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Anesthesia > Inhalant anesthetics > Flashcards

Inhalant anesthetics Flashcards

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

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

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
Q

Blood-gas partition coefficient is

A

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
Q

High blood gas coefficient means

A

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
Q

Low blood gas coefficient

A

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
Q

The blood gas coefficient determines and what happens when its low/high

A

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
Q

Minimum alveolar concentration is

A

“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
Q

MAC determines

A

A measure of the potency of a drug
Hgh MAC= less potent
Low MAC= more potent

31
Q

Notes about dosing anaesthetic gas

A

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
Q

MAC of dog for iso

A

1.3

33
Q

MAC for dog for sevo

A

2.4

34
Q

MAC for cat on iso

A

1.6

35
Q

MAC for horse on iso

A

1.3

36
Q

What are doses of inhalant anaesthetics measured as

A

are always in %
The percent of total gas is delivered that is in the anaesthetic; the remainder is usually oxygen

37
Q

Drugs with a lower MAC

A

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
Q

Drugs with a higher MAC

A

Less potent
Takes more durg to chive anesthesia
Easier to make fine changes in anesthetic depth
Newer drugs have a higher MACs

39
Q

Why it is important to know MAC

A

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
Q

Dosing using MAC

A

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
Q

How does drug concentrations change

A

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
Q

Factors that may alter dose of inhalant anesthetics

A

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
Q

Isoflurane (ISO) is and used when

A

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
Q

Chemical properties of iso

A

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
Q

Iso adverse effects

A

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
Q

How does iso depress the CO2 drive

A

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
Q

How does iso cause abortion

A

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
Q

How doe iso cause malignant hyperthermia

A

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
Q

Mask induction with iso

A

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
Q

Animal will fight induction with iso because:

A

Irritating to MM
Animal objects to distinct odor
Slow progression through stages 1 and 2

51
Q

Sevoflurane is

A

2nd most commonly used gas anaesthetics
Newer, $$$
Very similar to iso
Almost entirely exhaled by the lungs
No analgesia

52
Q

Chemical properties of sevo

A

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
Q

Adverse effects of sevo

A

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
Q

Advantages of using sevo (over iso)

A

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
Q

Nitrous oxide is

A

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
Q

When does nitrous oxide get used

A

Speeds induction and recovery
Analgesia
Significantly decreases the dose of other inhalant anaesthetics (up to 30% decrease in MAC)

57
Q

What to do with patients on N2O

A

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 (6 decks)

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