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Inhalant anesthetics Flashcards

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1
Q

Common inhalant anesthetics

A

Historical:
Diethyl ether (vomiting and explosive)
Chloroform (vomiting, nausea)
Halothane – no longer available
Current: isoflurane, sevoflurane
Human: desflurane

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2
Q

Delivery of inhalants

A

Liquid at RT, stored in a vaporizer
Vaporizer pressurises drug → turns into a gas at a specific partial pressure
A set amount of anaesthetic gas mixes with oxygen→ delivered to patient

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3
Q

Absorption of inhalants

A

Drug is delivered into the lungs when patient breathes in anaesthetic gas that is mixed with O2
Drug enters the alveolar sacs
Concentration of drug in the alveolar sac is higher than the concentration of drug in plasma
Drug diffuses across alveoli along the concentration gradient→ enters the circulation

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4
Q

Distribution of inhalant anesthetics

A

Once drug enters the plasma, it is rapidly distributed to the brain because
Drug is very lipid soluble
Brain is high in fat
Brain receivers lots of blood flow
Drug moves along steep concentration gradient: highest concentration in alveoli&raquo_space; blood» brain
So long as drug is being delivered to the lungs, will maintain brain levels
Drug is active so long as it is in the brain
(Not as rapid as the injectable anaesthetics)

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5
Q

Metabolism of inhalant anesthetics

A

With newer drugs, <0.1% of drug is metabolized by the liver
Drugs that require liver metabolism have an extended “hangover” effect
Benefit: pharmacokinetics are unaffected by liver disease

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6
Q

Elimination of inhalant anesthetics

A

99.99% eliminated via lungs in active from
When gas is turned off, the concentration gradient reverses
Highest concentration to brain → rapidly enters blood→ diffuses across alveoli into lungs→ exhaled
As drug leaves brain, patient wakes up

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7
Q

Speeding up elimination of inhalant anesthetics

A

Can increase the rate of elimination by increasing the concentration gradient b/w brain and lung/outside
Flushing the circuit (ie, removing drug from the lungs, mask/ETT, tubing)
Giving more 100% O2

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8
Q

Summary of inhalant drug movement

A

Diffusion rate is controlled by the concentration gradient between the alveolus and the blood
During induction, the concentration gradient is highest in the alveoli, lower in the bloodstream and lowest in the brain. So, the drug moves rapidly from the alveoli→ blood→ brain
When the anesthetic machine is turned off, the concentration gradient reverses. So, drug moves from brain→ blood→ alveoli
Maintenance is dependant on sufficient quantities of anaesthetic being delivered to the lungs
Takes time to reach therapeutic levels in the brain; but elimination is VERY rapid

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9
Q

Advantages of inhalants

A

Rapid elimination through the lungs
Easy and fast to alter the amount of drug in the brain by delivering more or less drug into the lungs: easy to adjust the depth of anesthesia
Good muscle relaxation
Very rapid recovery
Can use in patients with liver or renal disease
Patient is intubated and 100% O2 is available in the event resp depression or arrest

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10
Q

Disadvantages of inhalants

A

Takes a long time to induce
Expensive equipment required. Also requires trained personnel
NO analgesia
Hypotension (severe vasodilation) and moderate bradycardia
Hypothermia- related to the temp of the oxygen and heat loss through vasodilation (Note: there are other causes of heat loss on GA that apply to all drugs)

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11
Q

Precautions and adverse effects of inhalant anesthetics

A

Dose-dependent, reversible CNS depression
Decreased HR, cardiac output
Decreased RR and tidal volume
Always decreased under GA. Goal is to minimize the change
Vasodilation causing secondary hypoperfusion
Most vasodilation of all the drug classes discussed
Hypothermia
Cold 100% O2, vasodilation and lack of shivering
Can cause renal damage due to decreased BP
Watch old, renal patients, patients on drugs that affect kidneys

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12
Q

Induction with inhalant anesthetics

A

Never preferred
But, is acceptable in certain situation with cats and small dogs, exotics
Cannot find vein b/c fractious
Duration of GA required is much shorter than what injectable anesthetics provide
Ideally patient also has premed
Takes longer than injectable anaesthetics
Requires time to achieve effective levels in brain
Longer transition though stage 1 and 2 are unpleasant for the patient
Requires very high dose
Increases risk of adverse effects, especially vasodilation
Not indicated in LA

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13
Q

How to induce with anaesthetic inhalants

A

Induction can be chamber or mask
Space of the mask/chamber also needs to fill with certain amount of drug, before drug concentration gradient is high enough to move drug into blood
Chamber induction takes longer and can be very stressful

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14
Q

Inhalant is the Preferred maintenance anaesthetic because

A

1 choice for maintenance anaesthesia in all species given current techniques

Easy to maintain in therapeutic range for long periods
Can rapidly adjust depth of unconsciousness; can rapidly respond if patient is too light or too depp
Faster elimination and recovery than any of the injectables
Reminder: always keep anaesthetic time as short as possible. Longer anaesthetic times have an increased risk of complications, and have longer recovery time

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15
Q

Recovery with inhalants

A

Preferred
Smooth and rapid
Drug is (almost) entirely eliminated via the lungs by breathing out
Does not require waiting for liver metabolism
No redistribution to fat
Can accelerate the rate of elimination by providing the lungs with more oxygen or getting drug out of lungs faster (ie, flushing the system)

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16
Q

3 chemical properties of inhalant anesthetics

A

Vapour pressure
-How readily the drug evaporates
-Determines how it is delivered
Blood gas partition coefficient
-Affects how rapidly we can increase or decrease drug levels in the body
Minimum alveolar concentration (MAC)
-Used to calculate drug dose

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17
Q

Vapour pressure is

A

A measure of the ability to evaporate under normal atmospheric pressure
Remember the molecules enter the gas phase more readily under low pressures

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18
Q

Vapour pressure determines

A

Determines how a drug is to be delivered
In other words, it determines the type of precision vaporizer required
Precision vaporizers are canisters with a regulated internal pressure

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19
Q

Low vapour pressure drugs

A

These drugs DO NOT evaporate readily
There is very little of the drug that goes into gas form on its own; this limits the amount of drug that mixes with oxygen
Safe to give with a non-precision vaporizer because a minimal amount of the drug will be gas form at atmospheric pressure

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20
Q

high pressure vapour drugs

A

Drugs with high vapour pressure evaporate readily from liquid to gas
At atmospheric pressure, these drugs like to be present in gas form so likely to get a higher percent of drug mixed with oxygen

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21
Q

How to give high pressure vapour drugs

A

Must be given by a precision vaporizer
Precision vaporizers have high internal pressures; this limits the amount of drug that is present in gas form
Limits the amount of anaesthetic agent

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22
Q

What drugs have a similar vapour pressure

A

Halothane
iso

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23
Q

Iso as a high vapour drug

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A

At 20*C, at atmospheric pressure, isoflurane and oxygen would stabilise at a ratio of 31.5% isoflurane to 69.5% oxygen
Problem is 31.5% isoflurane is a toxic dose
Precision vaporizer si a high-pressure chamber that increases the local pressure so less iso is in gas form (goal 0-5%)

24
Q

How are vaporizers designed and what happens if temp is changed

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A

Precision vaporizers are designed to be only used with specific drugs
Exception: can use a halothane precision vaporizer for isoflurane because the vapour pressures of these two drugs is very similar
Extreme changes in the temp will alter the accuracy of the precision vaporizer

25
Blood-gas partition coefficient is
A measure of how well the inhalant anesthetic dissolves into blood Amount of drug in the blood relative to the amount of drug in the lungs Is associated with the speed of induction, speed of recovery and how rapidly one can change anaesthetic depth
26
High blood gas coefficient means
Older inhalation anaesthetics Requires a large amount of drug to build up in the blood relative to the lungs to have an effect Slower induction Slower recovery Tends to be more tissue accumulation of drug (with long anaesthetic procedures) which can increase the rate and duration of “anaesthetic hangovers”
27
Low blood gas coefficient
Newer drugs Does not require a lot of drug in blood (relative in the lungs) to have an effect Drug also moves from lungs to blood and back to lungs quickly Fast induction, faster recovery; can more rapidly alter the depth of anesthesia Less tissue accumulation; less residual effects
28
The blood gas coefficient determines and what happens when its low/high
the clinical use/effect of the drug Drugs with the lowest blood gas coefficient are preferable for mask or chamber induction Drugs with the lowest blood gas coefficient have the greatest ability to alter depth of anaesthesia Drugs with a higher coefficient have longer recovery times and patients may have anaesthetic hangovers
29
Minimum alveolar concentration is
“Sort of like” the ED50 Definition: the lowest concentration of an agent required to prevent a response to surgical stimulation in 50% of patients In other words, this is the dose of drug to obtain stage 3 plane 2 if ONLY using an inhalant (no other drugs)
30
MAC determines
A measure of the potency of a drug Hgh MAC= less potent Low MAC= more potent
31
Notes about dosing anaesthetic gas
Dose in measure % If a patient inhales 100% gas, the percent of this total amount that is made up of the gas anaesthetic Dose is different for every drug Dose is different for every species Dose may vary a bit among individual patients Dose is NOT affected by weight Because the inhalants don't really “fill” the body; they go to the brain pretty quickly
32
MAC of dog for iso
1.3
33
MAC for dog for sevo
2.4
34
MAC for cat on iso
1.6
35
MAC for horse on iso
1.3
36
What are doses of inhalant anaesthetics measured as
are always in % The percent of total gas is delivered that is in the anaesthetic; the remainder is usually oxygen
37
Drugs with a lower MAC
More potent Takes less drug to achieve anesthesia More likely to overdose Harder to make fine changes in anaesthetic depth Older drugs have lower MACs
38
Drugs with a higher MAC
Less potent Takes more durg to chive anesthesia Easier to make fine changes in anesthetic depth Newer drugs have a higher MACs
39
Why it is important to know MAC
MAC is used to determine the dose of an inhalant anaesthetic required for the “average patient” Giving the MAC dose should provide stage 3 plane 2 in 50% of patients
40
Dosing using MAC
Dosing is using balanced anesthetic protocols 1 times MAC provides light anesthesia (plane 1) 1.5 times MAC provides moderate surgical anesthesia (plane 2) 2 times MAC provided depp anesthesia (plane 3) Start around 1.5 x MAC, and adjust dose according to individual patient and depth of anesthesia
41
How does drug concentrations change
Depending on which drugs are used as part of the balanced anesthesia, the acceptable starting and maintenance dose of the gas may change Example: of a very strong premedication is used, may only need 0.5% to 1% iso to maintain a surgical plane of anesthesia When masking a cat, we start by giving 5% isoflurane
42
Factors that may alter dose of inhalant anesthetics
The following factors will decrease the dose of gas for that particular patient Very old, very young Preexisitng cardiovascular, resp disease Trauma, dehydration, shock, other illness Body temp Obesity Pregnancy If combined with other drugs (i.e., certain premeds, injectable anaesthetics)
43
Isoflurane (ISO) is and used when
Very commonly used The standard for maintenance anaesthesia Can be used for mask or chamber induction Have a pungent oder Good for detecting leaks Good muscle relaxation No analgesia >99.8% of drug eliminated from lungs 0.2% liver metabolism
44
Chemical properties of iso
High vapour pressure requires precision vaporizer Relatively low blood gas coefficient Faster induction and recovery than halothane; not as fast as sevoflurane or desflurane Patient responds in seconds to changes in dose MAC: 1.3% (D), 1.6%(C), 1.3% (EQ) Most patients will maintain a surgical plane of anaesthesia with a setting of 2%
45
Iso adverse effects
Vasodilation Always occurs. More severe than acepromazine 2* hypotension can cause delayed postop renal damage Dose dependent CNS depression, decreased HR, cardiac output, RR, tidal wave Remember that these occur with ALL the general anaesthetics Hypothermia Due to vasodilation, suppression of hypothalamus, lack of shivering and delivery via cold oxygen Irritating to MM Can cause patients to fight mask induction Can have very slight hangover effect due portion that requires liver metabolism Depresses the CO2 drive Potential abortion Malignant hyperthermia
46
How does iso depress the CO2 drive
Normally, high levels of CO2 are the brain's signal to make the body breathe. This is the CO2 drive. If you do not have a certain level of CO2, you actually stop breathing until those levels increase This signal can be lost with isoflurane May need to ventilate at proper dose 2x to 3x MAC can cause resp arrest in most patients (3% to 4.5% in a dog)
47
How does iso cause abortion
Risk to pregnant staff Rapidly crosses the placenta Casual association with spontaneous abortion (not proven); may be related to decreased memory Pregnant staff should not be around when mask inducing or chamber induction Allow fresh air to recirculate into room before entering
48
How doe iso cause malignant hyperthermia
Genetic predisposition; common in pigs; present (but rare) in all species Isoflurane triggers hypermetabolic state in skeletal muscle Muscle work so hard/fast they increase O2 demand and excess CO2 production Systemic acidosis due to increased CO2 and lactic acid production Also increased temp can cause fatal hyperthermia Response: stop iso, cool animal, support
49
Mask induction with iso
For mask or chamber induction, will typically use the highest dose setting on the vaporizer; 5% for isoflurane As soon as patient is induced, MUST decrease to appropriate maintenance dose (~2-2.5% for iso) or the patient will risk anaesthetic overdose
50
Animal will fight induction with iso because:
Irritating to MM Animal objects to distinct odor Slow progression through stages 1 and 2
51
Sevoflurane is
2nd most commonly used gas anaesthetics Newer, $$$ Very similar to iso Almost entirely exhaled by the lungs No analgesia
52
Chemical properties of sevo
High vapour pressure- requires precision vaporizer specific for sevo Blood gas coefficient lower than iso Less soluble in blood than iso; as a result, drug reaches the brain faster Faster induction and recovery Minimal drug dissolves in blood; therefore, drug is almost entirely eliminated by lungs MAC: 2.4% (D) 2.6%(C) 2.3%(Eq)
53
Adverse effects of sevo
CNS depression CV depression Vasodilation, decreased HR and cardiac output Vasodilation is slightly less than with iso Resp depression Decreased RR and tidal volume Does not turn off CO2 drive the way iso does; patients breathe better on sevo than iso
54
Advantages of using sevo (over iso)
Faster induction, faster recovery Can make smaller adjustments to dose and fine tune depth of anaesthesia better Especially important for the horses Range for sevo for maintenance is usually 2.5-4%. Range for iso for maintenance is 1.5-1.3% Faster response to change in dose No smell; not irritating to MM (preferred over iso for mask induction) Does not turn off CO2 drive
55
Nitrous oxide is
Aka laughing gas Use at 33% O2 +66% N2O 33% is the absolute minimum amount O2 that is require for a patient under GA
56
When does nitrous oxide get used
Speeds induction and recovery Analgesia Significantly decreases the dose of other inhalant anaesthetics (up to 30% decrease in MAC)
57
What to do with patients on N2O
Patients must be left on 100% O2 after turning of the N2O to prevent diffusion hypoxia As soon as N2O is turned off, drug flows out of the body and will accumulate in the lung. For a very brief period, there is actually going to be a high % of nitrous in the lungs Room air is only 21% oxygen and is insufficient to meet oxygen demands as he patient is recovering There are some contraindications

Anesthesia (8 decks)

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