Premedication Flashcards
List the groups of agents than can be used for premedication
- Alpha 2 agonists
- Phenothiazines
- Benzodiazepines
- Anticholinergics
What alpha-2 agonists are used for premedication in horses?
- Xyalzine
- Detomidine
- ROmifidine
What alpha-2 agonists are used for premedication in cats and dogs?
- Medetomidine
- Dexmedetomodine
- (replaced xylazine)
What alpha-2 agonists are used for premedication in cattle?
Xylazine and detomidine
Compare the use of xylazine and dexmedetomidine
- Xylazine unselective for a2 receptors, and have agonist effect at a1 receptors
- Reduced cardiovascular safety vs dexmedetomidine
What is the name of the licensed phenothiazine?
Acepromazine
Describe the use of acepromazine for premedication
- Used in combination with an opioid for their analgesic effects
- Called neuroleptanalgesia
- Leave animal undisturbed for 30-40 mins after administration
Compare the sedation of acepromazine and dexmedetomidine
Acepromazine provides less reliable sedation vs dexmedetomidine
Name the benzodiazepines used in premedication
- Midazolam
- Diazepam
Are benzodiazepines licensed for use as predmedication in the UK?
No
What are the benefits of benzodiazepines as premedication?
- Used alone produce minimal or no sedation in healthy cats and dogs
- Better for sick animals
What is a potential effect of benzodiazepines on healthy patients?
Excitation due to loss of learned “inhibitory” behaviour
Describe the usage of benzodiazepines for premedication
- Given in combination with other sedatives
- Often opioids
- Ketamine in compromised cats
What are the different formulations of benzodiazepines?
- Combined with propylene glycol
- Water soluble
- Milky
Why are benzodiazepines commonly given with opioids?
Both are cardiovascularly stable and in combination can provide reliable sedation
Name the anticholinergic premedicants
- Atropine
- Glycopyrronium
Describe the use of anticholinergics as premedicants
- Routine premedication with anticholinergics is unnecessary
- Used in past with irritant inhalants to reduce bronchial secretions and opioids to offset bracycardia
- Do not use with alpha 2 agonists to offset bradycardia
Describe the mechanism of action of alpha-2 agonists
- Act on a2 and a1 adrenoceptors
- At a2 receptors, bind to presynaptic receptors to have negative feedback on norepinephrine release
Describe the effects of premedication with alpha2 agonists
- Profound sedation (dose related)
- Synergistic with opioids or benzodiazepines
- Good analgesia through effects at spinal cord a2 receptors
- Improved intra-op analgesia
- Dramatically reduces dose of induction and maintenance agents required
- Biphasic effect on blood pressure (initial increase, followed by return to normal or slightly below)
- Heart rate decreased
- Reduction in cardiac output but not oxygen delivery
- Minimal effects on respiratory system in healthy animal
- Increased urine production
- Peripheral vasoconstriction
Describe the administration of induction agents following alpha-2 agonists as premedication
- IV
- Must be given slowly to effect
- Vein to brain circulation time is slowed due to lowered heart rate and cardiac output
Why do alpha2 agonists increase urine production?
Reduce vasopressin and renin secretion
How is a2 sedation and analgesia reversed?
- Administration of atipamezole (specific a2 adrenergic receptor antagonist)
- IM provides smooth recovery
- IV produces rapid, excitable recovery
- Ensure analgesia is supplemented with different classes of drugs
Describe the side effects of alpha2 agonists
- Body wide side effects
- Bradycardia
- Changes in blood pressure
- Increased urine production may be dangerous in anuria and distended bladder
- Mucus membranes blue-grey in colour
- Endogenous insuline secretion reduced leading to transient hyperglycaemia
- Liver blood flow and rate of metabolism of other drugs metabolised by liver reduced
- Small ruminants sensitive to a2 agonists
What is the benefit of reversal of alpha2 agonists?
Recovery period is noted to be a high risk time for anaesthetic complications
Describe the mechanism of action of phenothiazines
- Antagonism of a1 adrenoceptors
- Dopamine antagonist
Describe the effects of phenothiazines
- Long lasting and non-reversible
- Anti-arrhythmic
- Sedation and anziolysis (dose dependent)
- Larger doses have prolonged action
- NO analgesia
Describe the side effects of phenothiazines
- Peripheral vasodilation and fall in arterial blood pressure
- Reduction in body temp due to resetting of thermoregulatory mechanism and vasodilation
- Giant dogs more sensitive to effects
- Some boxer dogs very sensitive, low dose recommended
- Penile priapism in stallions
What are the contraindications for phenothiazines?
- Avoid in animals with marked cardiovascular disease, significant haemorrhage or animals in shock
- Hypotensive animals
- Breeding stallions
How can penile priapism following use of phenothiazines be treated?
Compression, slow IV administration of benztropine mesylate
What are phenothiazines commonly given in combination with and why?
- Opioids
- Improve quality and reliability of sedation
Describe the mechanism of action of benzodiazepines
Act on specific receptor site on GABA
Describe the effects of premedication with benzodiazepines
- Minor effects on cardiorespiratory system so can be used in animals with CV compromise
- Commonly used to manage convulsions (esp for animals presenting in status elipticus)
Describe the mechanism of action of anticholinergics
Parasympatholytics that block acetylcholine
Compare atropine and glycopyrronium
- Glycopyrronium does not cross BBB, slower onset
- Less wide eyed side effects vs those seen with atropine
Describe the side effects of anticholinergics as premedication
- Exacerbate bradycardia before HR increases
- Can thicken saliva and cause blockages (cats, ruminants esp.)
- Inhibit peristalsis (colic in horses)
- Bloat in ruminants
- Mydriasis, corneal drying, bronchodilation
Describe the routes of administration of alpha-2 agonist premedications
- Injection
- Oral also common in horses
What are the routes of administration of phenothiazine premedicants?
- Injection
- Gel
- Tablets
What are the routes of administration of benzodiazepines?
- Popylene glycol stings, IM administration
- Water soluble formulations can be IV or IM
- Milky formulations given IV (does not cause thrombophlebitis)
- Rectal diazepam in seizures
- IM diazepam in foals
What are the aims of premedication prior to balanced anaesthesia?
- Sedation and anxiolysis facilitating handling of animal
- Reduction of stress for the animal
- Reduction in amount of other anaesthetic needed
- Balanced anaesthesia faciliated
- Provision of pre-emptive analgesia
- Counter effects of other anaesthetic agent to be administered during procedure e.g. atropine to prevent opioid mediated bracycardia
- Contribute to smooth recovery
Describe the desirable properties of drugs used for premedication
- Reliable sedation and anxiolysis
- Minimal effects on CV system
- Minimal respiratory depression
- Provide analgesia
- Be reversible in order to hasten recovery
Describe the administration of any pre-medication drug
- Administer premed and allow to take effect, then induce anaesthesia
- In cats can administer premed and induction agent IM in one syringe
What is contained in a “Quad”?
- Ketamine
- Benzodiazepines
- Alpha2 agonist
- Opioid
What are some general considerations regarding premedication?
- Combinations depend on status of patient, action of drugs, and purpose
- Administer premed at time optimal for induction of anaesthesia and surgery
- IM administration over subcut
- Dose based on lean bodyweight rather than obese
- Record dose on chart
- Reduce dose in giant breed dogs to allow for effects of metabolic body size
Outline the factors that affect the choice of premedication
- Reasons for anaesthesia/sedation
- Duration of sedation required
- Procedure to be carried out
- Degree of pain expected from procedure
- Species and breed of patient
- Age of patient (lower alpha2agonist dose in geriatrics)
- Temperament of patient
- Condition and ASA classification of patient
List the common premedications used in dogs and cats
- ACP and opioid
- A2A and opioid
- A2A and benzodiazepine
- A2A with ketamine
- BZP with ketamine (cats)
- Opioid with BZP (cats)
- A2a and BZP and opioid
Compare the common premedications used in ASA1 dogs (ACP or dex/medetomidine)
- ACP: less reliable sedation, add opioid, no analgesia, vasodilation and hypertension, minimal respiratory effects, minimal drug sparing effects, not reversible
- Dex/med: potent sedation, potent analgesia, reduced cardiac output, narroer safety margin, only in normal CV patients, minimal respiratory effect, potent drug sparing effect, reversible
Outline the premedication of ASA2 dogs
- ACP and opioid combination most common
- May use dex/medetomidine depending on reason for ASA2 classification
Outline the premedication of ASA3 dogs
- ACP and opioid, depending on reason for ASA3 (vasodilation by ACP)
- Benzodiazepine and opioid (cardiovascular friendly, useful for cardiovascular disease or systemic abnormalities)
Outline the premedication of ASA 4 dogs
- Benzodiazepine and opioid
- May use dexmedetomidine post-op to reduce dysphoria, keep sedated with midazolam and methadone
Outline the premedication of ASA5 dogs
- Low doses or opioids or benzodiazepines
- Distinct premedication phase not usually required in very sick patients
- Co-induction technique combining midazolam with propofol or alfaxalone or ketamine (reduces dose of induction agent, improving cardiovascular stability)
- For icterus, liver failure, biopsy and euthanasia use fentanyl, and alfaxan for exploratory laparotomy
Outline the premedication of ASA1 cats
- Profound sedation by dexmedetomidine can be advantageous
- IV access after light sedation (may be difficult with ACP)
- Dex reversible
Outline the premedication of ASA2 cats
- ACP and opioid
- May be appropriate to use dexmedetomidine depending on reason for ASA2 classification
Outline the premedication of ASA3 cats
- ACP and opioid depending on evaluation of cardiovascular system
- Benzodiazpine and ketamine (higher dose of ket 10mg/kg IM will induce anaesthesia)
- When sedation/premedication required, lower dose of ketamine usually adequate
Outline the premedication of ASA4 cats
- Benzodiazpine and ketamine
- Opioid alone (cats with quiet temperament, e.g. buprenorphine, morphine/methadone)
- Combination of BZP and opioid does not usually result in reliable sedation (very profound)
Outline the premedication of ASA5 cats
- Low dose benzodiazepines and opioid around time of induction of anaesthesia contributes to balanced anaesthesia
- Co-induction as for dogs may also be appropriate
