Anaesthesia and premedication

Question 1

The principle of using multiple drugs to allow minimal doses is known as

Answer 1

Balanced anaesthesia

Question 2

Describe the sequence of anaesthetic events

Answer 2

1. Induction
2. Maintenance
3. Emergence/recovery

Question 3

Why minimise drug doses?

Answer 3

To reduce the side effects of the drug and enable balanced anaesthesia

Question 4

What is premedication?

Answer 4

A combination of drugs or single agent given prior to anaesthesia. This is a component of most anaesthetics.

Triple/quad protocols in cats are an exception to this.

Question 5

Why premedicate?

Answer 5

• For balanced anaesthesia
• To produce analgesia
• Sedation
• Anxiolytic/stress-reduction
• Agent-sparing effects
• Improved recovery

Question 6

True/false: under GA, patients experience pain.

Answer 6

False: under GA patients don’t experience pain, but they do experience nociception.

Response to this can make patients harder to manage under the anaesthetic.

Question 7

True/false: evidence suggests that pre-op analgesia has the same effect as analgesia given during/after the op.

Answer 7

False

Pre-op analgesia is more effective!

Question 8

Why is pre-op analgesia more effective than just post-op?

Answer 8

• Preventative analgesia
  
  • Decreases the pain after tissue injury
  • Prevent pathological modulation of the CNS (the development of hyperalgesia)
  • Prevents the development of a chronic pain state, which in turn:
    
    • Leads to increased cortisol, pro-inflammatory cytokines and slower wound healing
    • Increases the likelihood of the patient interfering with the wound

Question 9

What is this graph showing us?

Answer 9

That noxious stimuli can sensitise the nervous system response to subsequent stimuli.

Therefore, pre-op analgesia is a good thing!

Question 10

What does this figure show us?

Answer 10

The interventions that can modulate the activity in the pain pathway at different points.

Question 11

Mode of action: opioids

Answer 11

• Affect 4 main types of receptor in the CNS and peripherally:
  
  • Mu
  • Kappa
  • Delta
  • Nociceptin

Question 12

What can antagonise opioids?

Answer 12

Naloxone

Question 13

Advantages of opioids

Answer 13

✅ Provide sedation and analgesia

✅ Minimal CV depression → can be used in v sick patients

✅ Respiratory depression usually minimal; may be seen with fentanyl/high doses of methadone

Question 14

Disadvantages of opioids

Answer 14

❌ May see apnoea if given intra-op

❌ Reduce GI motility and gastric emptying

Question 15

Examples of opioids

Answer 15

Bold = most effective

• Methadone
• Fentanyl
• Pethidine
• Morphine
• Buprenorphine
• Butorphanol

Question 16

True/false: morphine is licensed in veterinary species.

Answer 16

False: morphine is not licensed in veterinary species.

Question 17

Features of butorphanol

Answer 17

• Provides good sedation
• Provides poor & short-lived analgesia
• May be good for birds as they have a high number of kappa receptors

Question 18

Effects of alpha-2 agonists

Answer 18

• Cause profound, dose-dependent sedation
• Biphasic CV effects
  
  • Initial vasoconstriction resulting in increased BP, then reflex bradycardia and reduction in cardiac output
  • After 15-20 mins, BP and HR return to normal or just below
  • Some patients may not be able to cope with these effects❗️

Question 19

What antagonises alpha-2 agonists?

Answer 19

• Atipamezole

This will antagonise sedative, drug-sparing and analgesic properties → consider what else you are prepared to give

Question 20

Advantages of alpha-2 agonists

Answer 20

✅ Useful in lively/aggressive animals at high doses

✅ Drug sparing: reduce amount of induction and inhalational agent required

✅ Provide good analgesia as there are alpha-2 receptors in the spinal cord. However, this is short-lived.

✅ Minimal respiratory depression

Question 21

Disadvantages of alpha-2 agonists

Answer 21

❌ Significant CV effects! Not suitable for sick patients/patients with CV disease.

❌ Increase the time taken for injectable anaesthetic to reach the CNS and brain. Inject slowly!

❌ Reduce blood flow to liver and therefore hepatic metabolism of drugs. Consider esp. in patients with liver disease.

❌ High degree of sedation: take care with brachycephalic dogs as there is a danger of URT obstruction.

❌ Can cause emesis esp. in cats

❌ Reduce endogenous insulin secretion → can cause transient hyperglycemia

❌ Cause increased urine secretion due to reduced renin and vasopressin secretion; consider in blocked cats, or the standing horse/cow who could slip.

Question 22

Examples of alpha-2 agonists

Answer 22

• Medetomidine
• Dexmedetomidine
• Xylazine (has a poor affinity for alpha-2 receptors in smallies, and causes high side effects)
• Detomidine
• Romifidine

Question 23

Mode of action: Phenothiazines

Answer 23

• Centrally acting, antagonises dopamine receptors.
• Also acts on alpha-1, muscarinic and H1 receptors.

Question 24

Characteristics and effects of phenothiazines

Answer 24

• Common premed component that might be combined with an opioid/alpha-2 agonist
• Contributes to sedation and tranquillisation

Question 25

Advantages of phenothiazines

Answer 25

✅ Causes minimal respiratory depression ✅ Antiarrhythmic via alpha-1 antagonism in the heart and reduction in sympathetic tone Clinical effects are still not really known.

Question 26

Disadvantages of phenothiazines

Answer 26

❌ No analgesic properties ❌ Takes 30-40 mins to take effect when given IM ❌ Lasts 6-8 hrs ❌ Profound vasodilation mediated by alpha-1 → can result in hypotension and hypothermia ❌ Might not be suitable for patients with sepsis, hypovolaemia or CV issues ❌ Some breeds are more sensitive than others: can get syncope (Boxers), susceptibility to CV effects (giant breeds). Duration of action may be prolonged in sighthounds → potentially consider lower doses ❌ Cannot be antagonised! ‼️

Question 27

Examples of phenothiazines

Answer 27

Acepromazine (ACP) is the only one licensed for veterinary use.

Question 28

Mode of action: **Benzodiazepines**

Answer 28

* Centrally acting via GABA_A receptor * Cause anxiolysis, hypnosis and sedation

Question 29

What antagonises benzodiazepines?

Answer 29

Flumazenil * This is via competitive inhibition and is short-lived * May not be stocked in practice as is expensive; uncommon to have in first opinion

Question 30

Advantages of benzodiazepines

Answer 30

✅ Minimal CV and respiratory depression → good for the compromised patient ✅ Drug sparing: reduce amount of induction and inhalational agent needed ✅ Contribute to muscle relaxation ✅ Cause amnesia in people; this might also be the case for veterinary patients ✅ Very good anticonvulsants

Question 31

Disadvantages of benzodiazepines

Answer 31

❌ Poor degree of sedation in healthy animals, especially if given IV ❌ No analgesic properties

Question 32

Examples of benzodiazepines

Answer 32

* Diazepam * Licensed in dogs and cats * Can only be given IV as painful and poorly absorbed via IM * Midazolam * Licensed in horses, commonly used in smallies * Can be given IM and IV

Question 33

Examples of NMDA receptor antagonists

Answer 33

* Ketamine * Methadone and pethidine (opioids) also demonstrate some NMDA receptor antagonism

Question 34

Advantages of NMDA receptor antagonists

Answer 34

✅ At high doses → dissociative anaesthesia = results in sensory blockade ✅ At lower doses, results in sedation ✅ At even lower doses, can provide analgesia ✅ Profoundly reduces dose of induce/inhalational agent required. Can serve as anaesthetic agent at high doses. ✅ Generally good CV stability

Question 35

Disadvantages of NMDA receptor antagonists

Answer 35

❌ Can be stimulatory to the CV system via noradrenaline release. This causes a transient increase in CO and BP. * Therefore unsuitable for some patients e.g. myocardial problems as may not be able to tolerate this. ❌ Variable dose-dependent respiratory depression. Not usually a problem at the doses we use. ❌ Short-acting: lasts 30-40 mins ❌ Very poor muscle relaxant, can lead to twitchiness ❌ Dries out corneas → must lubricate eyes ❌ Some patients who receive higher doses can become excitable and react to light/noise upon recovery

Question 36

Mode of action: **anticholinergics**

Answer 36

* Parasympatholytic/vagolytic: reduce the activity of the sympathetic nervous system * Used to be used often, now not so much

Question 37

Advantages of anticholinergics

Answer 37

✅ Reduce bradycardia: sometimes used intra-op or in premed if this is a concern ✅ Can be used to reduce respiratory tract secretions e.g. brachycephalic undergoing respiratory tract surgery

Question 38

Disadvantages of anticholinergics

Answer 38

❌ Cause mydriasis ❌ Can reduce gut motility

Question 39

Examples of anticholinergics

Answer 39

* Atropine * Glycopyrrolate

Question 40

Mode of action: **alfaxalone**

Answer 40

* Neuroactive steroid * Usually considered induction agent (IV), but can be used as a premed via IM route in sicker patients

Question 41

Advantages of alfaxalone

Answer 41

✅ Can be used in a patient that has rebuffed their premed i.e. significant sedation has not been achieved ✅ Few adverse CV effects May see mild vasodilation when given IV, not so much IM

Question 42

Disadvantages of alfaxalone

Answer 42

❌ Apnoea can be seen when given IV