Anesthesia

Sedation and Premedication

Goals of Sedation

Facilitate handling and positioning

• Increases comfort of patient
• Increases comfort and safety of handler

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Ultimate goal is to accomplish something:

• Exam, diagnostics, short procedure

Goals of Premedication

Is in preparation for subsequent general anesthesia

• Sedation and pre-emptive analgesia
• Proceeding to induction in a timely fashion
• Airway established
• To allow complete immobilization –major procedure/surgery

1. Safe handling – reduced stress

2. Provide analgesia preemptively

3. Balanced general anesthetic approach

• Lowers the dose of anesthetic induction drugs
• Smooth induction and endotracheal intubation
• Lowers the dose of inhalational anesthetics

Patient considerations

• Why are you doing the sedation?
• Duration, position, pain level, level of sedation required
• General health of the patient
• Cardiovascular, respiratory, liver, renal, endocrine, CNS, etc
• Ability to handle the animal before the sedation
• Necessity of the procedure to happen that day
• Last time animal has eaten

Route of administration

Most commonly dictated by two things

• Ease of patient handling
• Routes of administration available for each drug

For sedation most commonly considering either IV or IM

• Don’t forget oral for some medications that we don’t often think of

Pre-visit Pharmaceuticals (PVPs)

Gabapentin

Trazodone

Acepromazine

Dexmedetomidine (Sileo)

Commonly Used Sedatives in Small Animals

• Don’t forget orals!
• Phenothiazines - i.e. acepromazine
• Alpha-2 agonists - i.e. dexmedetomidine
• Opioids
• Benzodiazepines
• NMDA antagonists - i.e. ketamine
• Propofol
• Alfaxalone
• Anticholinergics

Phenothiazines - i.e. acepromazine

Mechanism of action

• Blocks dopamine receptors in the brain which is needed for wakefulness and motor activity

Dose - labeled dose is WAYYYY HIGHER than what I would typically use

• usually going to use 0.005-0.02 mg/kg

Uses - A major tranquillizer – reduces fear and anxiety; Is a sedative

• additional affects - antiemetic, may relax the urethra, is antiarrhythmic
• My main use of acepromazine is for post-operative dysphoria and anxious animals in hospital

Phenothiazines - i.e. acepromazine

Side effects - HYPOtension

• Will severely enlarge the spleen as well - not a problem but consider this when interpreting diagnostics

Contraindications

• Shock, weakness, dehydration, severely debilitated, major trauma to the head, advanced kidney disease, liver, or CV disease
• Is metabolized by the liver 🡪 no administration if severe liver disease

Good combo drugs - can go well with anything, but I don’t use it often

• If doing something painful requires an additional drug for pain management as acepromazine does nothing for pain!

Reversal - none

Alpha-2 agonists - i.e. dexmedetomidine

Mechanism of action - alpha-2 agonism increases inhibition in brain - leads to sedation

• lowering CNS levels of epinephrine and norepinephrine
• This allows arousal out of sedation if they are excited or stressed and so should be combined with other drugs if used in a very worked up animal

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Dose - is technically dosed by mg/m2 not mg/kg (i.e. use lower doses in bigger dogs)

• Dose choice depends on level of sedation, size of animal, combination of other drugs, how excitable the animal is, concurrent conditions, planned procedure, route of administration (generally higher IM/SQ, highest ™)
• Typically in dogs 3-20 mcg/kg
• Cats typically 5-15 mcg/kg
• Can be used as a CRI - dose of CRI generally low

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Uses

• All manners of sedation

Alpha-2 agonists - i.e. dexmedetomidine

Side effects

• Will raise the blood pressure due to peripheral alpha-1 and 2 stimulation
• Will be an initial raise and will then decrease to a more normal pressure
• Bradyarrhythmias
• Low heart rate but normal rhythm to AV block
• Other effects – vomiting, increased urine volume, change in uterine activity, decreased intestinal motility

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Contraindications

• Heart diseases impacted by increases in systemic vascular resistance (safe in cats with HOCM, but should have a diagnosis prior to making that call)
• Reduction in endogenous insulin release causing transient hyperglycemia and so avoid in diabetic patients
• Also contraindicated in cases with urinary obstruction as it increases urine volume

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Good combo drugs - goes well with anything

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Reversal - alpha-2 antagonist - atipamezole - equal volume as administered dexmedetomidine is a full reversal - IM only!!, only give IV in an emergency, is proarrhythmic

• Can give partial amounts to try and maintain sedation but decrease side effects

Alpha-2 agonists - i.e. dexmedetomidine

The heart rate is low, what do I do?

• First answer these questions:
• How low is low? Where did this patient start? What’s the blood pressure? Is it having an arrhythmia? Where are you at in the procedure/what are you doing next?
• If you decide it is time to do something about it:
• Reversal with atipamezole - IM if not an emergency, IV if an emergency
• Do not give atropine until reversal is given - will force the heart to work too hard and can cause myocardial ischemia
• If not an emergency and don’t want to reverse, can try lidocaine 1-2 mg/kg IV to treat AV block induced by dexmedetomidine - DO NOT DO THIS FOR ANY OTHER AV BLOCK, IT WILL MAKE IT WORSE/IS CONTRAINDICATED

Benzodiazepines - i.e. diazepam/midazolam

Mechanism of action - GABA agonist

Dose - 0.1-0.5

Uses - mild sedation, great for coinduction, muscle relaxation, anticonvulsant, appetite stimulant (cats)

• Midazolam can be used IM - don’t forget this option! CANNOT DO THIS WITH DIAZEPAM

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Side effects - no real severe side effects; can cause paradoxical excitement, particularly in young healthy animals

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Contraindications - liver dysfunction; glaucoma?

• metabolized solely by the liver, midazolam is a little better tolerated as the metabolites of midazolam are inactive while diazepams are active

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Good combo drugs - anything! My general use is coinduction with propofol/alfaxalone; classic ket/val

Reversal - flumazenil

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NMDA antagonists - i.e. ketamine

Mechanism of action - NMDA antagonist

Dose - high in Plumbs - practical doses - 0.5 mg/kg for analgesia as a bolus, or use CRI, higher doses 1-2 mg/kg for sedation

Uses - is a dissociative anesthetic

• Dream-like state of pseudo-unconsciousness - Eyes open, swallow reflex intact, can hear normally, intense muscle rigidity, hallucinogenic, disconnected from surroundings and pain
• Can spray this into the mouth of an aggressive animal - mix with dexmedetomidine

NMDA antagonists - i.e. ketamine

Side effects

• Generally minimal at low doses
• Sympathomimetic - increases heart rate, blood pressure, sympathetic nervous system stimulation
• If critically ill, cannot mount the sympathetic response - therefore the body does not show the above, instead you will see the opposite - bradycardia, hypotension, depression
• Muscle spasticity

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Contraindications

• Increased intraocular pressure, shouldn’t use it alone for anything, heart disease, hypertension, uncontrolled hyperthyroidism, careful with hepatic and renal dysfunction

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Good combo drugs

• Pretty well anything
• recommend adding muscle relaxant when administering to help with rigidity - i.e. benzodiazepine

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Reversal - None

Opioids - General

Mechanism of action - agonists or antagonists of the opioid receptors

• Mu (most important for pain), Kappa, Delta

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Dose

• Varies based on potency of the opioid you are planning on using

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Uses - almost any procedure has a use for an opioid

• Analgesia
• Sedation
• Euphoria
• Antitussive
• Antidiarrheal

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Good combo drugs - everything!

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Reversal - Naloxone

Opioids - General

Side effects

• Dysphoria - can happen with large doses of opioids, relatively common following anesthesia
• Can be treated with either sedatives - I commonly reach for acepromazine or very low dose dexmedetomidine
• Can be treated with reversal - either naloxone, or if a full mu you are trying to reverse, can use butorphanol (ideal if you don’t want to take away all of your pain management)
• Cardiorespiratory and other effects
• Increase vagal tone – reduced heart rate that is most pronounced with mu agonists that is exacerbated by inhalant anesthetic
• Can sometimes see an AV block, particularly with boluses of fentanyl
• Can prevent with anticholinergic - theoretically, but I have not experienced that in practice
• Does not cause clinically significant respiratory depression at the doses they should be used
• Only worry in sick animals when combined with other drugs
• This is what happens with an opioid overdose
• Gastrointestinal
• Increases segmentation, but decreases propulsion 🡪 leads to constipation
• Increases bile duct sphincter constriction
• Stimulation of the chemoreceptor trigger zone will induce vomiting
• Hyperthermia in cats

Opioids - Butorphanol

Mechanism of action - functional mu antagonist, kappa agonist

Dose - 0.1-0.5 mg/kg (I generally use 0.1-0.2)

Uses - sedation, antitussive, visceral pain via CRI?

• Great for non-painful procedures/diagnostics
• Great for sedation for respiratory cases

Opioids - Buprenorphine

Mechanism of action - partial mu agonist, kappa antagonist

Dose - dogs 0.01-0.04; cats 0.01-0.03 (may want higher like 0.05 for OTM)

• Cats SQ Simbadol - 0.12-0.24 mg/kg SQ for the 24 hr dose

Uses - pain management for medium pain procedures/trauma

• Do not administer if planning on doing surgery on a patient
• Due to the partial mu agonism and a very high binding affinity it will block other full mu agonists

Opioids - Hydromorphone

Mechanism of action - full mu agonist

Dose - 0.05-0.2 (I generally don’t go above 0.1 - I see more dysphoria, panting, etc., sedation if using it in an awake dog for pain)

Uses - Pain management for invasive procedures/trauma, sedation for painful procedures

Side effects - more vomiting and panting than other opioids

Opioids - Methadone

Mechanism of action - full mu agonist, NMDA antagonist

Dose - 0.2-0.5 (I like 0.2, find it is effective and well tolerated)

Uses - Pain management for invasive procedures/trauma, sedation for painful procedures

Side effects - I find less side effects than hydromorphone, but same things can happen

Opioids - Fentanyl

Mechanism of action - very potent full mu agonist

Dose - give a bolus, and then if need it any more than about 10 minutes, need to use as a CRI

• 2-10 mcg/kg/hr (can theoretically go up to 50, I have not…)
• I often run 3-4 mcg/kg/hr on an awake patient
• I usually run 4-10 mcg/kg/hr on an anesthetized patient

Uses - pain and sedation

Common protocols - Aggressive

Dexmedetomidine and Opioid

• +/- ketamine
• +/- alfaxalone

Sick

• Quite difficult
• Opioid + something
• Generally need something else but can be challenging to get them safely to a point where they can be handled depending on what additional diseases they have

Common protocols - Aggressive and Sick

Sick

• Quite difficult
• Opioid + something
• Generally need something else but can be challenging to get them safely to a point where they can be handled depending on what additional diseases they have

Consider using options that are reversible even if you are worried about the patient as stress can also decompensate a patient - can reverse if experiencing complications

• IM alfaxalone and IM midazolam can be helpful additions
• Acepromazone IM may be helpful but can’t reverse and is long-lasting, if worried about blood pressure before - avoid
• Can use dexmedetomidine and then reverse if experiencing complications

Common protocols - Diagnostics

Healthy

• Dexmedetomidine and opioid
• +/- alfaxalone vs propofol
• +/- acepromazine

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Sick

• Opioid and midazolam
• +/- judicious doses of alfaxalone vs propofol
• +/- low dose of acepromazine

Common protocols - Short procedure

Healthy

• Dexmedetomidine and opioid
• +/- ketamine

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Sick

• Opioid and midazolam