Premedication Drugs Flashcards
Purposes of premedication
- Sedation, analgesia
- Anesthetic sparing effect
- Reduction of stress and catecholamine release
- Redution of O2 demand
- Increased safety for animals and staff
- Decrease parasympathetic tone
Drugs classes used in premedication
- Anticholinergics
- Alpha 2 agonists
- Phenothiazines and butyrophenones
- Benzodiazepines
- Opioids
- Antihistamines and antibiotics
MOA Anticholinergics
Inhibit parasympathetic nervous system
Antagonize muscarinic acetylcholine receptors
Anticholinergic drugs
Atropine and glycopyrrolate
Atropine
Lipid soluble
Absorbed well IM/SC/PO
Crosses BBB and placenta
Dose: 0.01-0.04 mg/kg IV
Glycopyrrolate
Water soluble
Absorbed slowly IM/SC/PO
Does not cross BBB and placenta
Slower than atropine, half the dosage
Indications for Anticholinergics
Need for increasing the heart rate (opioid induced or reflex bradycardia)
Need for decreasing salivation and bronchial secretions
Contraindications for Anticholinergics
- Tachycardia
- Hyperthyroidism
- Most heart diseases
- Narrow angle glaucoma
Cardiovascular side effects of Anticholinergics
- Second degree AV block (bradycardia and cardiac arrect)
- Tachycardia
- Hypertension
What part of the heart do Anticholinergics effect?
SA node particularly
AV node is weakly effected
Medetomidine/Atropine disadvantages
Vasoconstriction, tachycardia, hypertension
Other effects of anticholinergics
- Relaxes lower esophageal sphincter
- Mydriasis
- Bronchodilation
- Dries airway secretions
- intestinal paralysis
- CNS toxicity
What drug treats anticholinergic CNS toxicity?
Physostigmine
Which Anticholinergic is preferred in rabbits?
Glycopyrrolate
Atropine is broken down too quickly
Which class has the strongest available sedative ability?
Alpha-2 Agonists
Do Alpha-2 agonists have specific antagonists?
Yes
MOA: Alpha-2 agonists
Competitive agonist of alpha-2 receptor (suppress NE release)
Location of alpha-2 receptors
- CNS presynaptic membrane
- Postsynaptic membrane
- Extra-synaptic sites
Alpha-2 agonist presynaptic effects
- Sedation
- Analgesia
- Reduction of sympathetic outflow from brain
- Reduction of stress response
Alpha-2 agonist postsynaptic effects
Vasoconstriction of arteries and veins
Alpha-2 agonist extra synaptic effects
- Inhibition of lipolysis
- Inhibition of insulin release- hyperglycemia
- Natural ligand with Epinepherine
- Interaction with stress response
Alpha-2 agonist CNS effects
- Species specific sedative (weak in pigs)
- Some analgesia- synergism with opioids
- Muscle relaxation
Alpha-2 agonist Cardiovascular effects
- Strong vasoconstriction- high SVR and BP
- Reflex bradycardia- low CO and tissue perfusion
- May develop hypotension
Should alpha-2 agonists be used with atropine?
No- reflex bradycardia does not need to be treated
Alpha-2 agonist Respiratory effects
- Mild respiratory depression (decreased RR)
- Increase in tidal volume
- Increased upper airway resistance
- V/Q mismatch in horses and ruminants- decreased PaO2
- Bronchoconstriction, lung edema, and hypoxemia in Ruminants
Which alpha-2 agonist is contraindicated in sheep?
Xylazine
Alpha-2 agonist GI effects
- Decreased salivation
- Decreased lower esophageal sphincter tone
- Decreased GI motility
- May induce vomiting- esp cats
Xylazine side effect in cattle
Uterine contractions and abortion
Alpha-2 agonist indications
- Sedation of aggressive animmals
- Sedation in the ICU
- Sedation to manage post-op airway obstruction
- Prevention/tx of seizures
Alpha-2 agonist contraindications
- Too young or too old
- Hemodynamic instability
- Severely debilitated patient
- Not suitable for most at risk patients
Alpha-2 agonist drugs
- Xylazine
- Medetomidine
- Dexmedetomidine
- Detomidine
- Romifidine
Alpha-2 antagonist drugs
- Atipemezole
- Yohimbine
- Tolazoline
Which alpha-2 antagonist should be used to counteract alpha-2 agonist drugs?
Atipemezole
Alpha-2 antagonist are always given what route?
IM
Emergency can be given IV
Which alpha-2 antagonist is the most specific?
Atipemezole
Xylazine dosages
Cattle ~10% of horses dose
Small ruminants between dog and horse dose
Xylazine duration
20-40min
Detomidine species used
Large animals
Detomidine route and duration
IM/IV/sublingual
90-120min
Romifidine species used
Horses
Romifidine route and duration
IM/IV
45-90min
Romifidine benefits
Produces less ataxia than xylazine and is preferred for standing procedures
Medetomidine dosing
USA- body surface
EU- body weight
Typically used in combination to reduce doses/adverse effects
Dexmedetomidine is ______ times as potent as medetomidine.
2x
Does dexmedetomidine have the same effects as medetomidine?
Yes
Medetomidine is a mixture of which two drugs?
Levomedetomidine and Dexmedetomidine
Phenothiazine drugs
Acepromazine, chlorpromazine, etc.
Which receptors do phenothiazines act on?
Dopamine, serotonine, alpha-1, histamine
Acepromazine duration
4-8 hours depending on dose
48 hours for liver patients
Phenothiazine CNS effects
- Sedation (weaker than apha-2 agonists)
- No analgesia
- Antiemetic
- Mild respiratory depression
Phenothiazine Cardiovascular effects
Vasodilation and hypotension (esp in hypovolemic animals)
Monitor bp very closely
Phenothiazine Other effects
- Antihistamine
- Antiarrhythmogenic
- Inhibit platelet function
- Penile prolapse in horses (not much of an issue but avoid anyway)
- Seizures (older drugs)
Phenothiazine indications
- Mild sedation for premed or post-op
- prevention/treatment of opioid dysphoria
- Prevention of emesis (morphine induced)
- Sedation for dogs with laryngeal paralysis
- Enhance xylazine in horses
Contraindications for phenothiazines
- Hypovolemia or hemodynamic instability
- Age extreme patients
- Von-Willebrand disease
- Boxers
- Breeding stallions
Butyrophenones are very similar to which other class?
Phenothiazines
Butyrophenone drugs
Droperidol and azaperone
T/F: Butyrophenones are more likely to cause behavioral side effects.
True
T/F: Butyrophenones are more likely to cause the same adverse effects as phenothiazines.
False, they cause less profound of the same side effects
Benzodiazapine MOA
GABA receptor agonists
Benzodiazapine effects
Sedative, anticonvulsant, muscle relaxant
Minimal CV and respiratory effects
No analgesia
Benzodiazapine drugs
Agonists- diazepam, midazolam, zolazepam
Antagonists- flumazenil, sarmazenil
Premedication mixtures with benzos
Opioids, alpha-2 agonists or both
Induction mixtures with benzos
Dissociative anesthetics, barbituates, propofol
Diazepam characteristics
- Lipid soluble
- Formulated in propylene glycol or lipid emulsion
- low chemical compatibility
Diazepam administration
Slowly IV
Poor absorption and pain on IM injection
Diazepam duration and metabolism
Metabolized in liver
1-4 hours
Midazolam characteristics
- Water soluble
- Good chemical compattibiliy
Midazolam differences from diazepam
- More potent
- Shorter acting
- Inactive metabolites
- Can be given IM/IV or MM
Opioid receptors
Mu: strong analgesia, respiratory depression, dependency
Kappa: weaker analgesia
Sigma: weaker analgesia
Location: brain, spinal cord, peripheral nerves, inflamed organs
Opioid classifications
Full agonists: activate receptors and trigger full tissue response
Partial agonists: activate receptors but do not trigger full tissue response even at high doses
Antagonists: bind to receptors but do not activate them
Concentration dependent
Opioid CNS effects
- Analgesia (acute pain)
- Decreasing MAC of inhalants (species dependent)
- Sedation (better with more pain)
- Excitation, dysphoria
- Tolerance
- Dependence
Opioid vomiting mechanisms
Stimulation of chemoreceptor trigger zone outside the BBB
After entering the brain they will inhibit the vomiting center
Water soluble > lipid soluble
Opioid cardiovascular effects
No direct negative inotropy or vasodilation
May reduce sympathetic outflow and indirectly reduce BP
Increase parasymathetic tone to decrease HR
Suitable risk for most patients, improves Cv function be allowing reduction in anesthetic doses
Opioid respiratory effects
Depression of respiration not as strongly as humans/primates
Antitussive effects, may inhibit airway protective reflexes
Opioid GI effects
Nausea, vomiting, defecation, obstipation, spasm of the sphincter or oddi
Opioids other effects
Hypothermia Post off hyperthermia Myosis in dogs, mydriasis in cats Inhibition of urination Noise sensitivity
Opioid indications
Premedication
Perioperative analgesia
Treatment of acute and chronic pain
Opioid administration
ALL THE WAYS
Morphine
- cheap and strong analgesic
- water soluble
- Slow onset, long duration
- High individual variety in elimination
- metabolized in liver
- may case histamine release esp in high doses
Hydromorphone, oxymorphone
- Strong analgesics (full mu)
- duration ~4hr
- Reliable metabolism
- better choice than morphine
Methadone
Similar to hydromorphone
Also acts as NMDA antagonist
Fentanyl
- Strong analgesic (full mu agonist)
- fast onset of duration
- may accumulate after long infusions
- bolus or CRI dosages
Remifentanil
- Similar to fentanyl with potentcy
- Short acting and does not accumulate
- disruption of administration may trigger strong pain
- boluses may cause sudden bradycardia
Butorphanol
- Partial antagonist on mu and kappa (mau antagonize full mu agonists)
- weak and short acting analgesic
- may worsen pain
- used for premed w/ benzodiazepines or alpha-2 agonist
Buprenorphine
- partial mu agonist
- stronger analgesic than butorphanol
- long acting slow onset
- often used in cats (less excitement and easy admin)
Tramadol
- weak analgesic
- inhibits NE and serotonin reuptake
- not scheduled
- can be given PO
Opioid antagonists
Naloxone: small animals to reverse respiratory depression
Naltrexxone: long acting, antagonize wild animals after long acting opioids
Antibiotics
- Cefazolin
- Amoxicillin clavulic acid
Give IV 20min pre-op and every 90-120min
Antihistamines
Diphenhydramine: H1 antagonist
H2 antagnoist: ranitidine, famotidine, cimetitide- increase pH of stomach
Given pre-men for mast cell tumor removal
