General anaesthetics

Question 1

What are the targets of GAs?

Answer 1

GABA and NMDA receptors

Question 2

What is the overall mechanisms of GAs?

Answer 2

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

Question 3

What are the 3 required components of general anaesthesia?

Answer 3

Unconsciousness
Analgesia
Muscle relaxation

Question 4

Why do we anaesthetise animals?

Answer 4

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

Question 5

How is GA achieved?

Answer 5

By using a combination of drugs:

• premedicant
• induction drug
• maintenance drug

Question 6

Define premedicant

Answer 6

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

Question 7

Define induction drugs

Answer 7

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

Question 8

Define maintenance drugs

Answer 8

• usually inhalational agents

- maintain anaesthetic state

Question 9

What are the stages of anaesthesia?

Answer 9

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

Question 10

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

Answer 10

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.

Question 11

What are IV anaesthetic agents typically used for?

Answer 11

• induction of anaesthesia

- occasionally to maintain anaesthesia

Question 12

What are the advantages to using IV agents?

Answer 12

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

Question 13

What are the disadvantages to IV induction?

Answer 13

Need IV access

Question 14

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

Answer 14

Total intravenous anaesthesia (TIVA)

Question 15

What are inhalational agents typically used for?

Answer 15

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

- occasionally to induce anaesthesia

Question 16

What are the advantages to inhalational agents for maintenace?

Answer 16

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

Question 17

What are the disadvantages to using inhalational agents for maintenance?

Answer 17

• equipment is required

- environmental pollution

Question 18

What are the advantages to using inhalational agents for induction?

Answer 18

IV access can be secured after induction

Question 19

What are the disadvantages to using inhalational agents for induction?

Answer 19

• envrionmental pollution

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

Question 20

Describe the pharmacokinetics of inhalational agents

Answer 20

• 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)

Question 21

Describe the pharmacokinetics of IV agents

Answer 21

• 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

Question 22

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

Answer 22

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

Question 23

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

Answer 23

• 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

Question 24

What are the properties of propofol?

Answer 24

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

Question 25

Describe the pharmacokinetics of propofol

Answer 25

- 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

Question 26

What are the clinical considerations when using propofol?

Answer 26

- pharmacological effect may be enhanced by hypoproteinaemia | - pharmacological effect is not prolonged if repeated doses are administered and in dogs with hepatic dysfunction

Question 27

What are the properties of alfaxalone?

Answer 27

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

Question 28

What are the properties of ketamine?

Answer 28

- 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

Question 29

What is total intravenous anaesthesia?

Answer 29

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

Question 30

What are the advantages to using TIVA?

Answer 30

- easy to administer - pharmacokinetics are known - inhalational agents may be unsuitable in some individuals - avoids risk to people administering drug

Question 31

Define minimun alveolar concentration (MAC)

Answer 31

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.

Question 32

Describe the properties of isoflurane

Answer 32

- 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

Question 33

What are the properties of sevoflurane?

Answer 33

- 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

Question 34

What are the properties of nitrous oxide?

Answer 34

- 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)