Anaesthesia and premedication Flashcards
The principle of using multiple drugs to allow minimal doses is known as
Balanced anaesthesia
Describe the sequence of anaesthetic events
- Induction
- Maintenance
- Emergence/recovery
Why minimise drug doses?
To reduce the side effects of the drug and enable balanced anaesthesia
What is premedication?
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.
Why premedicate?
- For balanced anaesthesia
- To produce analgesia
- Sedation
- Anxiolytic/stress-reduction
- Agent-sparing effects
- Improved recovery
True/false: under GA, patients experience pain.
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.
True/false: evidence suggests that pre-op analgesia has the same effect as analgesia given during/after the op.
False
Pre-op analgesia is more effective!
Why is pre-op analgesia more effective than just post-op?
- 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
What is this graph showing us?
That noxious stimuli can sensitise the nervous system response to subsequent stimuli.
Therefore, pre-op analgesia is a good thing!
What does this figure show us?
The interventions that can modulate the activity in the pain pathway at different points.
Mode of action: opioids
- Affect 4 main types of receptor in the CNS and peripherally:
- Mu
- Kappa
- Delta
- Nociceptin
What can antagonise opioids?
Naloxone
Advantages of opioids
✅ 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
Disadvantages of opioids
❌ May see apnoea if given intra-op
❌ Reduce GI motility and gastric emptying
Examples of opioids
Bold = most effective
- Methadone
- Fentanyl
- Pethidine
- Morphine
- Buprenorphine
- Butorphanol
True/false: morphine is licensed in veterinary species.
False: morphine is not licensed in veterinary species.
Features of butorphanol
- Provides good sedation
- Provides poor & short-lived analgesia
- May be good for birds as they have a high number of kappa receptors
Effects of alpha-2 agonists
- 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❗️
What antagonises alpha-2 agonists?
- Atipamezole
This will antagonise sedative, drug-sparing and analgesic properties → consider what else you are prepared to give
Advantages of alpha-2 agonists
✅ 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
Disadvantages of alpha-2 agonists
❌ 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.
Examples of alpha-2 agonists
- Medetomidine
- Dexmedetomidine
- Xylazine (has a poor affinity for alpha-2 receptors in smallies, and causes high side effects)
- Detomidine
- Romifidine
Mode of action: Phenothiazines
- Centrally acting, antagonises dopamine receptors.
- Also acts on alpha-1, muscarinic and H1 receptors.
Characteristics and effects of phenothiazines
- Common premed component that might be combined with an opioid/alpha-2 agonist
- Contributes to sedation and tranquillisation
Advantages of phenothiazines
✅ Causes minimal respiratory depression
✅ Antiarrhythmic via alpha-1 antagonism in the heart and reduction in sympathetic tone
Clinical effects are still not really known.
Disadvantages of phenothiazines
❌ 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! ‼️
Examples of phenothiazines
Acepromazine (ACP) is the only one licensed for veterinary use.
Mode of action: Benzodiazepines
- Centrally acting via GABAA receptor
- Cause anxiolysis, hypnosis and sedation
What antagonises benzodiazepines?
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
Advantages of benzodiazepines
✅ 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
Disadvantages of benzodiazepines
❌ Poor degree of sedation in healthy animals, especially if given IV
❌ No analgesic properties
Examples of benzodiazepines
- 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
Examples of NMDA receptor antagonists
- Ketamine
- Methadone and pethidine (opioids) also demonstrate some NMDA receptor antagonism
Advantages of NMDA receptor antagonists
✅ 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
Disadvantages of NMDA receptor antagonists
❌ 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
Mode of action: anticholinergics
- Parasympatholytic/vagolytic: reduce the activity of the sympathetic nervous system
- Used to be used often, now not so much
Advantages of anticholinergics
✅ 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
Disadvantages of anticholinergics
❌ Cause mydriasis
❌ Can reduce gut motility
Examples of anticholinergics
- Atropine
- Glycopyrrolate
Mode of action: alfaxalone
- Neuroactive steroid
- Usually considered induction agent (IV), but can be used as a premed via IM route in sicker patients
Advantages of alfaxalone
✅ 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
Disadvantages of alfaxalone
❌ Apnoea can be seen when given IV
