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LEAH FCP > Maintenance of anaesthesia > Flashcards

Maintenance of anaesthesia Flashcards

1
Q

What are the most common inhalation agents?

A

Sevoflurane & Isoflurane

(Halothane was old version)

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2
Q

Describe isoflurane

A

Lower solubility & safer cardiovascular profile than Halothane

Licensed in dogs, cats, horses, chinchillas, ferrets, gerbils, g pigs, hamsters, mice, rats, ornamental birds & reptiles

MAC = 1.28 (dog) 1.63 (cat) 1.3 %(horse), 0.9% (foal), 2.05% (rabbit)

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3
Q

What is Minimum Alveolar Concentration (MAC), and why is it important?

A

MAC: concentration of anaesthetic vapour in alveoli needed to prevent movement in 50% of subjects in response to surgical stimulus

Indicates potency of anaesthetic agent

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4
Q

Describe Sevoflurane

A

Faster induction, recovery & intraoperative modulation of anaesthetic depths than Isoflurane

More expensive

Not yet licensed in horses

MAC 2.2% (dog) 2.58% (cat) 2.3% (horse) 3.7% (Rabbit)

Induces dose-dependent cardiovascular depression to degree similar to that of isoflurane

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5
Q

Describe nitrous oxide (N2O)

A

Provides analgesia via endogenous opioid activation & NMDA receptor antagonism

Minimal cardiovascular & respiratory effects

Contributes to balanced anaesthesia

High MAC (>100%) means it can’t induce anaesthesia alone

Not a vapour, delivered as a gas
- Often used as adjunct in 2:1 (N₂O:O₂) ratio.

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6
Q

What are the health & safety concerns with Nitrous Oxide?

A

Risk of abuse in humans

Long-term exposure can cause:
- Bone marrow suppression.
- Potential carcinogenic effects.
- Contributes to environmental pollution.

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7
Q

How do vaporisers work?

A

Convert liquid anaesthetic agents (e.g., Isoflurane, Sevoflurane) into a vapour

Vapours are carried to lungs in oxygen or other carrier gases

Use temperature compensation to ensure consistent delivery.

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8
Q

What is the difference between a gas & vapour?

A

Gas has one defined state at room temp whereas vapour is substance that is in gaseous & liquid equilibrium at room temperature

Thus vapours are liquids vaporised in a carrier gas

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9
Q

What is the purpose of a vaporiser?

A

vaporiser adds anaesthetic vapour to fresh gas flow, ensuring output delivers set concentration of anaesthetic agent accurately

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10
Q

How does fresh gas flow through a vaporiser?

A

Fresh gas enters vaporiser & is split into 2 pathways:
- Bypass pathway: Gas remains vapor-free.
- Vaporising chamber pathway: Gas becomes fully saturated with vapour

At exit, 2 streams mix to produce desired concentration of anaesthetic agent

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11
Q

What controls the concentration of vapour output from a vaporiser?

A

vaporiser control dial adjusts splitting valve:
- Higher setting: More fresh gas flows through vaporising chamber, increasing vapour output
- Lower setting: More fresh gas bypasses chamber, reducing vapour output

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12
Q

What is the function of a wick in a vaporiser?

A

Wicks are added to vaporiser to increase surface area of contact between anaesthetic agent & fresh carrier gas

Allows vaporisation process to keep up with high fresh gas flow rates, ensuring adequate amount of anaesthetic vapour is produced

Without wicks, vaporiser may fail to deliver correct concentration of vapour at high gas flow rates

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13
Q

What happens to the temperature of an anaesthetic agent during vaporisation?

A

During vaporisation, anaesthetic molecules escape liquid, taking energy with them

This reduces energy in liquid, causing its temperature to fall

Lower temperature reduces ability of liquid to vaporise further

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14
Q

How do modern vaporisers manage temperature compensation?

A

Modern vaporisers include:

  1. Metal casings: Act as heat conductors, retainers & donors to maintain consistent liquid temperature
  2. Temperature compensating valves: Automatically adjust fresh gas flow to maintain correct vapour output
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15
Q

Why is temperature compensation important in vaporisers?

A

Prevents drop in vapour output caused by cooling of liquid during vaporisation

Ensures consistent anaesthetic concentration is delivered to patient

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16
Q

How does anaesthesia ‘hijack’ the physiological process of gas exchange?

A

During anaesthesia:
- 100% oxygen is delivered via endotracheal tube & breathing system
- Oxygen acts as carrier gas for anaesthetic agent
- Gas exchange in alveoli includes both oxygen & anaesthetic agent

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17
Q

What is the pathway of anaesthetic gas from inhalation to their effect?

A
  1. Inspired gas mixture enters alveoli
  2. Anaesthetic agent crosses alveolar membrane into arterial blood system
  3. Blood transports agent to tissues, including brain, where it induces anaesthesia
  4. Constant exhalation removes agent, helping maintain steady anaesthetic plane
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18
Q

What factors influence how quickly the anaesthetic agent enters the blood?

A

Ventilation rate: Faster breathing increases agent exchange in alveoli

Alveolar concentration: Higher concentration speeds up transfer into blood

Cardiac output (inverse relationship):
- High cardiac output (e.g., fit animals): Blood clears agent faster, making anaesthesia harder to maintain
- Low cardiac output (e.g., sick animals): Blood clears agent slower, requiring conservative dosing

Agent solubility (inverse relationship): More soluble agents take longer to reach effective concentrations in brain

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19
Q

How can MAC vary between patients?

A

Species-specific: Each species has different MAC values

Individual-specific: Factors like age, health & premedication can affect MAC

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20
Q

What factors can increase MAC?

A

Drug causing CNS stimulation (e.g. ephedrine)

Hyperthermia

Strong cardiac output (e.g. young/fit animal)

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21
Q

What factors can decrease MAC?

A

Premedication (e.g., opioids, alpha-2 agonists, benzodiazepines)

Hypothermia

Age (geriatrics)

Pregnancy

Severe illness or hypotension

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22
Q

Why do premedicated patients typically require less anaesthetic agent than the MAC value suggests?

A

Premedication drugs reduce brain’s sensitivity to stimuli, lowering required concentration of anaesthetic agents to maintain anaesthesia

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23
Q

What does blood:gas partition coefficient represent?

A

represents ratio of how much anaesthetic agent is dissolved in blood vs how much remains in gaseous form in contact with blood

LESS soluble agents (low coefficient) are washed away less quickly thus alveolar concentration rises FASTER

24
Q

How does the blood:gas partition coefficient affect anaesthetic onset & recovery?

A

Low blood-gas coefficient:
- Faster onset (induction) & recovery, as less of agent is dissolved in blood & more is available in gaseous form to cross blood-brain barrier

High blood-gas coefficient:
- Slower onset & recovery, as more agent is dissolved in blood & bound to proteins, limiting its availability to brain.

25
Q

Which type of blood-gas partition coefficient is preferred for rapid adjustments in anaesthetic depth?

A

low blood-gas coefficient is preferred because it allows faster changes in anaesthetic depth and recovery

26
Q

Why do overweight animals recover slower than lean ones?

A

Recovery is reverse of induction, so dependent on blood solubility, redistribution will have occurred into fat, which then acts as reservoir for anaesthetic so (depending on fat solubility) overweight animal will recover slower than lean one

27
Q

How do inhalation agents work?

A

Work within CNS by augmenting signals to Cl channels (GABA receptors) & K channels while depressing neurotransmission pathways

(Don’t know for certain)

28
Q

What are the cardiovascular effects of inhalation agents?

A

Dose-dependent cardiovascular depression due to:
- Decreased myocardial contractility (negative inotropy).
- Peripheral vasodilation, reducing systemic vascular resistance.
- Decreased arterial blood pressure.

29
Q

What are the respiratory effects of inhalation agents?

A

Dose-dependent respiratory depression:
- Reduced response to increased CO₂ levels & hypoxia
- Increased dead space ventilation due to bronchodilation

30
Q

How can you mitigate the negative effects of inhalation agents?

A

Use balanced anaesthesia to lower dose of volatile agents

Employ monitoring to adjust depth of anaesthesia & maintain stable cardiovascular & respiratory function

Calculate FGF correctly

Check all anaesthetic equipment before use

Know your drugs

TIVA/PIVA

31
Q

What are the physical properties of an ideal inhalation agent?

A

Non-flammable & non-explosive at room temperature

Stable in light

Long shelf life

Liquid & vaporisable at room temperature (low latent heat of vaporisation)

Stable with soda lime, plastics, & metals

Environmentally friendly: No ozone depletion

Cheap & easy to manufacture

32
Q

What are the biological properties of an ideal inhalation agent?

A

Pleasant to inhale & non-irritant, induces bronchodilation

Low blood:gas solubility for fast onset & recovery

High oil:water solubility for high potency

Minimal effects on other systems:
- Cardiovascular, respiratory, hepatic, renal, or endocrine

No biotransformation: Excreted unchanged via lungs

Non-toxic to operating theatre personnel

33
Q

Why is scavenging necessary during anaesthesia?

A

Excess gas is vented from breathing system via adjustable pressure-limiting (APL) or pop-off valve

This gas must be scavenged to prevent exposure of operating theatre personnel to waste anaesthetic gases

Can have passive or active scavenging

Annual audits done to measure exposure

Flush breathing systems before disconnection

34
Q

What is volatile capture technology (VCT)?

A

VCT captures waste volatile anaesthetic agents before they are released into environment

Technology aimed at reducing environmental impact of anaesthetic gases

35
Q

Why are horses prone to complications during anaesthesia with inhalants?

A

Horses are prone to hypotension when inhalants are used

Prolonged hypotension can lead to post-op myopathy, which may result in:
- Euthanasia due to severe muscle damage.
- Fractures or injuries during recovery.

36
Q

How does TIVA compare to inhalants for equine anaesthesia in terms of blood pressure?

A

TIVA causes less incidence & magnitude of hypotension compared to inhalants

37
Q

What is TIVA?

A

Total intravenous anaesthesia

38
Q

What are the advantages of TIVA?

A

Reduces pollution hazard from inhalant agents

Avoids repeated administration of toxic drugs

Quick induction, rapid reversal & superior recovery

Reduces postoperative nausea & vomiting

Easy to titrate drugs for stable anaesthetic plane

Independent of patient’s airway & breathing system

No need for anaesthetic vaporiser

Maintains cerebral autoregulation (important for brain disease)

Preserves hypoxic pulmonary vasoconstriction

Doesn’t trigger malignant hyperthermia

Can be used “in the field.”

39
Q

What are the disadvantages of TIVA?

A

Requires equipment like syringe drivers or infusion pumps for accurate drug delivery

Recovery relies on metabolism, which can prolong recovery in animals with hepatic disease

Can be expensive

May still produce unwanted side effects

Risk of accumulation with repeated dosing

40
Q

What are the desirable properties of TIVA drugs?

A

Rapid onset of action & smooth induction

Short duration & rapid metabolism

No active metabolites

Rapid clearance to avoid accumulation

Smooth, excitement-free recovery

Minimal impact on cardiovascular parameters

Provide unconsciousness, muscle relaxation & analgesia

41
Q

What are the 2 ways in which TIVA can be achieved?

A
  1. Top-up bolus injections (propofol, Alfaxalone, ketamine)
  2. Continuous rate infusions (propofol, Alfaxalone, “triple drip” in equine anaesthesia – useful for field anaesthesia)
42
Q

What is the goal of TIVA?

A

No inhalation agent

Produces much-diminished anaesthesia stress response compared with inhalation agents & thus physiologically superior method of anaesthesia

43
Q

What is Triple drip?

A

TIVA protocol commonly used for equine anaesthesia

Involves combination of 3 drugs that provide sedation, muscle relaxation & anaesthesia

44
Q

What is PIVA

A

partial intravenous anaesthesia

Combination of inhalation anaesthesia & IV drugs to achieve balanced anaesthesia

Reduces reliance on inhalants while providing better control of anaesthesia depth & minimizing side effects

45
Q

What are the goals of PIVA?

A

Reduce MAC

Reduce cardiopulmonary depression

Provide additional analgesia

Contribute to balanced anaesthesia

Less polution

46
Q

What are the commonly used IV agents in PIVA?

A

Opioids (e.g., fentanyl, methadone)

Ketamine: Provides analgesia & NMDA receptor antagonism

Lidocaine: Analgesic & anti-inflammatory properties

Alpha-2 agonists (e.g., dexmedetomidine): Sedation & analgesia

(many combinations possible)

47
Q

What are the disadvantages of PIVA?

A

Requires additional equipment

Cost may be higher than inhalant-only anaesthesia

IV drug side effects (e.g., bradycardia with alpha-2 agonists) must be managed

48
Q
A

Isoflurane & Sevoflurane

49
Q
A

Colour coded key for agent bottles
- Iso is purple & Sevo is yellow

50
Q
A

You cannot induce anaesthesia with it because the MAC is too high

There is hard evidence that long-term exposure to nitrous oxide can cause bone marrow suppression and teratogenesis (congenital malformations in the foetus)

It is usually used as an adjunct analgesic intraoperatively.

51
Q
A

The concentration of a vapour in the alveoli of the lungs that is needed to prevent movement in 50% of subjects in response to surgical stimulus.

52
Q
A

Sevoflurane

53
Q
A

All of the above

54
Q
A

Passive

55
Q
A

A drop in blood pressure due to peripheral vasodilation

LEAH FCP (49 decks)

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