Veterinary anaesthetic drugs and their uses Flashcards
What is balanced anaesthesia?
The term given to drug protocols designed to address each of the patient’s needs during each phase of anaesthesia. It focuses on a multimodal approach - smaller volumes of multiple drugs rather than large volumes of a few.
What are examples of routes of administration for anaesthetic drugs?
Subcutaneous, intravenous, intramuscular
What is multimodal anaesthesia?
The reliance of several smaller volumes of multiple anaesthetic agents rather than on one large volume of a single agent. This minimises the side effects of large doses of a single agent.
Define ANAESTHESIA
A state in which the animal is unconscious and unaware of its environment.
What makes up the triad of anaesthesia?
Hypnosis/amnesia, immobility/muscle relaxation/analgesia - all together combine to create reflex suppression
What are examples of types of anaesthesia
General, local
Which organs are most affected by anaesthetic drugs?
The liver, heart, brain and kidneys - this is because they receive the biggest blood supply
What is the blood brain barrier?
The barrier between the main bloodstream and the brain to protect it from harmful or unwanted substances in the circulatory system - it is semipermeable but highly selective and made up of endothelial cells.
The more lipid soluble a substance is, the faster it can cross the BBB.
What are examples of commonly used groups of injectable anaesthetics?
Barbituates, ketamine, phenols, steroid anaesthetics
What are barbituates?
A group of injectable anaesthetics that are known as ‘sedative anaesthetics’. They are fat soluble and fast acting, but have poor analgesic properties. They have a long duration of action but should only be given IV, due to their alkalinity causing tissue necrosis.
What is an example of a barbituate?
Thiopentone sodium (Thiovet) and Pentobarbital (no longer used for anaesthesia)
What are dissociative anaesthetics?
Agents which are used to achieve profound analgesia with a light plane of anaesthesia. They allow some maintenance of reflexes: the airway reflexes remain, the eyes stay open and the pupil dilated. Can affect recovery and cause euphoria or dysphoria.
What is an example of a dissociative anaesthetic?
Ketamine - it is an NMDA agonist, meaning it allows the action of NMDA, a glutamate receptor and one of the brain’s primary excitatory neurotransmitters.
What are the common effects of ketamine?
- is a good analgesic for muscle and bone
- patient’s eyes remain open
- pupils dilate
- muscle tone in the throat is retained (cannot be intubated)
- muscle fasciculations
- euphoric recovery
- increased blood flow
- apneustic breathing pattern
- muscle rigidity, tachycardia and mild respiratory depression
- can cause seizures if given alone to dogs
- cumulative effect
What is distinct about animals that are given ketamine when under its influence?
They retain their reflexes, and cannot be intubated
What patients is ketamine contraindicated for?
Head trauma patients, and dogs if not given alongside another anaesthetic agent (such as medetomidine or diazepam)
Define MYDRIASIS
Pupil dilation
Define APNEUISTIC
An abnormal breathing pattern that involves long gasping inhalations and insufficient, irregular exhalations.
What Schedule is ketamine classed as?
Schedule 2, as classified by the Misuse of Drugs Regulations 2001.
What is an example of a phenol anaesthetic agent?
Propofol
How long do the effects of propofol typically last for?
Usually between 5-8 minutes. It can affect the patient in as little as 6 seconds
What are some features of propofol?
- is a white emulsion, not water soluble, so cannot be diluted
- very lipophilic
- given IV, to effect. Should be given incrementally to prevent post induction apnea
- can be given as a CRI or part of a TIVA protocol (although no TIVA for cats)
- can cause severe respiratory and cardiac depression
- must be partnered with inhalational anaesthesia or continually topped up
Give an example of a steroid anaesthetic agent?
Alfaxalone (Alfaxan)
What are some features of Alfaxalone?
- can be used as an induction agent or as a sole anaesthetic
- should not be used in conjunction with other IV anaesthetics
- can result in disturbed recovery (paddling, muscle twitching etc)
- poor levels of analgesia
- slow wake up times
- no antagonist, so cannot be reversed
What is the wake up time for alfaxalone for cats?
45 minutes
What is the wake up time for alfaxalone for dogs?
25 minutes
What is local anaesthesia?
A type of anaesthesia used to interrupt signals from a specific nerve to the brain. It is typically used to prevent pain signals reaching the brain but may also affect the motor nerves and impact function.
What are some examples of local anaesthetic agents?
Lidocaine
Bupivacaine
Ropivacaine
What is the onset of action time for lidocaine?
1-2 minutes
What is the onset of action time for bupivacaine?
20 minutes
What is the onset of action time for ropivacaine?
20 minutes
What is the duration of action for lidocaine?
1 hour
What is the duration of action for bupivacaine?
6-8 hours
What is the duration of action for ropivacaine?
4-6 hours
What two groups can local anaesthetics be divided into?
The amide group and the ester group. They differ structurally
What is the amide group in terms of anaesthetics?
They are a type of local anaesthetic agent that undergo hepatic and lung metabolism.
What is the ester group in terms of anaesthetics?
They are a type of local anaesthetic broken down in the blood by enzymes
What are examples of anaesthetics within the amide group?
Lidocaine, bupivacaine and ropivacaine
What are examples of anaesthetics within the ester group?
Procaine
In what ways can local anaesthetics be administered?
Topically; onto skin, the larynx etc
Intradermal; line block during surgery
Peri-neural; nerve blocks or regional blocks
Epidural; spinal block, given around L7 to S1
What is the risk for overdose of local anaesthetics?
Can cause central nervous system and cardiac complication. Signs include drowsiness, tremors and seizures, arrythmias, hypotension and bradycardia. They may also cause irritation if injected into skeletal muscle and can cause nerve damage if not injected into the nerve sheath
What are inhalational anaesthetics?
A type of anaesthesia agent used to maintain unconsciousness in an animal, may be a gas or volatile liquid
What are some examples of inhalational anaesthesia?
Isoflurane, sevoflurane, halothane, nitrous oxide
What are the advantages of isoflurane?
- cleared quickly with a fast onset of action
- delivered with a carrier gas
- large range of administration rates, and rate can be easily changed
- patient is intubated during use, so there is full control over the patient’s airway
What are the disadvantages of isoflurane?
- metabolised by the liver - cumulative effects can damage
- can damage the kidneys
- dangerous to inhale, especially for pregnant people
- high cost machine
- cannot be used as an induction due to stress
- comes with risk of hypothermia due to use of oxygen
In what form does nitrous oxide come?
It is a vapour at room temperature
What is the use of nitrous oxide?
Used as a carrier gas; never as a standalone anaesthetic
What are the advantages of using nitrous oxide?
- provides good levels of analgesia
- reduces the need for isoflurane due to the ‘secondary gas effect’
- has some anaesthetic effects - good for multimodal anaesthesia
- does not suppress the cardiac or respiratory system
In what patients should use of nitrous oxide be avoided?
Any patient with disrupted oxygen intake due to closed space pooling of nitrous and poor oxygen circulation across the alveolar membrane. Particular examples include those with a pneumothorax or GDV
What colour cannister is nitrous oxide stored in?
Blue
What is a significant risk of using nitrous oxide?
As nitrous is much denser, it can take up the space required by oxygen, and dilute the concentration of oxygen in the alveoli, causing diffusion hypoxia
Which is heavier, nitrous oxide or oxygen?
Nitrous oxide
Define PHARMACOKINETICS
It is the affect of the body on a drug
What are the two features that should be known about inhalation anaesthetics?
Its mean alveolar concentration, and its blood/gas solubility coefficient
What is blood/gas solubility?
It describes the solubility of agents in the blood. If a substance is more soluble, it has a faster onset
What is minimum alveolar concentration?
It is a measure of potency. It is the concentration of inhaled anaesthetic within the alveoli at which 50% of people do not move in response to a surgical stimulus.
What is the b/g Co of isoflurane, sevoflurane and halothane? What do these mean?
Isoflurane - 1.39
Sevoflurane - 0.6
Halothane - 2.4
The higher the blood:gas coefficient, the slower the rate of induction.
What is the MAC of isoflurane, sevoflurane, and halothane?
Isoflurane - 1.3
Sevoflurane - 2.0
Halothane - 0.8
What is the ideal situation for inhalational anaesthetics?
The concentration of volatile gas in the alveoli mirror concentrations in the brain
What are common features of isoflurane?
A volatile liquid with a pungent smell - it is an irritant and unsuitable for mask induction. Is a myocardial depressant and causes dose dependent hypotension (RR and HR increase in response). Mostly excreted by the lungs, minimal effect on liver blood flow
What are common features of sevoflurane?
Suitable for mask induction as less of an irritant. Clear, colourless, odourless, non-flammable liquid. Can cause rapid induction, but cardiopulmonary effects similar to isoflurane. Rapidly excreted by the lungs with little effect on liver metabolism
What is a premedication?
Drugs that are used to calm a patient before the induction of general anaesthesia, making handling easier and safer
What are the benefits of giving a premedication?
- reduces catecholamine release
- relieves pain
- helps to achieve balanced anaesthesia - synergistic approach
- can reduce the negative side effects of other drugs
- calms the patient to aid a smooth induction
Define CATECHOLAMINES
A group of neurotransmitters that send signals throughout the nervous system. They make GA much riskier, as their presence can counteract the effect of anaesthetic drugs
Define a SEDATIVE
A type of anaesthetic drug that depresses activity of the central nervous system, reduces anxiety and induces sleep.
An example of a sedative are the benzodiazepines
Define a NEUROLEPT
A type of anaesthetic drug that sedates or tranquilises, tending to reduce nervous tension by depressing nerve functions
Define a TRANQUILISER
A type of anaesthetic drug that calms and relieves anxiety. Increasing potency leads to increased drowsiness. Most tranquilisers are potentially addictive
Define a HYPNOTIC
A class of psychoactive drugs whose primary function is to induce sleep or for surgical anaesthesia. This group is related to sedatives
Define a NARCOTIC
A type of anaesthetic drug that at moderate doses dulls the senses, relieves pain and induces profound sleep, but in excessive doses causes stupor, come or convulsions. Examples include opium and morphine
What are phenothiazines?
A class of anaesthetic agent that are antagonists at the central dopamine receptors, meaning they block dopamine. They are neurolepts, and cause nervous depression
Give an example of a phenothiazine anaesthetic agent
Acetylpromazine (ACP)
What is the affect of ACP?
Produces moderate sedation; higher doses do not increase sedation but increase length of action. Combined with an opioid to achieve neuroleptanalgesia
What are some advantages of using ACP?
- has antihistimine properties, can help reduce MCT
- decreases emotional activity
- is an anti emetic
- provides sedation
- spasmolytic
- has wide therapeutic index
What are some disadvantages of using ACP?
- can be unsafe for brachycephalics as they tend to react badly
- causes hypotension
- unsafe for epileptic patients as it reduces the threshold for seizures
- must be given with atropine
- has a long onset of action (around 45 minutes)
- sedation level cannot be increased, only length of time - ceiling effect
- can cause hypothermia
- can reduce tear production, especially in cats
In what types of patients should ACP NOT be used?
- hypotension animals, especially hypotensive cats
- brachycephalics, especially boxers
- epileptic patients
What is an alpha 2 adrenergic receptor agonist?
A type of anaesthetic agent that causes a decreases in the level of norepinephrine released in the brain, resulting in sedation and analgesia. They are potent sedatives that cause major cardiovascular and respiratory side effects
What is an example of an alpha 2 adrenergic receptor agonist?
Medetomidine and xylazine
What is the effect of alpha 2 adrenergic receptor agonists?
Potent sedatives, but with major cardiovascular and respiratory side effects.
In what patients should alpha 2 adrenergic receptor agonists be avoided?
In dogs with cardiac disease, respiratory disorders, liver or kidney disease, animals in shock, animals that are severely debilitated, animals that are stressed due to extreme heat, cold, or fatigue
What are some advantages of alpha 2 adrenergic receptor agonists?
- fast acting, depending on route of administration
- synergistic, often combined with an opioid such as methadone
- reversible, with atipamezole
- high levels of analgesia, until reversal given
- good muscle relaxant, can be paired with ketamine
What are some disadvantages of alpha 2 adrenergic receptor agonists?
- respiratory and cardiac depressant
- animals easily roused when used alone for sedation
- affects natural thermoregulation
- can cause vomiting, following injection and possibly recovery
- can cause vasoconstriction
What is an example of an alpha 2 adrenergic receptor antagonist?
Atipamezole, this reverses alpha 2 adrenergic receptor agonists and all its effects
How long after administering medetomidine can atipamezole be given?
15-60 minutes
What is a benzodiazipine?
A type of anaesthetic agent which have a calming effect; it enhances the inhibitory action of GABA. They are muscle relaxants, and anticonvulsants
What are some examples of benzodiazipines?
Diazepam and midazolam
What are some advantages of benzodiazipines?
- have no effect on the cardiovascular system
- can stimulate appetite
- can be used in young animals
- are reversible with flumazenil
- midazolam has higher amnesic effects than other benzodiazepines
What is the effect of a benzodiazipine?
Muscle relaxant, tranquiliser, anticonvulsant
What are some disadvantages of benzodiazepines?
- painful to inject
- can be unpredictable and cause voluntary excitement, which can result in aggression
- affects inhibition
- may relax respiratory muscles
What are opioids?
Potent analgesics that also provide sedation. Often included in a premed routine
What are some examples of opioids?
Methadone, morphine, pethidine, buprenorphine
What are the effects of opioids?
- analgesic effects
- sedation
- synergistic and can be paired with others
- provide neurolept analgesia when combined with ACP
- antitussives (specifically butorphanol)
- can cause nausea and vomiting
What schedule are most opioids under the Misuse of Drugs Act 1971?
Schedule 2, with the exception of buprenorphine which is schedule 3
What are some benefits of using an opioid?
Provide good levels of analgesia, sedation and synergy
What is buprenorphine?
A partial agonist opioid. It has a ceiling effect and its result is not increased by increased administration. A very potent analgesic. Should not be given with opioids due to competition for receptor sites
What is the duration of action of buprenorphine?
About 4-6 hours
What is the duration of action of methadone?
4-6 hours in dogs
What is the duration of action of pethidine?
1-1.5 hours
What is the duration of action of fentanyl?
15-20 minutes
What is the duration of action of butorphanol?
1-2 hours for analgesia
4 hours for sedation
What is neuroleptanalgesia?
Combines a neurolept sedative with an opioid
What are the advantages of neuroleptanalgesia?
- increased analgesia
- increased sedation
- decreased side effects
- no affect on cardiac output
What are the disadvantages of neuroleptanalgesia?
- causes vasodilation
- lack of muscle relaxation
- action of the neurolept much longer than the opioid
- can cause hypotension
- reduced threshold for seizures, especially if the animal is prone to
What is cyclo-oxygenase 1?
An enzyme that produces prostaglandins that promote inflammation, pain and fever. Also produce prostaglandins that protect renal haemodynamics, the stomach, platelet function and blood vessel constriction/dilation
What is cyclo-oxygenase 2?
An enzyme that also produces prostaglandins that promote inflammation, pain and fever.
What do cyclo-oxygenase 1 enzymes produce?
Prostaglandins that promote inflammation, pain and fever. Also produce prostaglandins that protect renal haemodynamics, the stomach, platelet function and blood vessel constriction/dilation
What do cyclo-oxygenase 2 enzymes produce?
An enzyme that also produces prostaglandins that promote inflammation, pain and fever.
What are NSAIDs generally indicated for?
They block the action of COX enzymes and therefore the production of inflammation, fever, pain etc. Often used for analgesia for a range of conditions
What are some adverse effects of NSAID use?
GI effects - vomiting and diarrhoea
Renal impact, especially for long term use
What is the use and effect of gabapentin?
Used for neuropathic pain, light sedation
What is the use and effect of tramadol?
Used as an adjunctive analgesic for chronic pain
What drug reverses the effects of opioids?
Nalaxone
What receptor sites does methadone effect?
Mu (pure) agonist
What receptor sites does morphine effect?
Mu (pure) agonist
What receptor sites does pethidine effect?
Mu (pure) agonist
What receptor sites does buprenorphine effect?
Mu (partial) agonist
What receptor sites does fentanyl effect?
Mu (pure) agonist
What receptor sites does butorphanol effect?
Kappa agonist and Mu antagonist
What receptor sites does ketamine effect?
NMDA agonist
What receptor sites does acepromazine effect?
Dopamine antagonist
What receptor sites does medetomidine effect?
Alpha 2 adrenoreceptor agonist
What receptor sites does atipamezole effect?
Alpha 2 adrenoreceptor antagonist
How long do alpha 2 adrenergic receptor agonists take to take effect when given IV?
3-5 minutes
How long do alpha 2 adrenergic receptor agonists take to take effect when given IM?
10-20 minutes
How long do alpha 2 adrenergic receptor agonists take to take effect when given SC?
Variable; lasts 60-90 minutes
