Preanesthetic Medications Flashcards
Many drugs are _____
Receptor mediated
- either agonists or antagonists
Most drugs that produce CNS depression are either _____ at inhibitory sites, or ____ at excitatory sites
Agonists; antagonists
Desired effect of most anesthetic type drugs
CNS
- few drugs are 100% selective
- most have adverse effects
Premeds
Drugs given before induction of anesthesia
- individual drugs are rarely ideal
- combinations are common
Indications
- calm, sedate, chemical restraint
- analgesia
- decrease drug requirements
- offset adverse drug effects
- smooth induction/recovery
Factors in choosing premeds
- temperament
- desired effects
- undesirable drug effects
- pain involved
- health status
- type of practice
- personal preferences
Classes of drugs
- anticholinergics
- tranquilizers
- alpha-2-adrenergic agonists
- opioids
- neuroleptanalgesics
- dissociatives
Anticholinergic - mechanism
Competitive antagonism of AcH at parasympathetic muscarinic cholinergic receptors
Anticholinergic - effects
- increased heart rate
- decreased secretions
- decreased GI motility
Anticholinergic - indications
- prevent or treat vagally mediated bradycardia
- decrease salivary and/or respiratory secretions
Atropine and glycopyrrolate are ______
Anticholinergic agents
Atropine
Crosses BBB and placenta
- may be more likely to cause tachycardia
- duration 0.5-1.5 hrs
- hydrolyzed rapidly in some
- useful in emergencies
Atropine cannot be used in _________
Ruminants or rabbits due to rapid hydrolyzation
Glycopyrrolate
Does not cross BBB or placenta
- duration 2-4 hrs
- less tachycardia
- less effective
- more expensive
Avoid anticholinergic agents in:
- ruminants and horses due to effect on GI motility
- animals with tachyarrhythmias
- questionable in routine cases
Anticholinergic are parasymptholitic
Can’t use heartrate as an indicator of anesthetic depth
Tranquilizers
- do produce calming
- do not provide chemical restraint
- do not provide analgesia
Acepromazine - mechanisms
Central antagonism of D2 dopamine receptors and H1 histamine receptors
- peripheral alpha-1 adrenergic blockade
Acepromazine - indications
Calming, potentiate other drugs, smooth induction/recovery
Acepromazine - effects
Tranquilization and vascular relaxation
- slow onset, long duration
Acepromazine pros
- smooth recovery
- minimal respiratory depression
- combine with opioids for neuroleptanalgesia
Acepromazine is contraindicated in _______
Hypovolemic shock
- antagonism of peripheral alpha-1 receptor activity
- prevents vasoconstrictive response to shock
Acepromazine - side effects
Potential for hypotension
- penile prolapse in horses
- slow heart rate in boxers
Trazodone
Serotonin antagonist reuptake inhibitor
- potent antagonism of serotonin 2A receptors, weak inhibition of postsynaptic serotonin reuptake
- used in humans as an antidepressant, anxiolytic, sleep aid, anti-obsessive agent
- popular adjunct for long-term treatment of anxiety in dogs and short-term post-surgery anxiolysis
Benzodiazepines - mechanism
GABA agonists
Benzodiazepines - effects
- decreased anxiety
- euphoria/dysphoria!
- muscle relaxation
- anti-seizure activity
Benzodiazepines - indications
Potentiate other agents for smoother induction and recovery
- decrease doses of other agents
Benzodiazepines - side effects
- wide individual/species variation
- not great when used alone (euphoria)
- mild respiratory depression
- minimal CV effects
- antagonist (flumazenil)
- controlled substance (class 4)
Midazolam
Versed
- water soluble, suitable for IM, SQ intranasal, rectal
Diazepam
Valium
- propylene glycol carrier
- IV only
Alpha 2 adrenergic agonists
Dose-dependent analgesia and sedation
- analgesia –> direct activation of spinal A2 dorsal horn receptors
- sedation –> noradrenergic A2 receptors in brainstem
- vagomimetic
- peripheral vasoconstriction
Where are alpha-2 receptors found?
Everywhere!
- brain
- spinal cord
- GIT
- pancreas
- uterus
- blood vessels
Alpha-2 agonists - effects
- sedation
- analgesia
- relaxation
- CV and respiratory depression
- vasoconstriction
- hyper, then hypotension
- diuresis
- hyperglycemia
- vomiting
Alpha-2 agonists - indications
Combined with other agents for short-term anesthesia or chemical restraint
- sedation/analgesia
- anesthetic premedication
Alpha-2 agonists - pros
- excellent analgesia
- sedation, NOT anesthesia!
- not DEA controlled
- antagonists are available
Alpha-2 agonists - side effects
- vomiting in small animals
- small animals may bite
- horses may kick
- ruminants: very sensitive (use lower dose)
- swine: very insensitive
Alpha-2 agonists as an abortigenic
Only xylazine in cattle
- 3rd trimester
- profound CV and respiratory depression
- bradycardia and secondary AV block
- vasoconstriction
Xylazine, detomidine, dexmedetomidine, romifidine are all ______
Alpha-2 agonists
Xylazine
Commonly used
- large and small animals
- most popular restraint drug in horses and ruminants
Dexmedetomidine
150x more potent than xylazine
- approved only for healthy dogs
- profound sedative effects
- with ketamine for injectable anesthesia
Detomidine and romifidine
Used in horses
- longer lasting than xylazine
- more potent (use less)
Which drugs reverse all effects of alpha-2 agonists?
Yohimbine, tolazoline, atipamezole
- causes hypertension, tachycardia
Opioids - mechanism
Inhibition of nociceptive information pathways
Opioids - effects
Analgesia, respiratory depression (increase CO2), bradycardia, sedation (dogs, primates), euphoria (cats, horses), vomiting
Opioids - indications
Analgesia, sedation, potentiate other agents, smooth induction and recovery
Opioids - pros
- minimal CV effects
- wide margin of safety
- useful in most species
- antagonists are available
Opioids - species indications
- sedation: dogs, primates
- analgesia
- excitation: likely in cats, horses at doses higher than used clinically
Opioids - side effects
- dose dependent respiratory depression
- bradycardia
- vomiting
- euphoria, dysphoria
- dogs pant excessively
- most are DEA scheduled
Classes of opioids
- full agonists
- partial agonists
- kappa agonists/mu antagonists
- antagonists
Full (mu) agonists
- morphine
- hydromorphone
- oxymorphone
- fentanyl
- methadone
- buprenorphine (partial)
Kappa agonist/mu antagonist
- butorphanol
- nalbuphine
Butorphanol
- short duration (45-60 min)
- minimal to moderate analgesia –> poor choice for post op pain control
- minimal sedation
- expensive
- DEA class 4
Naloxone
Opioid antagonist
- reverses ALL opioid effects
- can cause tachycardia, hypertension, pulmonary edema
- short duration of action
Naltrexone
Potent, long lasting opioid antagonist
- used in zoo and wild animal practice
Neuroleptanalgesics
Opioid and tranquilizer or sedative –> greater sedation and analgesia
- numerous combinations
- reduce dose requirements
Neuroleptanalgesia
Consider instead of single drugs
- fewer adverse effects, better restraint
- combo of tranquilizer or sedative and analgesic drug to achieve heavy sedation and analgesia
Dissociative anesthetics - mechanism
Interference with transmission between conscious and subconscious areas of brain
- antagonism of excitatory amino acid neurotransmitters
- N-methyl-D-aspartate, glutamate
Dissociative anesthesia - effects
Interruption of neurotransmission between thalamocortical and limbic areas
- muscle tone and eye reflexes remain
- ketamine, telazol
- maintenance of some motor, most autonomic tone
Dissociatives - indications
Chemical restraint or injectable anesthesia
Dissociatives - side effects
- poor muscle relaxation
- sympathomimetic actions (increase hr)
- poor recovery quality
- respiratory depression
- renal excretion in cats (dont give to renal failure patients!!)
- hepatic and renal in other species
Ketamine
Widely used
- wide margin of safety
- use in almost any animal species
- best in combos with tranquilizer, opioid, alpha-2 agonist
- DEA class 3
Telazol
Dissociative plus benzodiazepine
- tiletamine/zolazepam
- more potent than ketamine –> use less
- DEA class 3
Other premed drugs
- NSAIDS
- antihistamines
- neurosteroids
Aflaxan
Modulates GABA receptors
- chemically identical to progesterone, modified to not produce sex or glucocorticoid effects
