Top 5 Anesthetic Complications

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I stumbled upon a 2016 article by Dr Kate Cummings and Dr Lois Wetmore on common anesthetic complications entitled Top 5 Anesthetic Complications.  It’s such a simple, clear, and informative article that I had to share it.  According to Cummings and Wetmore, the five complications that commonly occur during anesthesia are hypotension, hypothermia, abnormal heart rate, hypoventilation, and difficult recovery.  If you’ve had any experience anesthetizing animals, I’m sure you responded to each of these the same way I did: “Yep.” “Oh yeah.” “Been there.” “Definitely.” and “Saw that yesterday.”  Here’s an overview of the 5 complications.

Hypotension

They define the minimum acceptable mean arterial pressure for anesthetized small animals is 60 mm Hg. Less than that is considered hypotension or low blood pressure. A study by Dr Anne Wagner and Andrea Gordon further explains that vital organs like the brain and kidneys have the ability to adjust blood supply to meet their metabolic needs, but only if the mean arterial blood pressure is above 60 mm Hg.  A survey of anesthetic records at the Colorado State University Veterinary Teaching Hospital indicated that 32% of dogs were hypotensive at some point during anesthesia.  A survey of twenty veterinary practitioners in Colorado published in 2002 suggests the percentage in private practice may be higher. In that survey, the only veterinarian who considered hypotension to be a problem during anesthesia was also the only veterinarian who regularly measured blood pressure in all her patients.  Coincidence?

Cummings and Wetmore describe the most common causes of low blood pressure during anesthesia, and include a very handy treatment tree labeled Hypotension Management.Top 5 Anesthetic Complications-Tx Tree

Hypothermia

Hypothermia is a complication near and dear to my heart.  DarvallVet focuses on developing innovations for patient warming, including the Darvall Heated Breathing Circuit that warms from within.  The article attributes much of the direct causes of hypothermia to the drugs used for sedation, analgesia, induction and maintenance of general anesthesia.  Additional causes include IV fluids, shaved fur, open body cavities, high oxygen flow rates and surgical prep solutions.   Additional studies report that most of patients’ lost body temperature occurs before they ever reach the surgery suite, during premeds and surgical prep.  Using a heated circuit from the moment of intubation prevents most of that temperature loss.  Preventing hypothermia is as simple as using a better breathing circuit.

Cummings and Wetmore report that hypothermia is known to have negative consequences on coagulation and overall immune function.  Dr Robertson elaborates in the Proceedings of the World Small Animal Veterinary Association World Congress of 2015 to say that as the core temperature falls there is a drop in blood pressure, changes in cardiac rhythm, and altered platelet function. Metabolism is slowed and liver function is impaired, delaying breakdown of anesthetic drugs which will prolong recovery times. In human studies, intra-operative hypothermia has been linked to increased post-operative wound infection, and humans report that waking up cold is extremely unpleasant.

Top 5 Anesthetic Complications- Cat copyAs important as it is to protect the patient’s body temperature, we are warned of the dangers of using warming devices that are not specifically designated for warming veterinary patients. Pictured here is thermal injury to a cat after direct contact with a warm fluid bag.

 

Abnormal Heart Rate

Abnormal heart rate and rhythm during anesthesia are distressingly common. The article stresses the need to monitor the heart with a combination of ECG, pulse oximeter, and stethoscope.  Cummings and Wetmore carefully explain common abnormal heart rates and rhythms, and provide a useful table of treatments for heart complications and others.  The table is organized by complication, then by drug name, and then dose range.  This section of the article is fact-packed, and merits extra time and attention.

Hypoventilation

Normal respiratory rates for dogs and cats vary based on size and positioning. Most dogs breathe at 6–10 breaths/minute and cats at 16–20 breaths/minute. Hypoventilation is actually difficult to understand at first, since we usually equate breathing with oxygen. Yet ventilation is defined by carbon dioxide.  It takes a mental moment to shift away from thinking about oxygen, and start thinking about carbon dioxide.  Capnography is discussed a lot in the practices I visit, as awareness of the value of monitoring CO2 increases, and the price of capnographs (carbon dioxide monitors) come down.  Dr Hendrix presents a good overview in her article Carbon Dioxide Monitoring in Anesthetized Animals.

Difficult Recovery

The take-home message of this section is that more than half of anesthetic deaths happen during recovery.  Just when you think you’re out of the woods, sometimes you’re just entering the hard part.  This highlights the need to continue close monitoring during the recovery period.  The article describes the two most common difficulties during recovery: the rapid, dysphoric recovery, and the prolonged recovery.  Hypothermia is a common contributor to the prolonged recovery, reminding us again of the importance of protecting our patient’s body temperature during anesthesia.

As Cummings and Wetmore stated in the article, anesthetic management of small animal patients relies on thorough patient assessment, diligent anesthetic monitoring, and supportive care into recovery. Being prepared to treat anesthetic complications offers the best outcome.  Their article provides a clear understanding of the 5 complications and an easy-to-navigate path toward recognizing them and treating them.


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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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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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Anesthesia for Pocket Pets

rats-ftr

With nearly 44 million exotic small animal pets in the US, sooner or later you’ll find yourself anesthetizing one in your practice.  And with pet rats becoming the ever more popular exotic pet of choice – especially in states where other exotic mammals are banned as pets – chances are you’ll be anesthetizing a rat.

In his article Oh, Rats! Why the Rodents Are Becoming Increasingly Popular Pets, Kenneth Miller recounts how he and his wife swallowed their misgivings and bought a pair of female rats for their daughter, who is allergic to more conventional pets.  He writes, “Ounce for ounce, Cookie and Blueberry have more personality than many conventional domestic animals. They run excitedly to their cage door when we enter the room. They enjoy being tickled and playing peekaboo behind the sofa cushions.”

He admits that as small critters go, there’s something about rats that generates particular “ardor” among their owners.  And according to statistics from the American Pet Products Association, that ardor has led the number of households with pet rats to nearly double in just a few years.  And families who love their pocket pets bring them to veterinarians for medical attention, anticipating the same level of care given to more conventional pets.

Fortunately when we are occasionally faced with anesthetizing a pocket pet, we can turn to the research industry that has pioneered rodent anesthesia, to provide us with normal physical parameters and suggestions for appropriate drug combinations.  But most of us rely on gas anesthesia for our clinical practices, and finding an effective gas anesthesia delivery system for rodents and pocket pets, outside of the research environment, is another story.

Despite the ongoing progress made by researchers in rodent anesthesia, there are still longstanding challenges anesthetists need to address.  Common among these are

  • Morbidity / mortality due to hypothermia
  • Waste gas contamination of the workplace
  • Long, uncontrolled inductions
  • Variable, unpredictable depth of anesthesia
  • High cost of anesthetic gas and oxygen

DarvallVet has developed innovative solutions to these longstanding challenges that have been adopted by such prestigious institutions as the new FDA White Oak research campus in Maryland, and AbbVie, home to 29,000 scientists, researchers, and manufacturing specialists located around the globe.  Now the same gas anesthesia delivery systems that reduce hypothermia and eliminate waste gas contamination in research institutions are available to clinical practice.  Whether your practice sees several pocket pets a year, or several a day, DarvallVet has the right breathing circuit for you.

ZDS Mask –

ZDS-MaskThe Darvall ZDS Mask is perfect for the clinical practice that sees only the occasional bird or pocket pet.  It’s also a great answer for anesthetizing small puppies and kittens.  Its clever design delivers fresh gas directly to the patient, preventing resistance to breathing and eliminating dead space.  As a matter of fact, ZDS stands for “Zero Dead Space”.  The ZDS Mask provides rapid and predictable anesthesia and is efficient enough to allow reduced flow rates, saving time, money, and protecting your patient’s body temperature.  It comes with two different sized diaphragms to prevent waste gas from leaking into the room.  The overall concept is simple enough that you could make one yourself just by looking at the picture, but it’s so reasonably priced that it’s hardly worth the effort.  It’s available in autoclavable and non-autoclavable models, although it’s the rare clinical practice that would need to spend the extra money on an autoclavable model.

ZDS Qube –

300-dpi-fatso-and-blockThe Darvall ZDS Qube is the all-around anesthesia workhorse for the exotics practice or practices that see a number of small pocket pets every week.  Its patented unidirectional low-flow design allows you to turn the oxygen flow rate down to a minimum (see your vaporizer’s recommended minimum oxygen flow rate) saving up to 80% of the cost of oxygen and anesthetic gas, as well as protecting your tiny patient from the how high flow rates syphon off body temperature.  To further maintain crucial body temperature in very small pets, the ZDS Qube is available in heated and non-heated models.  The Heated ZDS Qube warms the inspired gases, warming your patient from the inside out.

MasksThe ZDS Qube is an aluminum block weighing about a pound, adding security and stability to patient positioning.  There is a full range of interchangeable ZDS Qube masks and diaphragms available to minimize waste gas contamination and to fit any bird, pocket pet or positioning situation.  And like the ZDS Mask, the ZDS Qube provides rapid, consistent, and predictable anesthesia.

Developed for use in the best research institutions in the world, both the ZDS Mask and the ZDS Qube attach easily to your clinical gas machine without tools.  They’re simple to set up and simple to use, providing state of the art delivery of anesthetic gas to your clinical practice.

“They promptly won our hearts,” writes Miller about Cookie and Blueberry. Like half a million other American families, his has fallen in love with this less-than-conventional pet. Whether your practice sees the occasional rat or is an exotics specialty practice, it is now possible to borrow a page from the experts’ playbook and provide safer, predictable gas anesthesia to your clients’ pocket pets.white-rat-golden-teddy.jpg.838x0_q80

“To own a rat,” says Dale Burkhart, Vice President of the American Fancy Rat and Mouse Association, “is to know that forever your heart will walk outside your body on four little feet.”

 

 


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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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Let’s Just Mask Him Down

ZDS kitten

The Quizlet flashcard question asks, “What are the uses of mask induction?”  Quizlet  is an online study tool that uses flashcards to help improve your retention.  They call it a simple tool for learning anything.  The mask induction flashcard is part of a section for veterinary technicians on induction and patient prep for anesthesia.  The answer on the reverse side of the flashcard states: “When IV induction is difficult, mildly uncooperative cats and small dogs, neonates, small mammals, birds, debilitated patients subject to the negative effects of premedications or induction drugs.”

Mask inductions fit nicely into the “less is more” mindset we often adopt when it comes to anesthesia.  It seems safer because the fewer drugs we use the less that can go wrong, right?  In a recent article, John Jacobson DVM, DACVAA, lists these perceived advantages of mask inductions.

  • Inhalation anesthetics can be easily eliminated from the body with ventilation; they are not nearly as dependent on redistribution and metabolism for recovery as most injectable agents are.
  • The change in anesthetic depth is typically gradual compared with boluses of intravenous induction agents, giving the patient time to compensate for cardiovascular changes.
  • Intravenous access is not required.

All of this builds a pretty good story for mask inductions being fairly benign.  However, even Dr. Jacobson goes on to say that he avoids them.

“Significant cardiovascular and respiratory depression occurs under halothane and isoflurane anesthesia,” explains Dr. Nancy Brock, also a veterinary anesthesiologist. She states that although induction of anesthesia by mask is perceived by many as a safer method, gas anesthetic agents are not innocuous or inherently any safer or easier to control than injectable agents.

This cautionary information about mask induction is clearly outlined in the Veterinary Anesthesia and Analgesia Support Group (VASG) section on induction of anesthesia.  VASG is the well-respected online anesthesia resource founded by Dr. Bob Stein, a site that currently hosts over 20,000 visitors per month.  VASG states that mask inductions are not recommended for most patient groups.  Here are the reasons they list as why:

  • Increased patient stress, which could increase arrhythmic risk.
  • Unnecessary staff exposure to anesthetic agents.
  • Time required for complete induction of anesthesia is longer than compared to IV agents.
  • Prolonged period of unsecured airway with an increased risk of airway compromise or obstruction.
  • High concentrations of inhalant agents are required to achieve mask induction. Higher doses produce more cardiovascular and respiratory depression than seen with comparable doses of IV induction agents.
  • Contraindicated in brachycephalic patients.

In a special report on veterinary medical care guidelines for spay-neuter programs, the Association of Shelter Veterinarians also states that mask inductions should not be performed routinely and should be avoided whenever possible.  In addition to the reasons listed by VASG, they include the high cost of delivering high flows of oxygen and high concentrations of anesthetic gas directly into the scavenge system.  The report goes so far as to say, “If mask supplementation becomes frequent or regular, other options should be considered for patient and staff safety.”

I did a fair number of mask inductions early in my career.  In one practice, it was the standard protocol for induction of most cats and small dogs.  I had occasion to use a gas analyzer during that time.  A gas analyzer measures the actual concentration of anesthetic gas that’s delivered from a gas machine (as opposed to the concentration indicated by the vaporizer dial), and also the concentration of anesthetic gas exhaled from the patient.  That taught me that for a while after induction, the concentration of anesthetic gas exhaled by the patient was higher than the concentration of gas coming from the vaporizer.  For example, I had the vaporizer set at 1.5% for the surgery, but the patient was actually exhaling twice that concentration.  So my patient was over anesthetized until it could blow off the excess gas required for the induction.  That was an enlightening realization for me.  I had been inducing with anesthetic gas, the most hypotensive drug I use for anesthesia, in high concentrations, in an uncontrolled manner, to a patient who was not intubated and had no monitoring equipment on, and I thought it was the ‘safest’ plan.

Mask inductions have their place in our anesthesia toolbox.  But, if you routinely mask induce or use a chamber to induce patients, it’s time to reevaluate the safety, the cost, and the efficacy of that protocol. As the AAHA Anesthesia Guidelines for Dogs and Cats  warns, mask inductions should be reserved for situations where other alternatives are not suitable.  VASG is an excellent resource to begin marking a trail through the forest of anesthesia protocol possibilities. I know it can be difficult to change an anesthetic protocol.  We all become comfortable with a few ways to do things, and are reluctant to risk trying something different.  But VASG founder Dr. Stein suggests a way to gain confidence with a variety of advanced anesthesia techniques by periodically utilizing them on low risk patents.

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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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Rebreathing or Non-rebreathing?

1608c25a6eece6570a6242b4381b4f40Most veterinary practices have a policy in place to decide which patients use a rebreathing circuit and which use a non-rebreathing circuit during gas anesthesia.  In general, smaller patients use a non-rebreathing circuit and larger patients use a rebreathing circuit. Your hospital designates the actual cut-off from rebreathing and non-rebreathing, based on the weight of the animal.  But there’s more than the weight of the animal at play here, so let’s take a closer look at the decision to use a non-rebreathing circuit over a rebreathing circuit.

liar-scaleAnesthesia breathing circuits fall into two main categories, based on how they manage the patient’s expired CO2.  Rebreathing circuits use a carbon dioxide absorber to trap and remove CO2 so the patient can breathe gases that have been recycled through the machine.  Non-rebreathing circuits use high gas flows to washout expired CO2 from the circuit before the patient takes its next breath.

Regardless of the shape or configuration of rebreathing circuits, they all share five essential components:

  • Hoses
  • Rebreathing bag
  • Unidirectional flow valves
  • CO2 absorbent
  • Pop-off valve

By contrast, non-rebreathing circuits are comprised of only hoses. A rebreathing bag and pop-off valve are often part of a non-rebreathing circuit, but they are not essential to its function

The advantages of a rebreathing circuit make it the first choice for anesthesia.  Rebreathing circuits require lower gas flows which saves money for the practice as well as reducing pollution into the atmosphere by waste anesthetic gas. But the more significant advantages are to the patient.  Rebreathing circuits help keep patients warmer and help to retain moisture.  Non-rebreathing circuits require high gas flows and steal heat and moisture from the patient. And the loss of body heat and moisture are key complications of anesthesia that we face with every patient, especially smaller patients.

So why not put every patient on a rebreathing circuit?
That would be the ideal.  Unfortunately the one significant disadvantage of rebreathing circuits is that the combined five essential components of the circuit increase the resistance to breathing.  When you decide that your patient should be on a non-rebreathing circuit, you are actually deciding that you think your patient cannot overcome the resistance of a rebreathing circuit.  Not surprisingly, the one significant advantage of a non-rebreathing circuit is the minimal resistance to breathing.  July-20-2012-19-53-53-oiuImagine the difference in resistance between taking a drink from a soda straw, and taking a drink from a garden hose.  Although it takes only a small amount of energy to draw water up a soda straw, it’s still significantly more energy than it takes to get a drink of water flowing from a garden hose.  Similarly, it requires far less effort to take a breath from a non-rebreathing circuit than from a rebreathing circuit.  In order to draw a breath from a rebreathing circuit, the patient must be able to generate enough negative pressure to overcome the resistance of the hoses, lift the unidirectional flow valves and draw it through the soda lime.

Test the limits

Maybe it’s time to examine your hospital’s policy toward non-rebreathing circuits.  I’ve seen target weights set at 20 pounds, at 5 pounds, and at every weight in between.  And I’ve seen practices that don’t use non-rebreathing circuits at all.  They can’t all be right, can they?

Non-rebreathing circuits are expensive to use, flowing nearly ten times more oxygen and anesthetic gas into the scavenge system as rebreathing circuits, and they syphon off body heat and moisture from patients.  One way to extend the weight range for the use of non-rebreathing circuits into lower numbers is to challenge the efficiency of your current rebreathing circuit.

A simple first step

52-images-of-free-accounting-clipart-you-can-use-these-free-cliparts-to2q7u-clipartThe unidirectional flow valves and the CO2 absorber are significant sources of resistance in your circle rebreathing system, and that can be reduced considerably by evaluating your current system and replacing outdated parts of the gas machine.

However, a more accountant-friendly first step is to examine the tubing you use in your rebreathing circuit.  Hoses account for up to 50% of the resistance the patient has to overcome in order to take a breath.  And replacing the breathing hoses of your rebreathing circuit is not expensive at all.

004039ac1337829733123 copyResistance test results reported in a recent study show that the corrugations in standard breathing hoses and in the popular Universal “F” circuits used in many small animal practices today are sources of resistance to breathing.  The same test results show that removing the corrugations from the inside of the hose reduces resistance remarkably.  Removing the corrugations also allows the size of the tubing to be reduced, further improving the efficiency of the system.  Smooth wall circuits are even less resistant to flow than smaller corrugated pediatric circuits.

 

8349-SWT12Darvall has pioneered smooth wall circuits and has them available as heated and non-heated, in two sizes: 12mm for patients under 45 pounds and 16mm for patients over 45 pounds.  Both hold less volume than their standard 22mm corrugated counterparts which make them more efficient and makes your gas machine more responsive.  The smaller 12mm hose is so efficient that with some CO2 absorbers, you can use it on patients as small as 2 Kg.  And as a special bonus for those of us who wash breathing circuits, without corrugations nothing traps water inside the hoses.  Smooth wall circuits dry almost immediately.  That means no more swinging circuits over our heads to get them dry enough to use.

Non-rebreathing circuits cost your practice money and cost your smaller patients body temperature.  Examine your hospital policy toward non-breathing circuits and test the limits of your weight range.  A capnograph will indicate when a patient is inspiring CO2, and can be used to determine if a non-rebreathing circuit is necessary.  Using the most efficient hoses for your rebreathing circuits will go a long way toward lowering your target weights, and allow you to put smaller patients on rebreathing circuits.  Darvall offers smooth wall circuits in heated and non-heated.  Changing to smooth wall circuits is a great first step to improving the efficiency of your gas machine and allows you to use a circle rebreathing circuit on smaller patients than you normally would.

Follow these links for more information

Darvall Smooth Wall Tubing (SWT): Low Resistance & Volume 

Darvall Smooth-Wall Circuits – Efficient & Responsive Anesthesia Breathing Circuits

DarvallVet.com

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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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6 Tips to Success with Darvall Heated Breathing Circuits

With more and more practices turning to Darvall Heated Breathing Circuits to manage hypothermia during anesthesia (especially during National Pet Dental Health Month), we’ve put together a quick list of tips to help you get the most benefit from warming from within.

6 tips to Success

  1. Pre-Warm the Patient – Warming the patient in a heated cage or under a warming blanket before the procedure can prevent the loss of 1 – 2oF body temperature
  2. Pre-Warm the Circuit – Turn the heat controller on 5 minutes before induction so the tubing is warm and ready to go.
  3. Orange to Inspiration – Connect the hose with the orange collar to the INSPIRATION side of the breathing circuit. Get this wrong and you’re only warming the waste gas.
  4. Go Low Flow – Calculate the oxygen flow rate at 30ml/kg. Higher flow rates reduce its ability to heat the inspired gas effectively.
  5. Start at the StartHeated breathing circuits prevent hypothermia better than they treat hypothermia. Capture control of body temperature at intubation.  Use this circuit as the first breath and you’ll prevent the loss of 2 – 5oF body temperature.
  6. Maximize Continuity – A heated breathing circuit for each gas machine in your practice provides convenient and consistent patient warming throughout the hospital.

Here’s a presentation that dives deeper into each of the 6 Tips, with links to even more information.

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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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Hypothermia and the Veterinary Dental Patient

To highlight the importance of oral health care for pets, the AVMA designated February as National Pet Dental Health Month.  Clients who appreciate the safety and convenience of non-anesthesia dentistry may question the need to have their pet face the risks of anesthesia for a routine dentistry.  In answer, the American Veterinary Dental College (AVDC) states that veterinary dentistry includes scaling the surfaces of teeth both above and below the gum line, followed by dental polishing. Removal of dental tartar only on the visible surfaces of the teeth has little effect on a pet’s health, and provides a false sense of accomplishment. The effect is purely cosmetic.  The 2013 AAHA Dental Care Guidelines for Dogs and Cats says that general anesthesia with intubation is necessary to properly assess and treat the companion animal dental patient. AAHA states, “Cleaning a companion animal’s teeth without general anesthesia is considered unacceptable and below the standard of care.”

Although anesthesia for animals has come a long way and is safer now than it ever has been, we can’t dismiss our clients’ concerns about the risks of anesthesia.  A two-year study of nearly 200,000 pets published in 2006 ranked surgical procedures in the order of those most commonly resulting in anesthetic death.  Dental procedures ranked number three, in the top seven. Age, underlying systemic disease, length of anesthesia, and hypothermia are listed among the probable contributors to the greater anesthetic risk among dental patients.

A hypothermia study conducted by a research team from the Universidad CEU Cardenal Herrera in Spain was published in 2013 in Veterinary Record. The results showed that 83.6 percent of 1,525 dogs developed hypothermia as a result of anesthesia. A previous study performed by the same research team suggests that cats are even more likely than dogs to develop hypothermia while anesthetized. Almost 97 percent of cats develop hypothermia while receiving anesthesia, and kittens are at increased risk. A study published in the Canadian Veterinary Journal evaluating cats undergoing anesthesia for dentistry, supports the Spanish research team’s findings, indicating that unless steps are taken to conserve body temperature, a decrease of nearly 4°F may occur. These studies clearly indicate hypothermia is one of the most predictable complications of anesthesia, and therefore veterinary staff should continuously monitor the body temperatures of anesthetized animals and be proactive in preventing heat loss.

Preventing peri-anesthesia hypothermia has traditionally focused on skin warming and conserving body surface heat, but this often proves to be ineffective and can sometimes burn animals. The margin of safety from causing significant thermal injury is surprisingly narrow in animals. Skin can be burned from devices supplying constant surface heat of as little as 115°F for one hour. Hot tap water can be warmer than that. Sedated and anesthetized animals can’t move away from excessive heat, so containers of warm water, heated wheat bags or ‘on-off’ electric heat pads not specifically designed for anesthetized animals can cause severe burns. Thermostatically controlled constant warming devices with even heat distribution such as warm air blankets are safer.

Premedication drugs can cause mild hypothermia in dogs and cats, typically losing 1 – 2°F over thirty to sixty minutes before anesthesia induction.  This initial drop in core body temperature precedes the precipitous critical heat loss of an additional 2 – 5°F that occurs in the first fifteen to thirty minutes after induction.  If the patient is draped for surgery, then the rate of heat loss slows.  Many of us are surprised to realize that a patient could lose as much as 7°F before it has been anesthetized for very long.

Providing thermal support before anesthesia may sound counter-intuitive, but recent research shows that warming patients effectively from the time of premedication to the time of induction can prevent that initial drop in body temperature prior to induction, and slow the rapid heat loss immediately following induction.

Targeting the rapid heat loss after induction is particularly challenging.  In that early stage, patients are positioned and repositioned which results in poor heat transfer from heating mats beneath them.  Also stimulation during this early stage is often minimal. Combine that with the patient lying on a grate over a water table and having its face and head drenched with cold water, then supporting body temperature seems like a losing battle.  Here are some tips for warming dental patients suggested by Portland’s award winning veterinary hospital and training facility, DoveLewis.

  • Laying a patient on a towel over a water table provides more surface area to lose body heat.  Placing the patient on a solid surface like a mat will help.
  • Placing the patient on any type of approved heating pad will help slow heat loss.
  • Bubble wrap layers over the patient help retain heat
  • Baby socks on their feet retains heat
  • An emergency reflective blanket tented over the patient traps heat
  • Attempt to keep the head as dry as possible and take time to wipe it dry periodically.
  • Administering warm IV fluids can help
  • Warm inhaled gases slow heat loss

Warming inhaled gases can go a long way toward preventing peri-anesthesia hypothermia. Inspiring cold compressed oxygen from an anesthetic gas machine can be a major contributor to cooling anesthetized patients from the inside out, especially right after intubation. Normally the nose and pharyngeal mucosa transfer heat and moisture to inspired air and then recover much of the heat during expiration. Bypassing the nose and pharyngeal mucosa with an endotracheal tube results in the delivery of cold compressed gases directly into the lungs, with no chance of temperature recovery during exhalation. This can cost a 25 pound dog nearly 3000 calories of warming energy in the first hour of anesthesia alone. Warming the inspired gases to near normal body temperature from the moment of intubation is a great way to prevent the loss of core body temperature caused by the body’s attempt to warm cold inspired gases.  It literally warms from within.

The first heated breathing circuit for veterinary use was introduced to the United States in 2013.  Developed by Advanced Anesthesia Specialists of Australia, it is manufactured and distributed under the Darvall brand name.  Darvall’s heated breathing circuits have a heating element imbedded into the tubing of the inspiratory limb of the breathing circuit. A sensor molded in the tubing at the Y piece monitors gas temperature and a microprocessor controls heating. Closed loop feedback is provided by an esophageal temperature probe which enables the microprocessor to monitor the animal’s body temperature, and it turns off the heater if either sensor detects temperatures above the presets.

Anesthesia decreases the metabolic rate and inhibits muscular activity which contributes to hypothermia. Hypothermia may lead to dysrhythmias, hypotension, respiratory depression, bradycardia, coagulopathy, altered blood viscosity, and anesthetic drug overdose. Warming hypothermic animals recovering from anesthesia is a slow and laborious process often taking more than one to two hours. This delay affects the patient, the staff, and the efficiency of your workflow.  Heated breathing circuits offer a new and innovative way to capture control of a dental patient’s body temperature from the moment of intubation, and puts an effective new tool in your hypothermia-management toolbox.

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Resources to explore this topic further:

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Ken Crump AHT, AAS, 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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VSPN – A Great Online Resource for Veterinary Anesthetists

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Sometimes finding the answers to anesthesia questions can be tough.  But what if you had a whole group of friends you could count on, who understood the everyday challenges you face in practice and were eager to discuss them with you and help you find the answers you need.  The Veterinary Support Personnel Network (VSPN) has always provided that trusted circle of friends to me.  In this post I want to introduce you to VSPN, help you navigate the site, and get you started as a member of this free veterinary resource.

What is VSPN?

Maybe you’ve heard of VIN, the Veterinary Information Network.  VIN has pioneered providing instant access to vast amounts of up-to-date veterinary information, turning subscribing practitioners who might have felt like competitors into a community of over 53,000 professional colleagues.  VSPN is an extension of VIN, providing the same up-to-date veterinary information, tailored to the needs of veterinary support personnel.  One significant difference between VIN and VSPN is cost.  Unlike VIN, which is a fee-based subscription service, VSPN is a free community.  The Veterinary Support Personnel Network brings together people from all over the world to interact with each other, teach each other, and learn from each other.  As a VSPN member, you have access to thousands of colleagues who want to help you and your patients – 24 hours a day!

Let’s take a look around

When you click on this link, you’ll go directly to the VSPN.org home page.  One of my very few criticisms of VSPN is that the website layout looks a little dated and feels a little text-heavy.  To me, it still has the feel of a message board and navigation of the site is not as intuitive as I like.  New visitors may need a moment to get oriented.  But I guarantee that it’s worth the effort.  One of the first things to do here is to apply for membership.

vspn-home-pageYour membership application may take a couple of days to process, but in my experience the folks at VSPN are very responsive.

The Message Boards

You’ll have to wait until your application is approved before you can explore the content of the site, so let me give you a sneak preview of what’s in store.  I find one of the most useful resources is the VSPN Message Boards.  These are your gateway to discussions on current topics, management methods, drug protocols, and so much more. Discussions with your peers allow you to exchange ideas, get help with specific patient management problems or cope with stress of practice.

The Message Board folders cover medical and practice management topics, industry information, and so on. The Message Boards are searchable and customizable, allowing you to get the information you want in the way you want it.  Just click on ‘Message Boards’ in the top navigation bar and then open the drop-down menu labeled VSPN to choose your topics of interest.

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Of course, I’m partial to the Anesthesia folder where you’ll find lively and informative discussions about all aspects of the management of anesthesia, from anesthesia nerds around the world.  All VSPN members can initiate and participate in Message Board discussions. This interaction is what makes the VSPN community so useful to all of us as veterinary professionals.

The VSPN Notebook

notebook2One of the real treasures of this community is the VSPN Notebook®, an online handbook of easily-accessible information for hospital staff to use in practice every day. As an online version, it is regularly updated and amended to contain the most current information in the profession. We can all suggest additions to this online book to help make it fit our needs.  You can order a  hardcopy of the book from the VIN Bookstore to have as a ready reference in your practice.  I bought a copy for my team years ago, before it was offered online.  We always found it useful.

We’ve just scratched the surface!

nav-barNothing makes anesthesia easier than having good up-to-date information at your fingertips.  That’s why I wanted to introduce you to one of the most useful web communities I have found.  The VSPN worldwide community is made up of people just like us, who all pool our knowledge and experience to get help and to give help.  And our patients are better served by every question asked and answered.   Benefits for VSPN members include access to the online version of the VSPN Notebook®, community message boards, rounds sessions and an extensive library of past rounds transcripts and handouts. You’ll also find over 50 continuing education courses per year taught by leaders in the field of veterinary technology.  And sometimes you might just want to connect with people who understand the crazy world you work in.  Give VSPN a visit.

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Ken Crump  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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A Two Minute Overview of Peri-Anesthesia Hypothermia: The Problem & The Solution

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Go With The Flow – How to decide the oxygen flow rate for small animal anesthesia

Let’s begin this article by saying, “You’re right!”  Whatever oxygen flow rate you routinely use when anesthetizing your patients, whether it’s determined by careful calculation, by habit, or by setting it where you’re told to set it, “You’re right!”  This is one of those wonderful times when there is no universal agreement as to the proper flow rate for the various anesthesia breathing systems.  So, the pressure’s off.  You’re doing it right. [exhale]

Now that we’ve established there are different right ways to do it, we’re free to consider some new right ways to try.  But most of us are reluctant to tamper with the way we run anesthesia because we know anesthesia is part science and part magic.  And although we trust the science part, magic can be unpredictable. Before we attempt new right ways to do things, let’s take a look at some of the science behind how to decide the oxygen flow rate for small animal anesthesia.

How much oxygen does the patient need?

EllieThe oxygen flowing through an anesthetic gas machine is used to provide the patient with sufficient oxygen and to provide a carrier for the anesthetic agent.  Since the oxygen flow meter regulates the flow of oxygen per minute, we need to know how much oxygen our patient uses in a minute.  This is called the metabolic oxygen consumption rate per minute.  A safe estimate of oxygen consumption for small animal patients is 4 – 7ml/kg/min or 2 – 3ml/lb/min.  That means that my 14 pound cat Ellie is consuming around 40ml of oxygen each minute she monitors the dog’s behavior from her perch on the couch.

The kind of breathing circuit makes a difference

In general terms, there are two kinds of anesthesia breathing circuits: rebreathing and non-rebreathing.  Rebreathing circuits are most often circle systems that include a carbon dioxide absorber like soda lime.  The granular soda lime extracts exhaled carbon dioxide from the circuit which allows the gas to be breathed again or rebreathed.  Rebreathing circuits use lower gas flows, which decreases the cost and decreases pollution while it retains moisture and heat for the patient.  However the soda lime and unidirectional flow valves of a circle system are significant sources of resistance to breathing.  Smaller patients can’t overcome the resistance of a circle system and so you must use a non-rebreathing circuit.  Non-rebreathing circuits depend on high oxygen flow to remove exhaled carbon dioxide from the circuit between breaths.  High oxygen flow rates are inefficient, expensive, and carry heat and moisture away from the patient.  The decision to select a non-rebreathing circuit is often made by the weight of the animal, but it is actually a decision that the patient is too small to overcome the resistance of a rebreathing circuit.

Because of the simplicity of a non-rebreathing circuit, there is little debate over what recommended oxygen flow rate to use.  The AAHA recommended flow rate of 200ml/kg/min is generally accepted as appropriate.  That flow rate is 33 times more oxygen than is needed to meet a patient’s metabolic oxygen consumption each minute, but that high flow rate assures the patient will not rebreathe any of its exhaled carbon dioxide.

Rebreathing circuits offer a wider selection of flow rates.  The same rebreathing circle system can be operated as a closed system, a low-flow system, or a semiclosed system, depending on the selected oxygen flow rate.  They do not depend on the position of the pop-off valve.  The pop-off valve should always be fully open regardless of the oxygen flow rate.

The closed system flow rate meets the patient’s actual metabolic need for oxygen each minute.  It’s a style of anesthesia that demands a thorough understanding of the patient, the anesthetic agent, the gas machine, and the varying degrees of stimulation during the surgical procedure.  It’s best suited for anesthesia nerds.  Some variation of the semiclosed system is used in most small animal practices. The semiclosed flow rate well exceeds the patient’s metabolic requirement for oxygen, and a significant amount of excess gas is exhausted through the pop-off value.  The flow rate traditionally falls within 22 – 44ml/kg/min, most often settling at 30ml/kg/min.  Remember that the patient’s metabolic oxygen consumption rate per minute is less than 10ml/kg, so there is little concern that you won’t provide enough oxygen flow.

The vaporizer plays a role in your oxygen flow rate

AAHA also makes recommendations for the oxygen flow rate when using a rebreathing circle system.  The AAHA recommendation during the maintenance phase of anesthesia is between 200 and 500ml/min.  That flow is not calculated using the weight of the patient; it is the recommended setting of the flow meter.  They warn that the gas machine must be leak free for those flows to be effective, and concedes that flows that low may be out of the comfort zone of most of us.  However, they may not have considered the tendency for most vaporizers to over deliver at low flow rates.  Most vaporizers recommend oxygen flow rates not less than 500ml/min for accurate delivery of anesthetic gas.

Ready to try a new right way?

Whatever oxygen flow rate you routinely use when anesthetizing your patients, whether it’s determined by careful calculation, by habit, or by setting it where you’re told to set it, “You’re right!”  Now that we’ve taken a look at some of the science behind how to decide the oxygen flow rate for small animal anesthesia, maybe we’ll try another right way.

For more information, refer to AAHA Anesthesia Guidelines for Dogs and Cats

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Ken Crump is a writer and animal anesthetist and writes Making Anesthesia Easier for Advanced Anesthesia Specialists.  He makes dozens of Continuing Education presentations on veterinary oncology and anesthesia across the United States and in Canada.  Ken retired from Colorado State University in 2008. 

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