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

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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.


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


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 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.


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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Effective Patient Warming

A special guest blog post by Dr Colin Dunlop.

Colin Dunlop is a Diplomate of the American College of Veterinary Anesthesiologists. His research interests include hypothermia and prevention of anesthesia morbidity and mortality. He consults in anesthesia and critical care for small and large animal practices, biomedical research, and provides Continuing Education programs for veterinarians and veterinary nurses world-wide.



In my spare time I try to run a company that manufactures anesthetic delivery equipment and devices for patient warming, so please understand that some of the information I have included references specific warming devices, some of which we manufacture. The information sheets linked to this post try to fairly assess the capabilities of the various technologies and summarize information we have from published research and data from our in-house testing.  Written here is information I wrote in a hypothermia article some years ago.  It is simply to highlight the practicality of warming a hypothermic patient using IV fluid.   It is written in calories, and a Calorie is the heat required to raise 1 ml water by 1 degree Celsius.

Using the same information, you could also attempt to a very simplistic estimate of heat loss due to humidification of inspired air.  Heat conservation mechanisms (the nose!) play a big part, and intubation completely alters them. Recent work shows that the loss of heat of warmed gas from the Y-piece to the distal end of the endotracheal tube is up to 10 degrees Celsius!

IV Fluid Warmer Comparison

Not all IV Fluid Warmers are the same, and some can even be dangerous.  Test data from our evaluation of IV Fluid Warmers can be found here: IV Fluid Warmer Comparison. According to our tests, most are ineffective if their performance is tested 200mm downstream from the fluid warmer, which is equivalent to the distance to an anesthetized animal, draped for surgery.  Their performance is also affected by IV fluid flow rate – the larger the volume of fluid administered rapidly the less effective they are at warming.
Hanging bags of warm IV fluids in a cold operating room will produce the same kind of result as using an IV Fluid Line Warmer, as illustrated in the evaluation of IV Fluid Warmers linked above.

Hypothermia Review

The one-page guide linked here: Hypothermia Review summarizes information from various sources and includes an idealized graph that show heat loss occurs from the time of premedication, and that substantial heat loss occurs from induction to the time an animal is draped in surgery.  Once draped, heat loss tends to stabilize, but warming hypothermic anesthetized animals during surgery is very difficult. In fact even the best forced warm air heating systems, which are the most effective  way to deliver large volumes of heat, typically take 45minutes of “contact” before body temp begins to rise.  So using forced warm air heating devices to “increase” the body temperature during anesthesia for short procedures is probably not very effective. The rationale for pre-warming and preventing heat loss prior to draping for surgery is where our efforts should be focused.

Forced Warm Air Heating Review

The white paper linked here: FWAH Review and Cage Warming  shows the lag-time for warming and that not all these systems actually raise body temperature.  Also this paper makes the case for warming animals after premedication, before induction. Typically, warming animals during recovery, who became hypothermic in surgery animals, takes 1 to 2 hours of technician time.  Conversely, keeping animals at 37 degrees Celsius takes less than 45 min of “pre-warming”, and the blankets placed over animals in cages can be re-used.

Effective warming with fluids

Finally, use warm fluids effectively in severely hypothermic animals at the end of abdominal or thoracic surgery by pouring large volumes of warm (40 degrees Celsius) into the abdominal or thoracic cavity.  Be patient.  Wait several minutes before suctioning it out and then repeat this process three or four times, until the body temperature starts to rise. Then close the cavity.  At the same time use forced warm air heating, which will further increase the body temperature.

borderCalculating Calories and Warming With IV Fluids

A calorie (cal) is the amount of heat required to raise 1 ml (or 1 gm) of H2O 1 oC.
The specific heat of animal tissue is 0.83 cal/gm. Therefore a 10 kg dog requires 8,300 cal (8.3 kcal) to raise its temperature 1 oC.
Warming IV fluid administered during surgery:
A 10 kg dog administered IV fluid at 10 ml/kg/hr = 100 ml/hr.
If the fluid is warmed to 44 oC and the dog is 34 oC, then we can deliver:
(44-34oC =) 10 oC x 100 ml/hr = 1000 cal/hr

To Warm the 10 kg at 34 oC dog to 37 oC using IV fluid at 100 ml/hr requires:
(37-34 =) 3oC x 8,300 cal = 25,000 cal (approx) / 1000 cal/hr (from the IV fluid) = 25 hours!

Warming IV fluid may help prevent cold fluid exacerbating heat loss but is not effective for warming severely hypothermic animals.

Respiratory heat loss due to humidification is significant

During inspiration the nose and pharyngeal mucosa transfer heat and moisture to the air which is largely recovered during expiration, thus conserving heat. Air has a low heat capacity (0.24 cal/gm) and a low weight (1.3 gm/l). Saturated air holds 44 mg H2O/L at 37 oC which requires 24 calories. A 10 kg dog taking 20 x 100ml breaths/min ventilates 120 L/hr so requires (24 cal/L x 120 L/hr) = 2880 cal/hr for humidification. Intubation inhibits heat/moisture conservation via the nose, resulting in body temperature loss of about 1/3 oC/hr.

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IVECCS / ACVAA Conference – Washington, DC


We’re really excited to be exhibiting at the IVECCS / ACVAA conference being held at the Gaylord National Resort and Convention Center in Washington, DC.  It’ll be a great time to reconnect with old friends and show off what innovations Advanced Anesthesia Specialists brings to veterinary practice this year.  Spoiler Alert – we’re introducing a heated bain circuit this year.  Heated!  Yeah.  We’re excited about it too!

If you’re at the conference, stop by our booth #326 and say “hi”.

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Breathing New Life Into Veterinary Care

dealflow-bannerAdvanced Anesthesia Specialist’s Founding CEO receives recognition from the Australian Department of Industry and Science

dealflow is a quarterly magazine published by the Australian Government Department of Industry and Science. It showcases high-performing, small and medium sized Australian companies supported by the Accelerating Commercialisation element of the Government’s Entrepreneurs’ Programme.  The global efforts of Dr Colin Dunlop to improve anesthesia outcomes for small animals are applauded in the current issue of dealflow.  The article is reproduced below.

Better anesthesia for small animals.

Company founder Colin Dunlop anesthetizing a dog for a CT scan

Company founder Colin Dunlop anesthetizing a dog for a CT scan

Few companies can claim to help household pets while reducing greenhouse gases, but that’s what Advanced Anesthesia Specialists and its managing director Dr Colin Dunlop are doing.

The company is becoming a global leader in the design, manufacture and service of innovative veterinary anesthesia equipment, and continues to break new frontiers.

Dunlop says the mortality risk of anesthesia for animals under 20 kilograms, such as dogs and cats, is about 500 times higher than for humans. To improve survival rates the company has developed a new integrated anesthesia delivery system with Australian Government commercialization support.

Heated anesthesia breathing hoses reduce the risk of hypothermia during an operation

Heated anesthesia breathing hoses reduce the risk of hypothermia during an operation

The new system has three components. A Heated Smooth Wall Anesthesia tubing system, which warms the gas delivered to patients. This world-first system was released in Australia in 2014 and in UK and US markets this year. It helps to prevent hypothermia, the commonest complication of anesthesia and surgery.

“Hypothermia occurs in up to 85 per cent of anesthetized human infants and small animals,” Dunlop says.

The new Stingray anesthesia rebreathing circuit.

The new Stingray anesthesia rebreathing circuit.

The second component is the Stingray—the first low-flow, low‑resistance with rapid response rebreathing anesthesia circuit for patients under 20 kilograms. It improves on existing anesthesia technology and recycles exhaled breath, which also helps to reduce the risk of hypothermia.

The Stingray, which will be released to global markets in September this year, also reduces the release of environmentally harmful anesthetic gas into the atmosphere by up to 90 per cent.

The system’s third element is an anesthetic vaporizer which provides early warnings of problems during surgery. Dunlop says this novel system, which is in clinical trial stage, will help fill a gap in anesthetic training among veterinarians and veterinary nurses. It is due for release in mid-2016.

International usability is a vital element of the company’s design work. “We could never afford to design these products just for Australia, as the volume of potential sales here is too small to be cost-effective … we need to design equipment for use around the world,” he says.

Protecting intellectual property is also a company priority. “We have invested a lot of money and time in protecting the IP of our technology and devices and have over 13 families of patents, plus new patent applications lodged,” Dunlop says.

Australian Government Commercialisation Adviser, John Grew, has been assisting the company with its move into markets.

“Colin and his team have addressed the many challenges of refining the design, prototyping and pre-production of their products,” Grew says. “A lot of new IP has been developed and the products now coming to market reflect experience and insight.”

Advanced Anesthesia Specialists is owned by veterinarians and Dunlop’s main focus is to improve the odds for pets undergoing anesthesia in Australia and overseas.

“Our company is making a difference—by developing better, more sophisticated equipment we are improving outcomes for both veterinary staff, their patients and the environment.”

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The Cardiovascular System: A Pump, Some Pipes, and Fluid


One of my favorite mental pictures of the cardiovascular system is that of A Pump, Some Pipes, and Fluid. Broken down to this simple picture, it’s easier for me to interpret the information I gather as I monitor anesthetized patients.

A Pump

The heart is at the top of this minimalist’s view of the cardiovascular system by acting as a pump.  Its job is to pump blood around the body. The left side of the heart pumps oxygenated blood from the lungs to the rest of the body. The right side pumps stale blood from the body back to the lungs for a fresh supply of oxygen.

Some Pipes

The pipes, of course, are those estimated 60,000 miles of veins and arteries distributed throughout the body.  Most blood vessels can alter their size in order to accommodate the necessary flow of blood.  When a vessel’s interior grows larger to allow more blood flow, it’s called vasodilation.  When it shrinks down to decrease blood flow it’s called vasoconstriction.  Under normal circumstances, the vessels automatically vasodilate and vasoconstrict to help regulate blood flow through the body.  However, many anesthesia drugs alter the body’s ability to respond automatically in this manner.


It is generally accepted that most domestic animals have blood volumes of about 7% of their body weight (cats have a little lower percentage).  That equals about 70ml per kilogram or about 35ml per pound of body weight.  That means your 60 pound Labrador has a blood volume of about half a gallon.  When you consider that a half gallon of blood is pumped through 60,000 miles of blood vessels, you realize that it can’t be all places all the time.  The body is constantly making choices to route blood where it is most needed at any given point in time.

Pressure is the driving force for blood flow through capillaries that supply oxygen to organs and tissues. Blood pressure is needed to propel blood through vascular beds, with priority to those of the brain, heart, lungs and kidneys.  When I notice a drop in blood pressure, I immediately run through this simplified picture of the cardiovascular system. Why is the pressure dropping? Is the problem with the pump (ie not pumping hard enough or fast enough)? Is the problem with the pipes (ie vasodilation or positional occlusion of major vessels)? Or is the problem with the fluid (ie blood loss or vascular pooling)? The answers to these questions can help me anticipate a corrective treatment.

Anesthesia guidelines from the American College of Veterinary Anesthesia and Analgesia (ACVAA) and the American Animal Hospital Association (AAHA) urge us to monitor blood pressure during anesthesia, yet specialists say that blood pressure equipment alone is not the main ingredient to a smooth anesthetic event.  It’s the anesthetist’s knowledge that provides the greatest margin of safety for the patient.

Ken writerKen 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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