Maintenance of anaesthesia Flashcards
What are the most common inhalation agents?
Sevoflurane & Isoflurane
(Halothane was old version)
Describe isoflurane
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)
What is Minimum Alveolar Concentration (MAC), and why is it important?
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
Describe Sevoflurane
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
Describe nitrous oxide (N2O)
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.
What are the health & safety concerns with Nitrous Oxide?
Risk of abuse in humans
Long-term exposure can cause:
- Bone marrow suppression.
- Potential carcinogenic effects.
- Contributes to environmental pollution.
How do vaporisers work?
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.
What is the difference between a gas & vapour?
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
What is the purpose of a vaporiser?
vaporiser adds anaesthetic vapour to fresh gas flow, ensuring output delivers set concentration of anaesthetic agent accurately
How does fresh gas flow through a vaporiser?
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
What controls the concentration of vapour output from a vaporiser?
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
What is the function of a wick in a vaporiser?
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
What happens to the temperature of an anaesthetic agent during vaporisation?
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
How do modern vaporisers manage temperature compensation?
Modern vaporisers include:
- Metal casings: Act as heat conductors, retainers & donors to maintain consistent liquid temperature
- Temperature compensating valves: Automatically adjust fresh gas flow to maintain correct vapour output
Why is temperature compensation important in vaporisers?
Prevents drop in vapour output caused by cooling of liquid during vaporisation
Ensures consistent anaesthetic concentration is delivered to patient
How does anaesthesia ‘hijack’ the physiological process of gas exchange?
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
What is the pathway of anaesthetic gas from inhalation to their effect?
- Inspired gas mixture enters alveoli
- Anaesthetic agent crosses alveolar membrane into arterial blood system
- Blood transports agent to tissues, including brain, where it induces anaesthesia
- Constant exhalation removes agent, helping maintain steady anaesthetic plane
What factors influence how quickly the anaesthetic agent enters the blood?
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
How can MAC vary between patients?
Species-specific: Each species has different MAC values
Individual-specific: Factors like age, health & premedication can affect MAC
What factors can increase MAC?
Drug causing CNS stimulation (e.g. ephedrine)
Hyperthermia
Strong cardiac output (e.g. young/fit animal)
What factors can decrease MAC?
Premedication (e.g., opioids, alpha-2 agonists, benzodiazepines)
Hypothermia
Age (geriatrics)
Pregnancy
Severe illness or hypotension
Why do premedicated patients typically require less anaesthetic agent than the MAC value suggests?
Premedication drugs reduce brain’s sensitivity to stimuli, lowering required concentration of anaesthetic agents to maintain anaesthesia
What does blood:gas partition coefficient represent?
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
How does the blood:gas partition coefficient affect anaesthetic onset & recovery?
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.
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
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
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)
What are the cardiovascular effects of inhalation agents?
Dose-dependent cardiovascular depression due to:
- Decreased myocardial contractility (negative inotropy).
- Peripheral vasodilation, reducing systemic vascular resistance.
- Decreased arterial blood pressure.
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
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
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
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
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
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
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.
How does TIVA compare to inhalants for equine anaesthesia in terms of blood pressure?
TIVA causes less incidence & magnitude of hypotension compared to inhalants
What is TIVA?
Total intravenous anaesthesia
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.”
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
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
What are the 2 ways in which TIVA can be achieved?
- Top-up bolus injections (propofol, Alfaxalone, ketamine)
- Continuous rate infusions (propofol, Alfaxalone, “triple drip” in equine anaesthesia – useful for field anaesthesia)
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
What is Triple drip?
TIVA protocol commonly used for equine anaesthesia
Involves combination of 3 drugs that provide sedation, muscle relaxation & anaesthesia
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
What are the goals of PIVA?
Reduce MAC
Reduce cardiopulmonary depression
Provide additional analgesia
Contribute to balanced anaesthesia
Less polution
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)
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
Isoflurane & Sevoflurane
Colour coded key for agent bottles
- Iso is purple & Sevo is yellow
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.
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.
Sevoflurane
All of the above
Passive
A drop in blood pressure due to peripheral vasodilation
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
- Induction:
- Patient: Healthy, bouncy.
- Drugs: Acepromazine + opioid, Propofol/Alfaxalone.
- Equipment: IV catheter, ET tube, monitoring tools.
- Maintenance:
- Drugs: Isoflurane/Sevoflurane, NSAID.
- Equipment: Capnograph, thermometer, heat mat.
- Recovery:
- Calm area, monitor pain/vitals.
- Remove ET tube when reflex returns.
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
- Induction:
- Patient: Fractious, older.
- Drugs: Dexmedetomidine + Buprenorphine, Ketamine/Alfaxalone.
- Equipment: ET tube, BP/ECG monitors, IV catheter.
- Maintenance:
- Drugs: Isoflurane/Sevoflurane, dental blocks with local anaesthetic.
- Equipment: Scaler, suction, heat support.
- Recovery:
- Quiet, warm area.
- Reverse sedatives if needed.
- Monitor until fully alert.
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
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
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
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
How will you prevent hypothermia in rabbits?
Use warming blankets
Heat pads
Warm IV fluids
Minimize anaesthetic duration
How can you increase a rabbit’s blood pressure under anaesthesia?
Administer IV fluids
Reduce anaesthetic depth
Use vasopressors if needed
What drugs can be administered to ensure a quick recovery in rabbits?
Reversal agents (e.g. atipamezole if medetomidine was used)
Analgesics (e.g. buprenorphine, meloxicam)
GI stimulants if needed (e.g., metoclopramide, cisapride)
How long should you starve a bird before anaesthesia?
2-4h, but avoid prolonged fasting to prevent hypoglycemia
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
Why might a humeral fracture affect respiration in birds?
Humerus is connected to air sac system, so fractures can impair ventilation
How will you prevent hypothermia in birds?
Provide external heat sources (warm air, heating pads)
Minimize anaesthetic duration
Avoid wetting feathers excessively
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
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
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
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
