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Module 9: Respiration > General anaesthetics > Flashcards

General anaesthetics Flashcards

1
Q

What are the targets of GAs?

A

GABA and NMDA receptors

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2
Q

What is the overall mechanisms of GAs?

A

Decrease in neurotransmission in the CNS, leading to loss of consciousness

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3
Q

What are the 3 required components of general anaesthesia?

A

Unconsciousness
Analgesia
Muscle relaxation

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4
Q

Why do we anaesthetise animals?

A

To perform painful surgical or diagnostic procedures
To minimise patient suffering
To reduce risk to vet adn other individuals
To facilitate the procedure by immobilising the patient

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5
Q

How is GA achieved?

A

By using a combination of drugs:

  • premedicant
  • induction drug
  • maintenance drug
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6
Q

Define premedicant

A
  • drugs given prior to GA
  • typically a sedative-opiod combo
  • purpose is to calm and sedate the animal
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7
Q

Define induction drugs

A
  • usually IV
  • used to achieve transition from consciousness to unconsciousness
  • done rapidly
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8
Q

Define maintenance drugs

A
  • usually inhalational agents

- maintain anaesthetic state

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9
Q

What are the stages of anaesthesia?

A

Stage 1 - voluntary movement. GA inhibit inhibitory pathways causing initial voluntary movement
Stage 2 - involuntary movement or excitement
Stage 3 - surgical anaesthesia (divided into 4 stages)
Stage 4 - medullary paralysis

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10
Q

What is a characteristic of GAs which enable them to get to their site of action quickly?

A

High lipid solubility. Can readily cross cell barriers, and willl distribute faster around the body and have a faster effect on the CNS. Brain blood perfusion is v high and has a high lipid content which means get a rapid rise in concentration of the drug.

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11
Q

What are IV anaesthetic agents typically used for?

A
  • induction of anaesthesia

- occasionally to maintain anaesthesia

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12
Q

What are the advantages to using IV agents?

A
  • rapid, smooth induction (avoid stage 1 and 2)
  • rapid protection of the airway (intubate rapidly)
  • no environmental pollution
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13
Q

What are the disadvantages to IV induction?

A

Need IV access

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14
Q

What is the method by which IV agents can be used to maintain anaesthesia?

A

Total intravenous anaesthesia (TIVA)

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15
Q

What are inhalational agents typically used for?

A
  • maintenance of anaesthesia (once patient is in stage 3)

- occasionally to induce anaesthesia

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16
Q

What are the advantages to inhalational agents for maintenace?

A
  • delivery/elimination depends on ventilation (patient rapidly wakes up)
  • rapid adjustment of anaesthetic depth (breath by breath adjustment)
17
Q

What are the disadvantages to using inhalational agents for maintenance?

A
  • equipment is required

- environmental pollution

18
Q

What are the advantages to using inhalational agents for induction?

A

IV access can be secured after induction

19
Q

What are the disadvantages to using inhalational agents for induction?

A
  • envrionmental pollution

- takes longer and delay in securing the airways may be a problem in some cases

20
Q

Describe the pharmacokinetics of inhalational agents

A
  • Blood:gas coefficient (low gives rapid induction/recovery)
  • oil:gas coefficient (high gives high potency)
  • depends on alveolar ventilation rate and cardiac output
  • metabolism (liver, extenten depends on agent, potential toxic metabolites)
  • elimination (primarily by exhalation, determines duration of action)
21
Q

Describe the pharmacokinetics of IV agents

A
  • Redistribution - drug will redistribute throughout the body; as it equilibrates will get high level settlign in fat stores. As drug distributes away from CNS patient will begin to wake up
  • Metabolism - lipid soluble drugs aren’t easily excreted. Rate of metabolism is vital for determining whether a drug is suitable for TIVA
22
Q

What are the pharmacodynamic effects of GAs on the nervous system?

A
  • inhibits conduction of action potentials
  • inhibits transmission at synapses (low doses during induction)
  • reticular formation adn hippocampus are susceptible to action of GAs
23
Q

What are the pharmacodynamic effects of GAs on the CVS and respiration?

A
  • decreased contractility of isolated heart preparations
  • effects cardiac output and blood pressure
  • potentially arrhythmogenic (take care if administering catecholamines to counteract this)
  • decreased respiration
  • increased arterial CO2 tension
24
Q

What are the properties of propofol?

A
  • hindered phenol
  • oil at room temp. (administered in an emulsion)
  • enhances GABA transmission
  • short acting, smooth rapid recovery
25
Q

Describe the pharmacokinetics of propofol

A
  • highly plasma protein bound
  • large volume of distribution (>3L/kg)
  • phase II metabolsim - conjugaed in the liver prior to excretion in urine
  • rapidly cleared (>40ml/kg/min)
  • suitable for TIVA
26
Q

What are the clinical considerations when using propofol?

A
  • pharmacological effect may be enhanced by hypoproteinaemia

- pharmacological effect is not prolonged if repeated doses are administered and in dogs with hepatic dysfunction

27
Q

What are the properties of alfaxalone?

A
  • steroid anaesthetic
  • insoluble in water, presented in cyclodextrin vehicle
  • enhances inhibitory action of GABA
  • high therapeutic index
  • rapid induction
  • rapid metabolism
  • suitable for TIVA
28
Q

What are the properties of ketamine?

A
  • interrupts the association between the limbic and cortical regions (inhibits NMDA receptors)
  • dissociative agents produce a different qulaity of anaesthesia
  • sensory loss with analgesia
  • increased muscle tone
  • eyes open +/- slow nystagmus
  • active reflexes include laryngeal/pharyngeal
  • less profound CVS and resp depression
  • hallucinations/emergence delerium
  • commonly given with a sedative to offset other effects
29
Q

What is total intravenous anaesthesia?

A

Anaesthesia maintained by intermittent boluses or continuous infusion of an IV agent

30
Q

What are the advantages to using TIVA?

A
  • easy to administer
  • pharmacokinetics are known
  • inhalational agents may be unsuitable in some individuals
  • avoids risk to people administering drug
31
Q

Define minimun alveolar concentration (MAC)

A

The minimum alveolar concentration at which 50% of patients will not respond to a particular stimulus. Compares potency of different inhalational anaesthetics. Lower the MAC, the more potent the drug.

32
Q

Describe the properties of isoflurane

A
  • licensed in non-food producing animals
  • good speed of induction/recovery/change of depth
  • pungent odour (breath hold)
  • minimal metabolism
  • cardiac output is better maintained than haloflurane
  • less arrhythmogenic
  • most widely used inhalation agent
33
Q

What are the properties of sevoflurane?

A
  • licensed in dogs
  • rapid induction/recovery/change of anaesthetic depth
  • pleasant odour and minimal airway irritation so is suitable for induction
  • low rate of metabolism
34
Q

What are the properties of nitrous oxide?

A
  • colourless gas, no tase or odour
  • stored under pressure in containers
  • can’t use as sole agent as can’t get full anaesthesia with it on own and couldn’t give O2 with it (MAC >100%)
  • used as an adjunct
  • diffusional properties (helps to concentrate the effect of volatile agent as is v rapidly taken into body)
  • displaces N2 in body (is larger so will cause gas filled cavities to expand)

Module 9: Respiration (11 decks)

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