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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 increase surface area between anaesthetic agent and fresh gas.

Enhance vapour production, especially at high fresh gas flow rates.

Without wicks, vapouriser may under-deliver anaesthetic at high flows

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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

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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.

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25
Which type of blood-gas partition coefficient is preferred for rapid adjustments in anaesthetic depth?
low blood-gas coefficient is preferred because it allows faster changes in anaesthetic depth and recovery
26
Why do overweight animals recover slower than lean ones?
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
How do inhalation agents work?
Work within CNS by augmenting signals to Cl channels (GABA receptors) & K channels while depressing neurotransmission pathways (Don't know for certain)
28
What are the cardiovascular effects of inhalation agents? Why?
Dose-dependent cardiovascular depression due to: - Decreased myocardial contractility (negative inotropy). - Peripheral vasodilation, reducing systemic vascular resistance. - Decreased arterial blood pressure.
29
What are the respiratory effects of inhalation agents?
Dose-dependent respiratory depression: - Reduced response to increased CO₂ levels & hypoxia - Increased dead space ventilation due to bronchodilation
30
How can you mitigate the negative effects of inhalation agents?
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
What are the physical properties of an ideal inhalation agent?
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
What are the biological properties of an ideal inhalation agent?
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
Why is scavenging necessary during anaesthesia?
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
What is volatile capture technology (VCT)?
VCT captures waste volatile anaesthetic agents before they are released into environment Technology aimed at reducing environmental impact of anaesthetic gases
35
Why are horses prone to complications during anaesthesia with inhalants?
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
How does TIVA compare to inhalants for equine anaesthesia in terms of blood pressure?
TIVA causes less incidence & magnitude of hypotension compared to inhalants
37
What is TIVA?
Total intravenous anaesthesia
38
What are the advantages of TIVA?
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
What are the disadvantages of TIVA?
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
What are the desirable properties of TIVA drugs?
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
What are the 2 ways in which TIVA can be achieved?
1. Top-up bolus injections (propofol, Alfaxalone, ketamine) 2. Continuous rate infusions (propofol, Alfaxalone, “triple drip” in equine anaesthesia – useful for field anaesthesia)
42
What is the goal of TIVA?
No inhalation agent Produces much-diminished anaesthesia stress response compared with inhalation agents & thus physiologically superior method of anaesthesia
43
What is Triple drip?
TIVA protocol commonly used for equine anaesthesia Involves combination of 3 drugs that provide sedation, muscle relaxation & anaesthesia
44
What is PIVA
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
What are the goals of PIVA?
Reduce MAC Reduce cardiopulmonary depression Provide additional analgesia Contribute to balanced anaesthesia Less polution
46
What are the commonly used IV agents in PIVA?
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
What are the disadvantages of PIVA?
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
Isoflurane & Sevoflurane
49
Colour coded key for agent bottles - Iso is purple & Sevo is yellow
50
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
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
Sevoflurane
53
All of the above
54
Passive
55
A drop in blood pressure due to peripheral vasodilation
56
Describe the induction, maintenance & recovery of a routine castration in a 3 year old, healthy bouncy Labrador. With each of these events you consider the following categories: * Patient * Procedure * Drugs * Equipment / Staff
1. Induction: - Patient: Healthy, bouncy. - Drugs: Acepromazine + opioid, Propofol/Alfaxalone. - Equipment: IV catheter, ET tube, monitoring tools. 2. Maintenance: - Drugs: Isoflurane/Sevoflurane, NSAID. - Equipment: Capnograph, thermometer, heat mat. 3. Recovery: - Calm area, monitor pain/vitals. - Remove ET tube when reflex returns.
57
Describe the induction, maintenance & recovery of a dental in a fractious 10 year old Persian cat. With each of these events you consider the following categories: * Patient * Procedure * Drugs * Equipment / Staff
1. Induction: - Patient: Fractious, older. - Drugs: Dexmedetomidine + Buprenorphine, Ketamine/Alfaxalone. - Equipment: ET tube, BP/ECG monitors, IV catheter. 2. Maintenance: - Drugs: Isoflurane/Sevoflurane, dental blocks with local anaesthetic. - Equipment: Scaler, suction, heat support. 3. Recovery: - Quiet, warm area. - Reverse sedatives if needed. - Monitor until fully alert.
58
How long should you starve a rabbit before general anaesthesia?
Rabbits should not be starved before anaesthesia because they cannot vomit & prolonged fasting can lead to GI stasis
59
Why is reducing stress so important in rabbits?
Stress can cause increased catecholamine release, leading to cardiovascular instability, reduced gut motility, and increased anaesthetic risk
60
How will you ventilate a rabbit under anaesthesia?
Manual ventilation via a tight-fitting mask or endotracheal intubation; capnography should be used to monitor respiration
61
What equipment is used to monitor a rabbit under general anaesthesia?
Capnograph – Monitors CO₂ levels Pulse oximeter – Measures oxygen saturation ECG – Monitors heart rate & rhythm Doppler/oscillometric BP monitor – Measures BP Temperature probe – Prevents hypothermia
62
How will you prevent hypothermia in rabbits?
Use warming blankets Heat pads Warm IV fluids Minimize anaesthetic duration
63
How can you increase a rabbit’s blood pressure under anaesthesia?
Administer IV fluids Reduce anaesthetic depth Use vasopressors if needed
64
What drugs can be administered to ensure a quick recovery in rabbits? Give examples.
Reversal agents (e.g. atipamezole if medetomidine was used) Analgesics (e.g. buprenorphine, meloxicam) GI stimulants if needed (e.g., metoclopramide, cisapride)
65
How long should you starve a bird before anaesthesia?
2-4h, but avoid prolonged fasting to prevent hypoglycemia
66
How will you ventilate a bird under anaesthesia?
Positive pressure ventilation is recommended due to birds' lack of a diaphragm Intubation or air sac cannulation may be used
67
Why might a humeral fracture affect respiration in birds?
Humerus is connected to air sac system, so fractures can impair ventilation
68
How will you prevent hypothermia in birds?
Provide external heat sources (warm air, heating pads) Minimize anaesthetic duration Avoid wetting feathers excessively
69
How can you administer fluids to a bird under general anaesthetic?
IV fluids (jugular, ulnar vein) IO fluids (tibiotarsal bone) Subcut fluids for mild dehydration
70
How do you ventilate a tortoise under anaesthesia, and why is a ventilator needed?
Reptiles rely on active muscle contractions for breathing; positive pressure ventilation is needed as they do not breathe under deep anaesthesia
71
Why must a tortoise’s temperature be maintained near 30°C during anaesthesia?
Reptiles are ectothermic & low temperatures slow metabolism, affecting drug metabolism & anaesthetic recovery
72
What stimulates a reptile to breathe, and why is this important in GA recovery?
Hypoxia (low oxygen levels) rather than CO₂ levels This means that high O₂ levels post-GA can suppress breathing, so controlled oxygen weaning is essential

LEAH FCP (151 decks)

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