasing the volume
75
How is nitrous oxide metabolised
Most excreted by lungs
76
How is entonox presented
French blue cylinder Blue and white quarter shoulders 137 bar pressure, filling ratio 0.75,
77
Advantages of xenon as a voletile
Very cardiovascularly stable Non irritant Low blood gas coefficient so rapid onset and emergence Excreted unchanged Does not cause malignant hypothermia Not irritant
78
Disadvantages of xenon as a volatile
Very expensive Cannot be compressed to form liquid as critical temp 16o Hard to monitor Low potency (needs high concentrations to achieve Mac) High viscosity so needs high driving pressure

Primary Pharmacology (18 decks)

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