Anesthesia protocols. What do these drugs do?

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Milton Burro has a hoof abscess.  You have to be a certain age to know why Milton Burro is a funny name.  He’s my burro, but I don’t take credit for naming him.  And he’s not wearing a blindfold in this picture.  It’s a fly mask.  He can see through it.  I only mention that because I had to be told myself.

When I first started anesthetizing large animals, I was proud to say that I could tell a horse from a cow without first counting their toes.  Let’s just say I was more of a small animal guy.  I know a lot about horses now – but only when they’re laying down.  I still have a lot to learn about them when they are standing up.  I’m taking horsemanship lessons, so it’s getting better.

Milton’s doctor told me to soak his foot in warm Epsom salts solution and give him Bute twice a day.  I know that Bute is phenylbutazone, but I’m embarrassed to say – after all these years in vet-med – I had no idea what phenylbutazone does.  I never bothered to learn any more about it than its name.  I had to look it up.

I imagine this also happens with the anesthesia protocols I see taped to the walls in the hospitals I visit.  Everybody knows the drug names, the dose per pound of body weight, and what the bottles look like, but what does each drug do?  How does each drug fit into a balanced anesthetic regimen?

This stuff is hard, but I think I can help.  In this post, I’ve gathered information about drugs I commonly see used in private practice.  They are informally grouped in three categories, according to when they are likely to be used in an anesthesia protocol: Premeds, Induction, and Maintenance.

PREMEDS:

Acepromazine / Ace – Generic Name: Acepromazine – Benefit: Sedation

Advantages: Acepromazine is a potent tranquilizer and is probably the best drug in veterinary medicine for reducing anxiety.  Its onset of action is 30 – 35 minutes after SQ or IM injection. It can be given IV.  Its effects are long lasting and dose-dependent.  It offers some protection to the heart against certain kinds of arrhythmias.

Disadvantages: Acepromazine provides no analgesia and can cause dose-dependent hypotension.  It may contribute to patient hypothermia, and occasionally to aggressive behavior.

Antisedan – Generic Name: Atipamezole – Benefit: Reverses Dexmedetomidine

Advantages: Antisedan is a reversal agent specifically for alpha-2 agonist drugs like dexmedetomidine, and it is also very effective for reversing xylazine and detomidine.  Sedation, analgesia, and muscle relaxation are all reversed.

Disadvantages:  There is some anecdotal evidence of cardiac arrest following atipamezole administration while under gas anesthesia.  Care should always be taken when administering it under general anesthesia.

Atropine – Generic Name: Atropine – Benefit: Increases heart rate

Advantages: Atropine increases heart rate by inhibiting the effects of stimulation of the vagus nerve.  It also reduces salivation and respiratory secretions.  Its onset of action is 10 – 15 minutes after SQ or IM injection and its duration is about an hour.  It can be given IV.

Disadvantages: Atropine can cause tachycardia (rapid heart rate) and other arrhythmias.

Buprenex – Generic Name: Buprenorphine – Benefit: Pain management

Advantages: Buprenorphine is an opioid that provides long term analgesia (up to 12 hours) and mild sedation with no excitement.

Disadvantages: Because of its affinity for its receptors, it is difficult to reverse the effects with naloxone (see Narcan).

Dexdomitor – Generic Name: Dexmedetomidine – Benefit: Sedation and pain management

Advantages: The use of Dexdomitor markedly reduces anesthetic requirements of induction and maintenance drugs. Dexdomitor produces good sedation and analgesia, and there is evidence to suggest the sedation lasts longer than the analgesia.  Sedation and analgesia occur within 5 to 15 minutes, with peak effects at 30 minutes.  Dexdomitor may be reversed with atipamezole (see Antisedan), however once reversed it provides no analgesia.

Disadvantages: Dexdomitor reduces heart rate and initially causes vasoconstriction (increasing blood pressure) and then causes vasodilation (decreasing blood pressure). Due to the negative cardiovascular effects of Dexdomitor, be cautious when using it in dogs or cats with cardiovascular disease, respiratory disorders, liver or kidney diseases, or in conditions of shock, severe debilitation, or stress due to extreme heat, cold or fatigue.

Glycopyrrolate – Generic Name: Glycopyrrolate – Benefit: Increased heart rate

Advantages: Glycopyrrolate is very similar to atropine, but has a duration of about 4 hours and does not cross the blood brain barrier.  It is also less likely to produce tachycardia.

Disadvantage: At times the long duration can be a disadvantage as well, causing prolonged increased heart rate, dry mouth, etc.

Morphine – Generic Name:  Morphine – Benefit: Pain management

Advantages: Morphine is inexpensive and the gold standard by which all other opioid pain relievers are compared.  It works well administered SQ or IM in conjunction with acepromazine as a sedative/analgesic in dogs and cats.  Morphine is long lasting, providing good analgesia up to 6 hours.  Morphine is also an excellent choice for post-operative pain management.  The effects of morphine can be reversed with naloxone (see Narcan).

Disadvantages: As with the other opioids, dose-dependent excitation is seen in cats when administered high doses of morphine. Morphine can cause bradycardia (atropine and glycopyrrolate responsive) and respiratory depression. Respiratory depression may become severe at higher doses. Morphine commonly causes vomiting and defecation when used as a premed.  Combining morphine with acepromazine will reduce the likelihood of vomiting and defecation. Avoid IV administration of morphine as it may cause a release of histamine which can cause cardiovascular collapse.

Narcan – Generic Name: Naloxone – Benefit: Reverses opioids

Advantages: Reliably reverses respiratory depression produced by most opioids. Micro-doses may limit the degree of reversal to only reversing respiratory depression and sedation, while maintaining analgesia.

Disadvantages: At higher doses it will reverse the analgesia produced by opioids. Narcan’s duration of action is not as long as some opioids (like morphine), so the reversal agent could wear off over time and the effects of the opioid may return.  Narcan does not reliably reverse Buprenex because Buprenex has such a strong affinity for the mu receptor.

Numorphan – Generic Name: Oxymorphone – Benefit: Pain management

Advantages: Oxymorphone is an opioid offering good sedation and good analgesia.  It has a long duration of action with a peak effect lasting 1 to 3 hours.  It is ten times more potent than morphine. The cardiovascular effects of oxymorphone are similar to morphine, but less profound. The effects of oxymorphone can be antagonized with naloxone (see Narcan).

Disadvantages: Oxymorphone can cause bradycardia. Panting is often produced and is not changed by depth of anesthesia.  Panting may be reduced by administration of acepromazine. Despite a high respiration rate, oxymorphone causes respiratory depression. Oxymorphone will produce an exaggerated response to loud noises.  High doses may cause excitatory behavior, especially in cats and its use IV is generally not recommended in cats. It occasionally induces vomiting or defecation. Oxymorphone is more expensive than morphine.

Torbugesic / Torbutrol / Torb  – Generic Name:  Butorphanol – Benefit: Pain Management

Advantages: Butorphanol is a type of opioid that provides moderate visceral analgesia and potentiates the action of other anesthetic drugs.  Duration of action is very short in dogs, and moderate in cats.  Butorphanol is reversible with naloxone (see Narcan).

Disadvantages: Butorphanol does not reliably provide sedation when used alone, but good sedation is produced when used in combination with a tranquilizer.   It is not as effective for severe pain.

Valium – Generic Name: Diazepam – Benefit: Muscle relaxation

Advantages:  Valium is used most frequently to potentiate the effects of other anesthetic drugs. It induces muscle relaxation.  It is also used to treat acute seizure activity.  Valium can be reversed with flumazenil.

Disadvantages: Alone, Valium does not produce sedation in most animals.  Due to its propylene glycol preparation, IM injection may result in pain and its absorption from IM injection may be unreliable.  It does not mix well in the same syringe with others drugs, and may form a precipitate when mixed.

Versed – Generic Name: Midazolam – Benefit: Muscle relaxation

 Advantages: Versed is a muscle relaxant similar to Valium.  It is water-soluble, which minimizes irritation at the injection site when given IM or SQ.  Unlike Valium, it mixes well in the same syringe with other drugs.  Versed offers excellent sedation and muscle relaxation in birds.  Versed can be antagonized with flumazenil.

Disadvantages: Versed does not produce sedation in normal dogs and cats, but may sedate depressed patients.  It may cause agitation and irritability in calm dogs and cats when administered alone.

INDUCTION DRUGS:

Alfaxan – Generic Name: Alfaxalone – Benefit: Induction of anesthesia

Advantages: Alfaxan is another anesthetic with a rapid onset and short duration of action with minimal side-effects. In general its clinical use and properties can be compared to propofol. Similar to propofol, Alfaxan is an induction agent that, because of its short half-life in dogs and cats, is suitable for repeated bolus injections or a continuous rate infusion (CRI).  Unlike propofol, Alfaxan has little or no cardiovascular effects when given in the normal dosage. Alfaxan can be safely combined with premeds.

Disadvantages: Rapid IV administration of Alfaxan causes apnea, and it is recommended to administer it slowly IV; over a minute or so. Alfaxan should not be considered a significant analgesic. Recovery from Alfaxan can be agitated, especially when little or no premeds are used.  Cats recovering from Alfaxan seem to be extra sensitive to outside stimuli, and the recovery should be in a quiet, darkened room.

Ketalar / Ketaset / Vetalar – Generic Name: Ketamine – Benefit: Induction of anesthesia, pain management

Advantages: Ketamine administered IV induces smooth and rapid anesthesia.  It can be administered IM as a premed or induction agent to aggressive cats and dogs. Ketamine is an appropriate induction agent for sighthounds.

Disadvantages: Ketamine increases salivation, muscle rigidity, and is painful on IM injection. Vocalization, and delirium or seizure-like activity during recovery have been reported following higher doses.  Atropine or glycoppyrolate will control the excessive salivation. To decrease the muscle rigidity, administer a muscle relaxing drug like Valium or Versed with the ketamine.

 Propoflo – Generic Name: Propofol – Benefit: Induction of anesthesia

 Advantages: Propofol is an ultra-short acting intravenous injectable anesthetic agent.  It produces a smooth, rapid induction and is unlikely to cause arrhythmias.  The soy bean oil and egg protein emulsion of its base causes no reaction if inadvertently injected perivascularly. It provides a brisk recovery. Propofol can be used as an induction agent or can be delivered by constant rate infusion with minimal cumulative effects, to maintain anesthesia for short procedures.

Disadvantages: Propofol causes dose-dependent apnea and hypotension. These can be minimized by slowly administering the drug over 30 ‑ 60 seconds.  Cumulative effects after prolonged infusions (resulting in prolonged recovery from anesthesia) are reported in cats. Occasional aberrant muscle twitches are observed following IV administration to dogs. Propofol has poor analgesic properties.

Telazol – Generic Name: Tiletamine-zolazepam – Benefit: Induction of anesthesia

 Advantages: Telazol is a combination of the drugs tiletamine, a drug similar ketamine, and the muscle relaxant zolazepam, which is similar to Valium.  It has a more rapid onset of action following IM injection, and longer duration of anesthesia compared to a combination of ketamine and valium. It provides good muscle relaxation and is an appropriate induction choice for sight hounds. It is a very useful drug for sedation of aggressive dogs.  It comes as a powder in a single vial and needs to be reconstituted.  Since it needs to be reconstituted, it can be reconstituted to concentration.  This flexibility makes it an ideal capture drug for field anesthesia.

Disadvantages: Increased salivation in dogs and cats, pain on IM injection in cats, vocalization during recovery in dogs, and delirium or seizure-like activity during recovery have been reported as side effects.  Recovery from Telazol in dogs tends to be rough.  Cats have long recoveries which are occasionally rough. Refrigeration is recommended after reconstitution to prolong shelf-life.

MAINTENANCE DRUGS:

Isoflurane – Generic Name: Isoflurane – Benefit: General anesthesia

Advantages: Isoflurane produces rapid induction and rapid changes in anesthetic plane, with minimal metabolism.  It provides good muscle relaxation and usually does not cause cardiac arrhythmia.  Very little isoflurane is metabolized by the body and the effects go away by breathing out the gas.

Disadvantages: It is a potent respiratory depressant.  Isoflurane also causes dose dependent hypotension, largely attributed to vasodilation.

SevoFlo / Sevo – Generic Name: Sevoflurane – Benefit: General anesthesia

Advantages: Sevoflurane’s low solubility produces rapid induction, rapid changes in anesthetic plane, and rapid recovery. Sevoflurane does not appear to sensitize the heart to arrhythmias, although it will increase heart rate above resting values.

Disadvantages: Sevoflurane causes dose-dependent respiratory depression and hypotension, similar to isoflurane. At a surgical plane of anesthesia, it decreases mean aortic blood pressure, stroke volume, and cardiac contractility.  It will also cause systemic vasodilation.

This is by no means a comprehensive review of all the anesthetic agents you might find in a small animal practice.  This is just a thumbnail sketch of the drugs I most often see on protocol sheets.  It’s the kind of information I wanted to know about Bute, before I gave it to Milton.  For more comprehensive information about these drugs and other drugs not listed here, visit the online Veterinary Anesthesia and Analgesia Support Group (VASG).  VASG is a site maintained by veterinary professionals who have made a commitment to anesthetic and pain management excellence.


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Ken Crump AAS, AHT is a writer and animal anesthetist, and writes Making Anesthesia Easier for DarvallVet, a division of Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary anesthesia and oncology across the United States and in Canada.  Ken retired from the Veterinary Teaching Hospital at Colorado State University in 2008
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2 Responses to Anesthesia protocols. What do these drugs do?

  1. Rick Trexler, RVT, CBET says:

    Ken,

    I enjoy your articles. As an old-timer myself having been in veterinary medicine for over thirty years now, I always find it amusing to see the quizzical looks on younger technicians and veterinarians faces when older terminology is used. That being said I think you should include the older generic name for acepromazine which is acetylpromazine as you will on rare occasion still run across it in the literature.

    Also since you are including the opioid reversal why not include the bezodiazepine reversal, flumazenil?

    Rick Trexler, RVT, CBET

    • Ken Crump says:

      Hi Rick – I’m glad there are a few of us “more mature” types out there. Thanks for the suggestions. I thought long and hard about including flumazenil as a stand alone drug on the list. But it’s so crazy expensive and so rarely used, that I doubt many practices have it on hand anyway. I opted to just refer to it rather than list it.

      Great minds, tho…
      Ken

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