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Jorge FCP > Maintenance of anaesthesia > Flashcards

Maintenance of anaesthesia Flashcards

1
Q

What are the most commonly used inhalation agents?

A

Sevoflurane
Isoflurane

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2
Q

What are types of breathing systems that are no longer used?

A

Magill
To and Fro

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3
Q

Describe the features of isoflurane

A

Most commonly used agent currently
Lower solubility than Halothane
Different CV depression to Halothane
Studies have shown significantly fewer anaesthetic deaths

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4
Q

Describe the features of sevoflurane

A

Induction, recovery, and intraoperative modulation of anaesthetic depths, notably faster than halothane and isoflurane
More expensive
Not yet licensed in horses
induces dose-dependent cardiovascular depression to a degree similar to that of isoflurane

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5
Q

What are the MAC values of isoflurane?

A

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

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6
Q

What are the MAC values of sevoflurane?

A

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

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7
Q

What is the function of nitrous oxide (N2O) in veterinary?

A

Can be used to get a balanced anaesthesia
Allows you to lower inhalation agent

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8
Q

How are inhalation agents delivered?

A

Vaporised convert liquid agents (iso, sevo) into vapours to be delivered to the lungs

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9
Q

What can’t N2O be used to induce anaesthesia?

A

very high MAC > 100%

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10
Q

How does a vaporiser work?

A

Fresh Gas Entry:
- Fresh gas enters the inlet of the vaporizer.

Splitting of Gas:
-The gas is divided into two pathways:
a. Bypass Pathway: The fresh gas flows through this pathway without encountering any vapor.
b. Vaporizing Chamber Pathway: The fresh gas is directed to the vaporizing chamber.

Control by Splitting Valve:
- The splitting valve determines how much fresh gas goes through each pathway.
- The position of the control dial on the vaporizer adjusts this split.

Saturation with Vapor
- Fresh gas entering the vaporizing chamber becomes fully saturated with anesthetic vapor.

Mixing of Gases
- At the exit end of the vaporizer, the bypass gas (vapor-free) and the chamber gas (fully saturated with vapor) mix together.

Output Adjustment by Control Dial
- The amount of vapor in the final output depends on how much fresh gas was directed through each pathway.
- Higher Control Dial Setting: More fresh gas goes through the vaporizing chamber, resulting in a higher anesthetic concentration.
- Lower Control Dial Setting: Less fresh gas passes through the vaporizing chamber, resulting in a lower anesthetic concentration.

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11
Q

What is the functions of wicks in vaporisers?

A

The agent rises into the wicks, increasing the surface area of contact between the fresh carrier gas and the agent
Improves efficiency

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12
Q

Describe temp compensation in vaporisers

A

When anaesthetic liquid vaporises, it loses energy, causing its temperature to drop, which reduces vaporisation. Modern vaporisers address this with two key features:
- Metal Construction: The vaporiser is surrounded by metal, which acts as a heat conductor, storing and transferring heat to maintain a stable temperature.
- Temperature Compensating Valves: These valves automatically adjust the flow of fresh gas to ensure consistent vaporisation, regardless of temperature changes.

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13
Q

How are inhalation agents delivered?

A

Via ET tube => alveoli => arterial blood => brain (and other tissues) => venous blood => alveoli => exits

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14
Q

What factors affect the speed at which the agent conc rises in plasma?

A

Ventilation - high RR encourages exchange of gases quicker
High agent conc in alveoli
CO - high CO decreases rate of delivery

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15
Q

Why does it take longer for inhalation agents to take effect in fit animals?

A

Higher CO => clears agent out of system efficiently => requires more agent to anaesthetise

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16
Q

What is MAC?

A

Minimum alveolar concentration
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

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17
Q

What factors increase MAC?

A

Strong CO
Drugs causing CNA stimulation
Hyperthermia

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18
Q

What factors decrease MAC?

A

Old age
Disease
Analgesics
Hypothermia
Pregnancy
Other drugs

19
Q

What is MAC used for?

A

used to compare the strengths/potency of anaesthetic vapours
can also be used to contribute towards decision making during anaesthesis

20
Q

What is the blood:gas partition coefficient

A

The ration of the inhalation agent that stays in the blood vs the agent that stays in gaseous form close to the blood
Want it to be low (e.g., sevoflurane) - takes effect faster and faster recovery

21
Q

What does a high blood:gas partition coefficient mean?

A

High ratio means the agent is more soluble in blood - clings to proteins and travels with those rather than crossing tissues in gaseous form e,g, BBB

22
Q

Why would an overweight patient take longer to recover from anaesthesia?

A

Fat acts as a reservoir for anaesthetic agent (dependent on blood solubility)

23
Q

How are inhalation agents thought to work?

A

work within the CNS
augmenting signals to chloride channels (GABA receptors) and potassium channels while depressing neurotransmission pathways

24
Q

What are the negative effects of inhalation agents?

A

dose-dependent cardiovascular and respiratory depression

25
Q

How do inhalation agents cause cardiovascular depression?

A

By direct myocardial depression (negative inotropy)
By causing peripheral vasodilation
By decreasing vascular reactivity and impairing tissue autoregulation (tissue perfusion becomes more dependent upon the “driving” (systemic arterial Bp)
Via CNS depression and reducing autonomic tone

26
Q

How do inhalation agents cause respiratory depression?

A

By a depressed ventilatory response to CO2
By a depressed hypoxic pulmonary vasoconstriction
By almost abolishing the ventilatory response to hypoxia
By causing bronchodilation (which increases dead space)

27
Q

How can negative effects of inhalation agents be mitigated?

A

Balanced anaesthesia
COSHH
Scavenging
Calculating FGF correctly
Anaesthetic plans
TIVA/PIVA

28
Q

What are the risks of inhalation agents on us?

A

Increased risk of miscarriage
Bone marrow suppression and teratogensis (congenital foetal malformations) following long-term N2O exposure

29
Q

What are the ideal features of an inhalation agent?

A

Stable
Preservative free
Non-inflammable
Cheap
Ozone friendly
Non metabolised
Non-toxic
CVS effects free
Analgesic

30
Q

What is passive scavenging?

A

Fluosorber – (absorbs the inhalants, but not nitrous oxide)
Work by gravity (put them on the floor)
Has a weight limit

31
Q

What is VCT (volatile capture technology)?

A

Captures and absorbs anaesthetic agents that would otherwise be released into the environment.

32
Q

Describe the risk of inhalants used in equine practice

A

Horses are prone to hypotension when inhalants are used
Prolonged hypotension is likely to cause post op myopathy
Post op myopathy may necessitate euthanasia or cause a fracture or injury in recovery
Incidence and magnitude of hypotension is less with TIVA

33
Q

What is TIVA?

A

Total intravenous anaesthesia

34
Q

How is TIVA achieved?

A

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

35
Q

What are the advantages of TIVA?

A

Reduces the pollution hazard as environmental contamination is common with inhalation agents
Avoids repeated administration of toxic drugs
Quick induction and reversal along with superior recovery
Reduced postoperative nausea and vomiting
Easy to titrate drugs
Provide stable plane of anaesthesia and completely independent of patient’s airways and breathing system
No anaesthetic vaporiser is needed
Improved cerebral blood flow autoregulation for patients with brain diseases
Maintains hypoxic pulmonary vasoconstriction response
Not known to trigger malignant hyperthermia
Can be carried out “in the field”

36
Q

What are the disadvantages of TIVA?

A

Disadvantages of TIVA
Equipment needed, such syringe drivers or infusion pumps for accurate administration
Drugs will require metabolism for recovery from anaesthesia, which may be prolonged in animals with hepatic disease.
Can be expensive
Still produces unwanted side effects
Can be accumulative

37
Q

What are the desirable properties of TIVA drugs?

A
  • Rapid onset of action and smooth induction;
  • Short duration of action;
  • Rapid metabolism;
  • No active metabolites;
  • Rapid clearance from the body so accumulation does not occur;
  • Smooth, excitement-free recovery;
  • Little or no effect on cardiovascular parameters; and
  • Provides unconsciousness, muscle relaxation and analgesia.
38
Q

What is the goal of TIVA?

A

No inhalation agent! Produces a much-diminished anaesthesia stress response compared with inhalation agents and therefore, considered a physiologically superior method of anaesthesia.

39
Q

What is the TIVA Triple Drip used in Equine practice?

A

Detomidine
Guaifenesin
Ketamine

40
Q

What is PIVA?

A

partial intravenous anaesthesia

41
Q

What are the goals of PIVA?

A

Reduce MAC
Reduce cardiopulmonary depression
Provide additional analgesia
Contributes to balanced anaesthesia
Less pollution

42
Q

What drugs are used for PIVA?

A

Inhalant +
- Lidocaine (CRI)
- Ketamine (CRI or ‘top ups’)
- Alpha 2 agonist (CRI)
- Opioids

43
Q

What is used to ensure the wrong agent is not filled into a vaporiser

A

A colour coded key for the agent bottles

44
Q
A

Jorge FCP (60 decks)

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