Low Flow Anesthesia & Patients Waking Up – Part 4

“Why would my patient wake up just because I reduced my oxygen flow rate?”

We originally posed the question in Part 1 of this series. It’s a common challenge that often leads to abandoning the practice of low flow anesthesia. It’s also a problem overlooked in the AAHA Anesthesia Guidelines for Dogs and Cats, when they talk about oxygen flow rates for circle systems and recommend, “During the maintenance phase, total O2 flow rate should typically be between 200 and 500 mL [per minute].” I agree with that recommendation, but there’s more to the story. The short answer to the question is that the patient wakes up because it’s not getting enough anesthetic gas.

In Part 2 and Part 3 of this four-part series, I filled in some of the background you need for rest of the story. In this final Part 4, we’ll look at how the flow of oxygen through a vaporizer determines the actual dose of anesthetic the patient receives.

Again, stay with me. This gets a little fussy, but I guarantee it’s worth it. And if you want to play along, grab a calculator – math is involved. But I promise it’s no more complicated than calculating a tip at a restaurant. As a bonus, at the end of this post you’ll find a link to the entire series compiled as one printable PDF.

The most important take-home message from Part 3 is that the anesthetic gas coming from a vaporizer is a percentage of the total oxygen flow. That percentage doesn’t actually tell us the dose of inhalant we are presenting to the patient, it just tells us that a known portion of the gas flowing from the vaporizer is inhalant.

Grab your calculator and we’ll break this down to its parts.

Let’s say that you have your oxygen flow set at 1 liter per minute flow, and your vaporizer set at 1.5%. With those settings, how many milliliters of volatilized anesthetic are you presenting to the patient every minute? I’m pretty specific about the wording of that question. Administering anesthetic gas is not like administering any other drug. When you draw up a syringe of a drug and inject it into a patient, you know that the patient is getting all of the drug in the syringe. Therefore, you know how much drug is administered. When administering anesthetic gas, the patient uses significantly less than the total amount of the oxygen flow, and therefore oxygen and inhalant are being exhausted through the pop-off valve. So we can only calculate the amount of inhalant being presented to the patient each minute. To determine that actual dosage of inhalant administered to the patient, you’d need to use an agent monitor. Agent monitors, by the way, are great tools to understand how all of this works. If you have one anywhere near you, play with it. They’re awesome.

Back to the question. You have your oxygen flow set at 1 liter per minute flow, and your vaporizer set at 1.5%. How many milliliters of volatilized anesthetic are you presenting to the patient every minute?

You’re being asked what 1.5% of 1 liter is. The metric system is simple when you remember all of the rules. In this case, it’s helpful to know that 1 liter = 1000 milliliters. Another useful hint when talking about numbers is whenever you hear the word “of”, it means “times”. So the numeric expression is 1.5% X 1000 milliliters. Or 0.015 X 1000 = ?

The answer is 15ml. The entire thought could be stated like this: With the oxygen flow set at 1 liter per minute, and the vaporizer set at 1.5%, you are presenting 15 milliliters of inhalant anesthetic to the patient each minute.

Is the patient using all of that 15ml of inhalant anesthetic presented each minute? No. And remember, we don’t know how much of the 15ml it’s using.

Now we decide to try low flow anesthesia. We turn the oxygen flow rate down from 1 liter per minute to 500ml per minute, and leave our vaporizer dial setting at 1.5%. How many milliliters of volatilized anesthetic are we presenting to the patient every minute now? The numeric expression is now 1.5% X 500 milliliters. Or 0.015 X 500 = ?

The answer is 7.5ml. With the oxygen flow set at 500ml per minute, and the vaporizer set at 1.5%, we are presenting 7.5 milliliters of inhalant anesthetic to the patient each minute. Even though the vaporizer dial setting is the same in each instance, the actual amount of anesthetic gas presented to the patient at 500ml oxygen flow is half of the amount presented at 1 liter flow. Half. It’s not surprising the patient may become more lightly anesthetized.

When we challenge ourselves to step out of our comfort zone by reducing the oxygen flow rate during anesthesia, it’s important that we see the broader picture and anticipate our response to changes in the patient. The patient will often become more lightly anesthetized and may even wake up. This often leads anesthetists to abandon the idea of low flow anesthesia. But now that we understand the relative potency of anesthetic gas, and how a vaporizer’s delivery is tied to the oxygen flow rate, we can anticipate that the patient may get lighter, and we can be prepared to increase the vaporizer dial setting.

There are many great tools for today’s veterinary anesthetist, but the most important monitoring tool is the person sitting at the head of the patient. AAHA summed up their Anesthesia Guidelines with this closing statement, calling out the key to success: “Successful anesthetic management requires trained, observant team members who understand the clinical pharmacology and physiologic adaptations of the patient undergoing anesthetic procedures, as well as the use of anesthetic and monitoring equipment.”

Click on this link for all 4 Parts on a single printable PDF.

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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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Low Flow Anesthesia & Patients Waking Up – Part 3

For Part 2 in this series, click on this link.

“Why would my patient wake up just because I reduced my oxygen flow rate?”

In this series we dive deeply into answering that question. We’re approaching the answers from three directions:

  1. We looked at the anesthetic gas itself in Part 2
  2. We’ll look at how a vaporizer works in this post
  3. We’ll look at the flow of oxygen through a vaporizer in Part 4

In this post we explore the role of the vaporizer, and how it turns a liquid into a precise concentration of gas. The big concepts of the process are pretty simple to understand if you don’t get too tied up in the details. The picture below is used by one of our engineers to explain how vaporizers work. We’ll use it to follow the flow of oxygen through a vaporizer and see how it picks up anesthetic.

Wow. So many colored arrows going every which way. Let’s break it down and follow just the oxygen. First we’ll look at just the top half of the vaporizer.

I’ve stripped away everything but the oxygen flow in this top-half view. I have highlighted the “Concentration Control Dial” label and showed what turning the setting to OFF will do to the oxygen flow. Notice the oxygen just flows straight through the vaporizer with no oxygen diverted into the lower half of the vaporizer.

In this view, everything is the same except the highlighted “Concentration Control Dial” is turned ON. Notice a percentage of the flow is diverted into the lower half of the vaporizer, while the majority of the flow still passes directly through and out of the vaporizer. Now let’s slide the image a little lower and follow the percentage of the total flow of oxygen that was diverted into the bottom of the vaporizer.

The dial at the top of the vaporizer – the one we use to set our desired percentage of anesthetic gas – actually diverts a percentage of the oxygen flow downward, through the wick and into the sump holding the liquid anesthetic. There, it becomes saturated with volatilized anesthetic and travels through baffles and up the vaporizer to rejoin the main stream oxygen flow.

In this view we see the saturated oxygen moving through baffles and up through the wick to rejoin the main flow. Once it merges with the main flow of oxygen, the combined gases flow out of the vaporizer to the breathing circuit. The percentage of the flow of oxygen that was diverted from the mainstream and into the sump of the vaporizer has now been saturated with anesthetic gas. When it recombines with the main flow, that diverted percentage becomes the percent of anesthetic gas you set with the vaporizer dial. In other words, if you set the vaporizer dial at 2%, then 2% of the main oxygen flow is diverted down into the vaporizer, through the wick, into the sump where it’s saturated with anesthetic gas, and back up to rejoin the main oxygen flow. As it mixes into the main flow, it changes the concentration of the gas out-flowing from the vaporizer. The mixed gas leaving the vaporizer is now 98% oxygen and 2% anesthetic gas.

It’s interesting to learn the general principles of how a precision vaporizer works, but the question we’re trying to answer is why a patient might wake up simply because you turn down your oxygen flow rate. The most important message to take from this section is that all of this concerns a percentage of the total oxygen flow. In the final part of this four-part series we’ll look at the role that oxygen flow rate plays on anesthetic delivery, and how 2% of one oxygen flow rate does not equal the anesthetic delivery of 2% of another oxygen flow rate.


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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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Low Flow Anesthesia & Patients Waking Up – Part 2

For Part 1 in this series, click on this link.

“Why would my patient wake up just because I reduced my oxygen flow rate?”

The short answer is that your patient isn’t getting enough anesthetic gas. We’ve established that even at a lower oxygen flow rate, the patient gets more than enough oxygen (see the post Go With The Flow ). If the patient gets enough oxygen, and the anesthetic gas is mixed in the oxygen, why does it not get enough anesthetic gas?

Over the next three posts in this series we will dive deeply into answering that question. We’ll approach the answers from three directions:

  1. We’ll look at the anesthetic gas itself
  2. We’ll look at how a vaporizer works
  3. We’ll look at the flow of oxygen through a vaporizer

In this post we’ll explore how to calculate an appropriate dosage of an anesthetic gas. We’ll look at anesthetic gas as a drug. Stay with me. This gets a little fussy.

A Brand New Gas

We all have our routines we follow when we’re using an anesthetic gas we’re familiar with. We set the vaporizer on a number that’s comfortable. And whether your starting point is determined by careful calculation, habit, or doing what you’re told, it usually works out just fine. They’re all OK methods when we’ve had some experience with the gas we’re using. However, it falls apart when we make changes to the any parts of our routine (like changing the oxygen flow rate) or when we use a gas for the first time.

For the sake of example, let’s look at a gas that I just made up: C3PO. It’s so new, it doesn’t even have a name – just a bunch of letters and numbers. This revolutionary new anesthetic gas has countless advantages, no disadvantages, it’s safe for all patients, and costs less than water. It’s the kind of gas I would make up if I was making up an anesthetic gas (which I just did). The only drawback is that you will need a new vaporizer. Luckily, the distributor gave you the vaporizer.

What do you need to know about this gas before you are ready to use C3PO?

On your list will be how much to use. You are comfortable administering anesthetic gas through a vaporizer. And you’re comfortable turning the dial up a little or down a little according to your patient’s responses during a procedure, but you may have forgotten why your vaporizer dial settings start out higher with Sevoflurane than with Isoflurane, or why the settings for Isoflurane start out higher than for Halothane.

MAC – The Great Equalizer

To jog your memory, let’s revisit the way relative potency is established for different anesthetic gasses. Relative potency is comparing the effectiveness of one gas to another. That’s where MAC comes in. MAC stands for Minimum Alveolar Concentration, but that’s only the first three words of the definition. The full definition is the “Minimum Alveolar Concentration required to prevent purposeful movement from a noxious stimulus.”

I’ve always liked how concise that definition is; just a handful of important words. To start, it tells you that it wants to determine the least amount of gas needed at the smallest part of the lungs, the alveoli, where gas exchange occurs in the blood. Next it describes the response it’s looking for. It doesn’t look for subtle responses like an elevated heart rate or increased respiration rate, it defines the response as movement. And more specifically, purposeful movement. And finally it describes the stimulation as noxious. On a scale of degrees of stimulus, noxious ranks pretty low. More like annoying than painful. So, another way to write the definition of MAC might be “the least amount of gas required to keep a subject from pulling its foot back when you pinch its toe.” As a matter of fact, that exactly describes a MAC study. The noxious stimulus is usually a toe pinch. The purposeful movement is usually the subject pulling its foot back.

It’s a concise definition of a somewhat non-specific comparison. To make it even more non-specific, MAC is an ED50. That means that it establishes an effective dose (ED) for only 50% of patients – therefore ED50. And there are several things that can affect a patient’s requirement for the MAC of anesthetic gas. Those things include the patient’s physical status (a very sick animal won’t need as much anesthetic gas as a bouncy, healthy one), sedatives and analgesics that the patient may have received, the degree of surgical stimulation, and of course the patient may just be in the other 50% group. Knowing the MAC of a gas is as useful as knowing what neighborhood your friend lives in: it might not get you to their front door, but it helps narrow the search.

To apply what we know about MAC to the operating room, we have to start with logic, then adjust to the individual patient and procedure. Once we know the MAC of an anesthetic gas, we know how much gas is required to prevent a patient from reacting to a toe pinch. Since the stimulation in surgery is likely to be greater than pinching a toe, logic dictates that the patient will need a higher dosage of anesthetic gas. In other words, the patient will need more than [1 X MAC] of the anesthetic gas you’re using. But how much more?

From Theory to Practice

It is generally accepted that [1.25 to 1.5 X MAC] is required for surgery – also known as surgical MAC. The question may be asked like this: “What factor of MAC is required for surgery?” And the answer is 1.25 – 1.5 MAC.

The MAC of commonly used anesthetic gases are published. For reference in this example, the MAC of isoflurane is ~1.4%, and the MAC of sevoflurane is ~2.1%. The gas that I made up, C3PO, has a MAC of ~9%. So, if we are to calculate the surgical MAC of each of these three gases, it would look like this:

  • Isoflurane = surgical MAC of about 1.75% to 2.1%
  • Sevoflurane = surgical MAC of about 2.6% to 3.2%
  • C3PO = surgical MAC of about 11.25% to 13.5%

Of course, we have to remember the limitations of MAC. The patient’s physical status, the other drugs it has on board, the degree of surgical stimulus, or that it may just fall into the other 50% group who needs more or less anesthetic gas to achieve the same effect, all impact the actual amount of gas needed. Bearing all of those variables in mind, we know that isoflurane delivered at 1.75% should achieve the same effect as sevoflurane delivered at 2.6%. And if we are presented with a gas we’ve never used, and we know the MAC of that gas, we know the neighborhood where we can start looking for the dosage we need to achieve the effect we want.

In this post, we’ve broken down some of the mysteries behind administering an effective dosage of anesthetic gases. The big question we’re answering in this series is why a patient might wake up simply because you turn down your oxygen flow rate. The short answer is that your patient wakes up because it doesn’t get enough anesthetic gas. This section sheds light on how much anesthetic gas is enough, and how to calculate enough gas, regardless of the gas you’re using. Part three of this four-part series looks at your vaporizer and how oxygen flow rate affects anesthetic gas delivery.


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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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Low Flow Anesthesia & Patients Waking Up – Part 1

“Why would my patient wake up just because I reduced my oxygen flow rate?” At first, this question perplexes many of us who try low flow anesthesia. Later we realize that if our patients don’t get lighter when we reduce our oxygen flow rate, we have likely been over-anesthetizing them in the past. In this four-part series we’ll be taking a closer look at the advantages and disadvantages of low flow anesthesia, and dive deeply into how it all works.

“Quick and Dirty” Calculations

It’s no secret that I am a fan of low flow anesthesia. When your patient is on a circle system, there are some distinct advantages to reducing the oxygen flow rate: patients have a tendency to stay warmer during anesthesia, it helps retain moisture, it’s an efficient use of anesthetic gas, it costs about 90% less than using a non-rebreathing circuit, and it produces about 90% less pollution. Low flow anesthesia yields significant advantages to your patients, your practice, and your environment.

I am careful here to mention low flow anesthesia in the context of a circle system. Low flow techniques cannot be used with a non-rebreathing circuit. You’ll find more information about circuits in my blog post Rebreathing or Non-Rebreathing. That post will help you decide when to use a circle system. It’s more often than you may think. And to clarify further, here are some “quick and dirty” calculations for oxygen flow rates that are generally accepted as “low flow”.

  • 35 pounds and under? Set oxygen flow rate at 500ml / min
  • Over 35 pounds? Calculate oxygen flow at 30ml / kg / min
  • [Hint] You don’t get to 1 Liter flow until you get to a 70 pound Labrador

There’s more information about oxygen flow rates in my blog post Go With The Flow – How to decide the oxygen flow rate for small animal anesthesia.

OK. Let’s do this!

Now that you’ve decided to challenge a long standing more-is-better oxygen philosophy, and you’re ready to try low flow anesthesia, this is a good time to ask about the downsides. In other words (as we’ve all asked all too often), “What could go wrong?”

When you change something as fundamental as the oxygen flow rate, you have to expect that other things may change as well. Here’s a short list of challenges you may see.

  1. You may scare your DVM.
  2. Your patient may inspire CO2.
  3. Your patient may wake up.

Anything out of the ordinary related to anesthesia is going to capture your doctor’s attention. So talk with them before you make changes to the way you normally do things. Explain the advantages and disadvantages of low flow oxygen delivery, and review your understanding of what to expect. Your doctor may also have experience to share with you. It’s a great way to put everyone on the same page.

If you monitor CO2 during anesthesia with a capnograph, you’ll be able to easily detect inspired carbon dioxide by watching the baseline of the waveform during inspiration. Notice how the wave form depicted below does not return to “0” at inspiration, but rather continues to migrate upward. At each inspiration phase the wave form should return to zero. If it doesn’t, the capnograph is detecting carbon dioxide in the inspired air.

There are a few reasons this may happen but for the sake of this conversation, number one on the list is insufficient oxygen flow. And it’s easily resolved: turn up the oxygen flow rate. Since you may be trying low flow anesthesia for the first time, you may want to scrap the whole idea right now and go back to an oxygen flow rate you are more comfortable with. Resist that urge for a little longer. Turn the oxygen flow rate up just a little, and wait for a few breaths. Repeat until the wave form comes back to base line on inspiration. It won’t take long.

If a capnograph is still on your wish list, all is not lost. Hypercapnia (elevated levels of carbon dioxide) has some pretty identifiable signs. If you are trying low flow anesthesia and your patient is experiencing rapid, shallow breathing (tachypnea) and/or red mucous membranes, hypercapnia is a pretty good guess as to the cause. Fortunately, the remedy is the same as if you had a capnograph: slowly turn up the oxygen flow rate until it resolves.

Going back to the first on the short list of challenges you may see (scaring your DVM), keep your doctor in the loop through all of this so everybody stays on the same page.

The third of the three challenges – Your Patient May Wake Up – is due simply to an insufficient amount of anesthetic gas. Over the next three posts in this series we will dive deeply into answering the question, “Why would my patient wake up just because I reduced my oxygen flow rate?” We’ll approach the answer from three directions:

  1. We’ll look at the anesthetic gas itself
  2. We’ll look at how a vaporizer works
  3. We’ll look at the flow of oxygen through a vaporizer

Next up, understanding how to calculate an appropriate dose of a gas – a look at anesthetic gas as a drug.


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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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Where to get answers to your anesthesia questions

Screenshot (3)I typed the title of this post into a Google search engine and got 13,700,000 results in 0.60 seconds.  When I modified my search to where to get answers you can trust to your anesthesia questions, I got only half as many results, and it took 0.05 seconds longer.  But still, having 7,000,000 results to sift through is daunting.  As excited as I am to live in a time when I can get answers to any question in fractions of a second, the real question then becomes whose answers can I trust?  In this blog post I am going to introduce two valuable resources for anyone who does animal anesthesia.

VSPN

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Maybe you’ve heard of VIN, the Veterinary Information Network.  VIN pioneered providing instant access to vast amounts of up-to-date veterinary medical information, turning subscribing practitioners who might have once 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 medical information, yet 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!

When you click on this link, you’ll go directly to the VSPN.org home page.  One of the first things to do here is to apply for membership.  Your membership application may take a couple of days to process, but in my experience the folks at VSPN are very responsive.  It’s free to become a member, and the application process is in place only to assure all of us that we are exchanging ideas and experiences with others who share our background and interests.  It can be tough to work in this industry sometimes and it’s nice to have a place to go where everybody understands the pressures.

Nothing makes anesthesia easier than having good up-to-date information at your fingertips, provided by colleagues and experts in the field. The VSPN worldwide community is made up of people just like us, who 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 to VSPN membership 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.

VSPN is a great place to find out others’ experience with Feliway for improving anxiety in cats, or to learn tips for getting finicky dogs to eat.  Should you increase the IV fluid rate to treat hypotension during anesthesia? Or deliver additional IV fluids as a bolus?  Which is better? Why?  Who has tried the new heated breathing circuits?  Do they work?  Do you like them?  And then sometimes you might just want to connect with people who understand the crazy world you work in.  Give VSPN a visit.

NAVAS

Where VSPN covers every imaginable aspect of veterinary practice, the North American Veterinary Anesthesia Society (NAVAS) is a new collaboration of veterinary anesthesia specialists. As a member of NAVAS, you have access to anesthesia experts from the American College of Veterinary Anesthesia and Analgesia (ACVAA), the European College of Veterinary Anesthesia and Analgesia (ECVAA), the Association of Veterinary Anesthetists (AVA), the Academy of Veterinary Technicians in Anesthesia and Analgesia (AVTAA), and more.  And the best part is that you don’t have to be a specialist to join.  Never has the knowledge and experience of so many specialists in the field of veterinary anesthesia been so available to regular people like you and me.  The heart of the NAVAS mission is to support all animal care givers engaged in providing veterinary anesthesia and analgesia.

NAVAS is a much needed bridge of veterinary anesthesia specialties, expanding their ability to better meet their mission of improved quality of animal anesthesia.  Not only is it a collaboration of anesthesia experts, VetBloom – a premier online education platform – has also teamed up with NAVAS to provide structured certificate and RACE-approved CE training courses in all aspects of veterinary medicine.  Whether you are looking to continue your education with online veterinary courses, or you’re a practice in search of a systematic set of educational materials, VetBloom has developed and tested the training that you are after.

NAVAS will continue to expand its offerings in coming months to include podcasts, journal club, literature reviews, how-to videos, species-specific drug formularies, and anesthesia safety recommendations. Combine all of this with their forums, a NAVAS blog, and other members-only resources, and you have a valuable one-stop compendium of veterinary anesthesia expertise.  Just follow this link to the NAVAS website.  It definitely deserves a look.

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When I searched Google for how to end a blog post, it found 2 billion results in about half a second.  I have to admit that I didn’t dive too deeply into the 2 billion.  I decided to “Do a recap of the main message,” so we can all get back to our lives.  I set out to introduce two valuable resources for anyone who does animal anesthesia. I guess we’re done here.


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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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When to Change Soda Lime

Soda Lime Usage

Your anesthetized patient exhales carbon dioxide.  But you already know that.  And you know that you are exhaling carbon dioxide as you’re reading this.  You may not have thought about it in awhile, but you also know how carbon dioxide gets in your lungs so you can exhale it. Carbon dioxide is the waste gas that is produced as part of the body’s energy-making processes. The lungs and respiratory system facilitate oxygen absorption into the body, and enables the body to get rid of carbon dioxide in the air breathed out.  To extend our glance at this system of gas exchange to a global view, plants absorb carbon dioxide that we exhale and they exhale oxygen.  It’s a pretty efficient relationship.

Exhaled carbon dioxide is handled differently in the two different types of anesthetic breathing systems.  With a non-rebreathing system (Bain, Mapleson, etc), the exhaled carbon dioxide is managed with high oxygen flow rates.  The oxygen flow rate must be high enough to move the exhaled carbon dioxide out of the circuit and refill the circuit with fresh gas before the patient takes its next breath.

Exhaled carbon dioxide is managed differently in a rebreathing circuit.   By adding a carbon dioxide absorber into the circuit – thereby taking the exhaled carbon dioxide out of the breathing circuit – we are able to use lower oxygen flow rates which saves oxygen, anesthesia gas, patient body temperature, and money.  The carbon dioxide absorber most commonly used is a soda lime canister.

Soda lime is a granular mixture, consisting predominantly of calcium hydroxide, together with small amounts of potassium hydroxide and sodium hydroxide.  A color change indicator is also usually in the mixture which lets you know when the soda lime is spent and no longer absorbing carbon dioxide.

Since soda lime absorbs carbon dioxide, it stands to reason that it will reach its capacity at some point, and will need to be changed.  How do we know when that is?

There is a color change indicator in soda lime that many rely on as a signal to change the soda lime.  And it’s not to be ignored.  However, it can’t be the definitive indicator of saturated soda lime because the color changes back to white after prolonged disuse.  So, if the color changed on yesterday’s late shift but the soda lime wasn’t changed after that shift, when you come in the next morning, the color will have turned back to white and you wouldn’t know it needs attention.

The manufacturers of soda lime set the specific recommendations for changing the soda lime – and again, these recommendations are not to be ignored.  They recommend that soda lime be changed after 6 – 10 hours of use.  Although that’s a wide range of time, and it doesn’t take into account the size of your absorber relative to the size of the patients (larger animals on smaller-capacity absorbers will saturate the soda lime more quickly), there is a take-home message here: record the time of use somewhere on the gas machine.  Follow this link to download a chart that you can print and attach to your machine each time you change the soda lime.  It’ll help a lot.

Watching for color change and using a chart to record the time of use are great guidelines.  The final test is in your hands – literally.  Feel the soda lime granules.  Fresh soda lime crumbles easily when you crush it between your fingers.  Exhausted soda lime feels hard like tiny gravel between your fingers.

Put your gas machines on a regular maintenance program.  Set a time interval that you and your crew will break down the machines, clean them, and test them to be sure everything is working the way it should.  The interval will depend on how often you use the machines.  If they are used everyday, consider a weekly interval.  If they are used only occasionally during the week, the interval could be stretched out to monthly.  The important thing is that the maintenance day for the machines is set and regular.  Between maintenance days, watch for the soda lime color to change and tick off the time of use on the soda lime chart. But during each regular cleaning, be sure to test the soda lime with your fingers, and let that be the definitive indicator to change the soda lime.

For more information about carbon dioxide absorbers, click on the links below.

Soda Lime Usage Chart download

Sodasorb Manual of CO2 Absorbtion


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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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Are you ready for Dental Month?

toothDid you know that February 9th is National Toothache Day? No, really.  It’s a thing.  Along with National Toothache Day, American Heart Month, Black History Month, Valentine’s Day, and Creative Romance Month (again, it’s a thing), February boasts the AVMA-sponsored National Pet Dental Health Month.  It’s a time when many pet owners schedule their pet’s annual checkup.  Now is a perfect time to review ways to meet some of the challenges that come with anesthetizing pets for dental procedures.   At the end of this post, and interspersed throughout it, you’ll find links to more information that you might find useful.

Veterinary anesthesia is always improving and is safer now than it ever has been.  But we can’t dismiss our clients’ concern about the risks of anesthesia for dentistry.  After all, they are not unfounded. In a two-year study of nearly 200,000 pets, dental procedures ranked number three as a surgical procedure likely to result in death. Patient age, underlying systemic disease, length of anesthesia, and hypothermia are listed among probable contributors to the greater anesthetic risk among dental patients. Of those contributors, we have most control over preventing hypothermia.  Studies show that 84% of anesthetized dogs and 97% of anesthetized cats experience hypothermia. These studies clearly indicate hypothermia is one of the most predictable complications of anesthesia, and veterinary staff needs to be proactive in preventing heat loss and to monitor body temperatures continuously.

Preventing hypothermia has traditionally focused on skin warming and conserving body surface heat, but remember 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. Here are some tips for safely warming dental patients suggested by Portland’s award winning veterinary hospital and training facility, DoveLewis.

  • Place the patient on a solid surface like a mat. Laying a patient on a towel over a water table provides more surface area to lose body heat.
  • Place the patient on any type of approved heating pad.
  • Bubble wrap layers over the patient to 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.

Breathing cold oxygen from an anesthetic gas machine can be a major contributor to cooling anesthetized patients, especially in the early stages, 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. An endotracheal tube bypasses the nose and pharyngeal mucosa and delivers cold gases directly into the lungs. That leaves no chance of temperature recovery during exhalation. This costs 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 and delivering it 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 by Advanced Anesthesia Specialists of Australia. These heated circuits have a heating element embedded 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 a patient 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.  The heated circuit is distributed by DarvallVet in North America.  Heated breathing circuits offer a new 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.

In addition to surface warming with an approved veterinary warming device (for instance a warm air blanket system) and the use of a heated breathing circuit, there are other ways to protect your patients from losing body heat.  High oxygen flow rates are not only expensive and an inefficient use of anesthetic gases, they also rapidly siphon body temperature from your patient.  Selecting a circle system rather than a non-rebreathing, and then running the circle system at a lower oxygen flow rate is an excellent way to prevent heat loss from your patient.  Visit the DarvallVet website to learn more ways to help keep your patients warm before, during, and after anesthesia or give them a call at 866-931-3292 to discuss any of your anesthesia challenges.  Their anesthesia staff has over 35 years’ experience as animal anesthetists.

Here’s a list of additional referenced posts on this subject (click title to view):

 


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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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ECG and Me – What do I need to know?

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Look at a list of recommended anesthesia monitoring tools. You’ll always see an ECG, usually near the top of the list.  The 2011 AAHA Anesthesia Guidelines for Dogs and Cats even lists it first on their list, although I can’t tell if there is significance to the order of the list.  But it’s safe to say that an electrocardiogram is highly recommended.  At one time or another we’ve all watched an ECG wave form crawl across a screen.  And we’ve all used little sayings to remember how to hook up the electrodes.  Sayings like “white right, snow over grass, brown ground, and smoke over fire…”.  Frankly I struggled to memorize all of those cute little ditties until I realized the placement location is written on each electrode.  Then I promptly forgot them all.  For the longest time, that’s all I really knew about an ECG: where to connect the electrodes.

It turns out, even that tidbit of knowledge is flawed.

Since I’m using an ECG monitor on my anesthetized patients, what should I really know about it?  Let’s start with the basics of what the ECG tells me and what it doesn’t tell me (but I may think it does).

  • ECG and EKG mean the same thing. The first ECG/EKG was manufactured in Germany where all things ‘cardiac’ begin with the letter “K”
  • It tells you that there is electrical activity at the heart
  • It graphs a tracing of the heart’s electrical activity
  • It does not tell you that the heart is responding to the electrical activity
    • The ECG does not tell you that the heart is beating

That last bullet – it doesn’t tell you that the heart is beating – was a bit of a wake up call for me.  How can that be?  The answer goes to the previous bullets: the ECG graphs the heart’s electrical activity, but it doesn’t tell you that the heart muscle fibers are responding to the electrical activity.  Oh, it will in time, but not immediately.  At least not at my level of skill interpreting ECG wave forms.

As a veterinary technician who does anesthesia, where should my level of skill interpreting ECG wave forms be?  I find electrocardiography fascinating, and I’ve spent long hours with a cardiologist looking at wave forms and cardiac ultrasound images.  But as an anesthetist, I only need to know one thing about an ECG wave form: what normal looks like.  And if it looks anything other than “normal”, I draw the doctor’s attention to it.

A normal ECG wave form repeats a PQRST-1series of ‘blips’ in a row.  Each normal blip has a designated letter identifier.  The full complex contains the waves “P, Q, R, S, and T” with Q, R, and S usually combined and referred to as “QRS”.  What each wave indicates with reference to the heart’s activity, is a conversation for another time.  The important thing for us is that we see each of the lettered waves appear, and in order.

All of that said, sometimes the “P” wave is missing.  Sometimes the “T” wave looks upside down.  In other words, sometimes normal doesn’t look exactly normal, and it will take a little time, practice, and conversations with your DVM to recognize when a deviation from normal is significant.

PVC RaW EcG

But once we’ve established what a normal ECG is supposed to look like, it gets pretty easy to recognize what abnormal looks like.   For instance, the image on the left is very obviously abnormal.  You can easily see the normal order of the P, QRS, and T waves is interrupted by a very abnormal wave complex.  This merits the attention of the doctor.

So the responsibility is on us to establish a readable ECG wave form to start with. Picture1 copyIf our initial ECG wave looks like the one on the right, we have no hope of identifying anything normal or abnormal.  It’s not enough to clip the leads to the animal.  We need an ECG tracing we can use.

Let’s talk about clipping the electrodes to the animal, because this sometimes requires some creativity.  Earlier I mentioned that the one thing I knew about the ECG (where to clip the electrodes) is flawed.  It helps to understand ‘flawed’ by realizing what the ECG actually does. The ECG detects and graphs electrical activity between two electrodes.  That’s all.  Most practices use an ECG with three electrodes, which reads the electrical activity between any two, and the third just has to be in contact with the body.  The electrodes are labeled RA (Right Arm), LA (Left Arm) and LL (Left Leg).standards

Now, stay with me because this is where it gets a little fussy.  The two electrodes between which the ECG reads electrical potential are determined by the “LEAD” you select on the ECG machine.  The standard leads are described in the picture on the left: Lead I, Lead II, and Lead III.  Most ECG machines default to read Lead II, so unless you actively change that setting, your ECG will default to read the electrical activity between the electrodes labeled RA and LL.  That means the electrode labeled LA need only be in contact with the body.

In order for the ECG to read a Lead II, the heart must be between standards copythe RA and LL electrodes.  Again, the LA electrode can be anywhere, just as long as it is in contact with the body.  To illustrate, imagine you decided to clip the RA electrode near the paw of the right foreleg, and the LL electrode a little farther up the same leg (as shown). You would not get a readable tracing of a Lead II because the heart is not between the two electrodes.  But that’s the only thing you have to remember about placing electrodes: the heart must be between the two electrodes that are reading electrical activity and the third electrode must be in contact with the body.  Follow that rule and you’ll get an ECG tracing you can use every time.  That leaves us a lot of opportunity to be creative about where we place the electrodes.  And often that can be really helpful.

Picture1This patient requires that we be creative about where we place the ECG electrodes.  The right forelimb is to be amputated.  With your ECG set to a Lead II, where would you attach the RA, LL, and LA electrodes so they would not interfere with the surgery, but would still offer a useful ECG tracing?  There are any number of correct answers to this question.  My choice would be to clip the LL electrode to the cat’s left hind leg, and then clip the RA and LA electrodes together and slide them into the cat’s mouth.  I would not clip them to the tongue or oral mucosa (ouch!).  I would just slide them into the mouth.  The moist oral cavity provides good contact to the electrodes, and having them clipped together assures that the LA electrode makes contact with the body.  The heart is between the two electrodes that the ECG is reading (RA and LL) so I will get a useful tracing. And I will have access to all three electrodes throughout the surgery in case they need adjustment.  Simple.  Creative.  Effective.

We are not cardiologists, so perfectly placed electrodes and carefully positioned patients are not necessary for us to get good information from an ECG tracing.  That allows us to “hack” the placement of electrodes to suit awkward situations we sometimes find our patients in.  As long as we learn to recognize abnormal wave forms and draw attention to them when we see them, and remember to keep the heart between the right two electrodes, the ECG is a simple and useful monitoring tool for the veterinary anesthetist.

 


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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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Why Monitor CO2?

My guess is that you have just purchased a vital signs monitor that includes the ability to monitor CO2.  Kudos to you.  That single purchase has improved the quality of your anesthesia monitoring exponentially.  Nicknamed the “Anesthesia Disaster Early Warning System,” the ability to monitor expired CO2 in real time is said to be responsible for the reduction in death rates during general anesthesia in human medicine from 1 in 5,000 in 1983 to 1 in 300,000 in 2005.  It’s no coincidence that the AAHA Anesthesia Guidelines for Dogs and Cats and Banfield’s comprehensive new book Anesthesia and Analgesia for the Veterinary Practitioner: Canine and Feline (and others) recommend monitoring CO2 during anesthesia as standard of practice.

Measuring exhaled CO2 is noninvasive and can tell us quite a lot about our patient’s cardiovascular and ventilatory status.  But we can get useful information just by detecting CO2, long before it’s measured.  For example, detecting CO2 in expired gas confirms that the endotracheal tube is in the trachea and not in the esophagus.  If you’ve placed an endotracheal tube and your CO2 monitor does not detect any CO2, then the tube is very likely in the esophagus and not the trachea.  It’s good practice to connect the capnometer to the endotracheal tube as soon as intubation is complete – even before you tie the tube in place.  Regardless of your level of skill at intubation, it’s always a comfort to confirm you have been successful.

When I was first began navigating the value of monitoring CO2 in an anesthetized patient, I locked onto the basics.  I learned that CO2 defines ventilation, although I wasn’t sure exactly what that meant.  The easiest way to understand the relationship between carbon dioxide and ventilation is to let go of the notion that breathing is about taking in oxygen.  It took a little while to remove the word ‘oxygen’ from my brain when thinking about breathing.  Taking in oxygen is an important function of breathing, but the stimulation to take a breath is most often triggered by CO2.  Eventually I learned to look at carbon dioxide values when trying to determine a patient’s breathing status, or ventilation status.  Normal values for end-tidal carbon dioxide (ETCO2) are between 35 mmHg and 45 mmHg.  That generally means that if the carbon dioxide reads greater than 45 mmHg, then CO2 is building up in the lungs.  Since the body rids itself of CO2 when it exhales, a buildup of CO2 indicates the body isn’t exhaling often enough.  In other words, the patient is hypoventilatingconscious-breathing-carbon-dioxide-controls-breathing edit Simple, simple.  To correct hypoventilation, I can breathe for the patient a few extra times each minute until the CO2 is back to within normal limits.  If the capnograph reads below 35 mmHg, then the opposite is true.  There is not enough buildup of CO2 in the lungs and so the patient is hyperventilating.  Although not quite as simple to resolve, the approach to treating hyperventilation is logical: get the patient to breathe less often so CO2 has a chance to build up.  This illustration (at right) shows the influence of carbon dioxide on ventilation.

As much as I love the circular illustration of carbon dioxide’s influence on breathing, the top of the illustration simply says, “Carbon dioxide is produced.”  To step beyond the basics and increase the value of monitoring CO2 during anesthesia, we have to look closer at the top of the circle.

“Carbon dioxide is produced” everywhere in the body.  Carbon dioxide is a byproduct of metabolism, so it is being dumped into the blood stream from literally everywhere in the body.  It is then transported to the lungs, triggering the breathing center, and is exhaled.  Examining how the carbon dioxide in the blood stream is transported to the lungs is key to recognizing the relationship between ETCO2 and the cardiovascular system.  Transportation of carbon dioxide to the lungs is dependent on the heart. The heart pumps blood.  So, there is a significant relationship between ETCO2 and cardiac function. To take that relationship to the extreme, if the heart is not beating, then blood isn’t moving.  If blood isn’t moving, then CO2 is not brought to the lungs.  If CO2 is not brought to the lungs, then the capnograph can’t read it.  You see where this is going, right?

breath-baumanThe relationship of carbon dioxide and the cardiovascular system is actually much deeper than just plus-or-minus CO2 going to the lungs.  There is a lot to discover as you integrate the use of a capnograph in your practice of anesthesia.  You can experience some of the cardiovascular effects of CO2 yourself, simply by holding your breath for awhile.  The changes you may notice – the urgency to breathe (which you once thought to be a need for oxygen, but now realize that it’s the need to get rid of CO2), your increased heart rate, the pounding of your ears (indicating increased cardiac output) – are all related to changing levels of carbon dioxide in your blood.  It’s commonly said that CO2 drives the cardiovascular system, and there’s a lot of truth to that.

Let’s take a broad look at how your brand new, fresh-out-of-the-box “Anesthesia Disaster Early Warning System” End-tidal CO2 monitor is a non-invasive, low risk assessment tool for the veterinary anesthetist.

  • It tells you that the endotracheal tube is in the trachea
  • It detects –
    • Extubation
    • Disconnection
  • Cardiac arrest
    • Faster than SPO2
    • Faster than ECG
  • Indicates changes in cardiac output
  • Respiration rate
  • Detects inspired CO2
    • From dead space
    • From circuit misfit (resistance)
  • Indicates ventilation status
    • Hypoventilation
    • Hyperventilation
  • Can be useful to assess the effectiveness of CPR efforts

There are many resources online to further your understanding of carbon dioxide and capnography.  Here’s a short list of resources I have called upon.

Capnography in Dogs

Dead Space – Cause, Effect, & Management Basics

AVMA 2017: Anesthesia Monitoring With Capnography

 


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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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Nerve Blocks for Veterinary Oral Surgery

KentalYou’ve been to the dentist, right?  Yeah, me too.

February is right around the corner, and in honor of AVMA-sponsored National Pet Dental Health Month, this post is about making dental anesthesia easier for you and your animal patients by remembering to use regional anesthesia to eliminate pain before, during, and after oral surgery.

We sometimes forget that we have all experienced some degree of pain similar to what our animal patients might experience during a dental procedure.  So we can draw on our own experiences to be proactive for the animals in our care.  For instance, I have often recited the anesthesia mantra, “pain is easier to prevent than to overcome,” but all I really have to do is to remember that my dentist applies a regional nerve block inside my mouth before he fires up the drill.  Despite the fact that I always steel myself against the needle’s approach, I appreciate his timing.

Regional nerve blocks contribute to multi-modal pain management by interrupting the impulse transmission along the pain pathway, which inhibits the pain response. One of the greatest benefits of a regional block is that we can maintain our patients at a much lighter plane of general anesthesia, thereby significantly reducing some risks associated with general anesthesia.  When surgical pain is fully controlled with local anesthesia, we are able to use anesthetic gas for what it does best: patient restraint. Regional blocks also provide smoother recoveries because the pain impulse never reaches the cerebral cortex, so even when the animal is fully awake, there is no recognition of pain.

To Begin

The internet and YouTube are full of tutorials on where and how to place regional anesthesia for dental procedures in animals.  And by now we’ve all learned how to qualify sources and use the resources we find online appropriately.  For this overview, I lean heavily on a 2014 article by Dr Brett Beckman entitled Nerve Blocks for Oral Surgery in Dogs.  In the article, Beckman provides step-by-step technique with photographs, as well as drugs, dosages, do’s-and-don’ts, and tips.  Whether your role is to place the blocks or to assist, Beckman’s article is worth the read.

Tools of the Trade

One of the beautiful things about regional nerve blocks for oral surgery is that you don’t need any special equipment to place them.  Here’s your short list of supplies to gather.

  • Syringe (sized to the infusion volume)
  • Fine gauge needles
  • Local anesthetic of choice
    • Bupivacaine is a long-acting anesthetic frequently used for regional anesthesia.  Interestingly, a 2016 study showed that adding buprenorphine to the bupivacaine significantly increased the time many animals were pain free.
  • Optional: a canine skull or other visual guide to anatomic landmarks
    • I always used Miller’s Guide to the Dissection of the Dog

Types of Nerve Blocks

maxresdefaultNerve blocks are commonly used in four regions of the oral cavity.   Beckman suggests that the nomenclature for these blocks is confusing in that the name of a block may refer to the region that it blocks or it may be named according to the actual nerve that is blocked.  He offers a simplification and clarification of nomenclature to describe the region affected rather than the nerve blocked.  Beckman describes the four most common blocks as follows:

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A quick directional review

  • Rostral maxillary block
    • Known as infraorbital block
    • Affects bone, teeth, and soft tissue in the mouth from the maxillary third premolar rostral to the mid-line.
  • Caudal maxillary block
    • Affects bone, teeth, and soft tissue in the mouth from the last molar rostral to the mid-line, including the soft and hard palates.
  • Rostral mandibular block
    • Known as mental block
    • Affects bone, teeth, and soft tissue in the mouth from the mandibular second to third premolar rostral to the mid-line.
  • Caudal mandibular block
    • Known as inferior alveolar block
    • Affects bone, teeth, and soft tissue in the mouth from the mandibular third molar rostral to the mid-line.

Whether you are placing nerve blocks yourself or assisting someone else, remembering to use this valuable tool will enhance patient safety during the surgery and patient comfort afterward. Local blocks are easy to administer and require no special equipment to perform. Their use is paramount in providing the best patient care for oral surgery.


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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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