Anesthesia - An Informal Chat

On February 15th, 1999 (note this was some years ago, not all information may be the most current) Dr. Cliff Swanson, Professor of Anesthesiology at North Carolina State University/Veterinary Teaching Hospital, volunteered his time to the Raleigh Kennel Club to answer questions from the members. Dr. Swanson began by providing a brief history of the use of various methods of anesthesia by the veterinary profession.

Over the last two decades the practice of veterinary medicine has evolved in its use of inhalation gas anesthesia. Twenty years ago you could walk into any veterinary practice, open the door, and you would likely smell a very sweet odor.

What was producing that odor was a drug called methoxyflurane. It was a very safe drug that did not demand constant monitoring of the patient. However, there are serious environmental health considerations in regard to the sub-clinical exposure of veterinary medical personal to methoxyflurane, particularly the risk of kidney disease. In addition, 20 years ago, veterinary practices werenít as good at scavenging the gas anesthetic, that is, preventing its escape elsewhere other than into the patient, as they now are.

Another inhalation anesthetic in human medicine was halothane. There was a tremendous resistance to using halothane by veterinary practices. There were two reasons for this reluctance - halothane was more expensive and, unlike methoxyflurane, it required careful monitoring of the patient. This higher degree of vigilance demanded the presence of somebody to be present and monitoring the patient all the time.

Isoflurane became available for human use 15 to 20 years ago. Then, between 6 to 10 years ago, the company that produced isoflurane went through all the FDA rigmarole required to have it approved for use in horses and dogs. In addition the company came up with a brilliant marketing scheme to persuade private veterinary practices to begin using isoflurane. The deal was, if a practitioner agreed to buy a certain number of cases of the drug over a multi-year period, the company would give them the several thousand-dollar vaporizer required to administer the drug.

There was also a great deal of continuing education concerning isoflurane. It is safer than halothane, to a degree. It is very predictable like methoxyflurane. And, for certain kind of patients, isoflurane offers a distinct advantage in that it requires very little liver function to dissipate. It goes away very quickly, almost entirely by being exhaled. Less than 1% of isoflurane shows up as metabolites. It has now become the mainstay of almost every veterinary practice for gas inhalation anesthesia.

Question: What anesthesia protocol would you recommend for caesarian sections?

Answer: Anesthetic drugs must cross many membrane layers to penetrate the brain, referred to as crossing the blood-brain barrier, in order to sedate an animal. The membranes that separate the puppy from the damís uterus, the placental barrier, is similar in vasculature to the blood-brain barrier.

Thus when anesthetic drugs are administered to the dam they pass from the higher concentration level in the damís blood into the pup's circulation. Consequently, to minimize any negative effects on the puppies (such as depression of respiration and/or heartbeat), a drug that can be reversed quickly (i.e. neutralized), as soon as the puppies are removed from the bitch, is desirable.

Narcotic drugs like morphine can be reversed with the administration of another drug, which makes the action of the narcotic drug, go away. Therefore opiates tend to be good drugs to use in Cesarean sections because their side effects can be quickly controlled if things begin to go bad. If a pup comes out that is not breathing well because of the morphine given to the bitch the morphine's action in the puppy can be reversed. It is often standard procedure that when a pup is taken by Cesarean section somebody will shoot a little of the opiate reversal drug, called naloxone, into the pup's mouth.

The two drugs that most people think about using in Cesarean sections that go away quickly are propofol and isoflurane. Propofol is an injectable drug and is relatively new having only been around for the past six or seven years. Propofol has seen increasing use among private practitioners. The main virtue of propofol is that it is very, very rapidly metabolized and disappears from circulation very quickly. It is an anesthetic induction drug that you inject into a vein to make a patient go to sleep.

One can continue to give small amounts of propofol to keep a patient asleep. There are reasons both pro and con for doing this. The drug does pass through the placental barrier and enters the puppy's circulation. Because it is relatively rapidly broken down it therefore doesnít suppress the puppies for a very long time. However, with repeated dosing of the bitch, it can cause some puppy depression and that is one reason not to maintain anesthesia on propofol by continuously giving it intravenously. Therefore, my general preference is to initially administer propofol to induce sleep, and then switch over to isoflurane gas to maintain anesthesia.

Question: Are there any tricks to using propofol concerning how much to use?

Answer: The biggest dosage problem with propofol is that if too much is given, too quickly, even if it is the "right" amount in terms of the total dose but is just administered too quickly, you end up with a patient that is not breathing very well, or may stop breathing, called apnea. The remedy to this problem is to introduce an endotrachial tube and ventilate the patient and the patient will eventually start breathing. This is similar to the problem of inducing anesthesia with barbuturic drugs (such as sodium pentobarbital) which have been the standard for 50 years but are also associated with the same risk of apnea. So long as this side effect is expected, and the veterinary professional is prepared to deal with it, it does not present a problem.

In terms of the cardio-vascular system propofol is not that different from a lot of the other drugs. It does depress blood pressure and there can be variable effects on heart rate. Once again, the alert veterinarian can successfully deal with these side effects. Like any other drug there are individual animals that may have an idiosyncratic reaction. That is, despite proper administration of the drug, something weird may happen for an unexplained reason. The chance of this is less than 1%.

Question: Are there levels of unconsciousness you can take a bitch down to so that the bitch is not in pain but the least amount of drug is moving across the placental barrier to the puppy?

Answer: In answer let me explain something about how anesthesia works. Generally speaking we talk about there being four stages of anesthesia.

1) Analgesia - This first stage is where pain killing begins to occur. Methoxyflurane is a very good analgesic. It's pain killing properties are so good, in fact, that it can do so at sub-anesthetic levels. You can inhale a little bit of the stuff and it will take away certain kinds of pain before it knocks you out.

2) Excitement phase - The cerebrum is the part of the brain that we think with, that we are conscious with, and it exerts a lot of inhibitory action on some lower centers of the brain. When you remove the action of the cerebrum, which is what happens in this stage of anesthesia, what you see is a lot of the uninhibited excitatory activity such as - thrashing around, what looks like seizuring, and vocalizing.

The reason we use intravenous drugs now to put patients to sleep quickly is because we want to move through this excitatory phase as rapidly as possible. Aside from the physical carrying on that may be observed in this excitatory phase there is also some internal vital organ function activity that can be disruptive or erratic when a patient becomes really excited. Blood pressure regulation and heart rate, for instance, can be thrown out of whack if we let this excitement go on for too long. We want to try and avoid that.

3) Surgical Anesthesia - This is where we want to maintain the patient during surgery. In this stage we maintain muscle relaxation, decrease pain, and the patient remains asleep. There are gradations of light, moderate, and deep sleep within this stage.

4) Medullary depression - This stage refers to the lower centers of the brain. Those that are responsible for all the things you donít have to think about like keeping your heart going, blood pressure regulation, and all the other body functions that maintain us in a normal state of health which normally happen automatically.

 

The trick in anesthesia is to stay in the area of surgical anesthesia without straying into medullary depression. The goal is to use enough anesthetic that we maintain all the desirable things about anesthesia: some muscle relaxation, lack of pain sensation, and no waking up and remembering everything. So, the question was, can you maintain a light plane of anesthesia? A situation where you have a bitch that isnít feeling any pain, has the right degree of muscle relaxation so the surgery can be done and basically all these desirable things without getting too much drug so that you are not overly depressing the puppies any more than you have to.

The answer is - the trick of all anesthesia is to give just enough medication so that we stay as close as possible to a light degree of surgical anesthesia without leaving this stage of anesthesia and straying back into the second, excitatory phase or too far into medullary depression.

Almost everything that we do when we are monitoring certain things in anaesthetized patients relates to us trying to get a handle continuously on where we are within this stage of anesthesia. In particular how close we are to crossing over into medullary depression because you can give, and give anesthetic, and eventually knock out the functioning of the centers in the brain that maintain all this other necessary functions, like blood pressure regulation, and then you have a dead patient. To maintain this delicate balance we are collecting information every 5 or 10 minutes or so from the patient and trying to make decisions about how much anesthetic do we really need to be using. Some certain situations require more vigilance than others.

Question: When you have finished a Cesarean section, using propofol, how long should it be before the bitch is coming out of it?

Answer: If you are just maintaining the bitch on propofol infusions - beginning with a big slug of it and then dribbling it in so she stays asleep, within 15 or 20 minutes of the end of the procedure you have an awake patient. As an example, on one occasion, there was a patient in which inhalation anesthesia could not be used and this patient was maintained asleep on propofol for 7 hours. Within 15 minutes of the conclusion of the procedure the patient was awake and walking about.

Is it the best drug to use with a c-section by itself? That would depend on the age and condition of the bitch. Likely, use of propofol alone, is not the best option. Most of the time with anesthesia we use a combination of 3 to 4 drugs. One drug may be used to sedate, take the pain away, another drug gets used intravenously to knock the patient out, and another, perhaps gas anesthesia, to maintain the patient asleep. Not every drug has every desirable property - by combining drugs a balance of the pros and cons of each drug can be reached.

Question: What is currently the best protocol for quick, non-invasive procedures, such as OFA X-rays?

 

Answer: Medetomidine is a terrific drug for some patients. It is a very, very powerful sedative and analgesic; especially when it is combined with opiates such as morphine or oxymorphone. We can give such a combination and have a patient that is breathing, has really good blood pressure, is very relaxed, very sedated, easily handled and stretched into the appropriate position. For a patient thatís got less than ideal hips, perhaps has some arthritic pain and so forth, this is an analgesic so you can get them into position and work with them and there is not a lot of carrying on. There are some dosage considerations but that is the general case. One really great thing about medetomidine is that there is another drug, atipamazol, which is the reversal drug for medetomidine, and, in fact they are sold together. With an injection of atipamazol the patient on medetomidine is awake momentarily.

Caution - However, medetomidine is NOT a good drug for any animal, any species, that has any kind of cardio-vascular problem. So, if youíve got a dog that has a congenital heart defect or an older patient with regurgitive mitral valve problems, for instance, this is not a good drug for them. The reason being that it causes the vasculature to constrict, which is why the blood pressure goes up, and then the heart has to work harder to pump the blood out.

Question: Given the newer drugs available, does that mean that my Whippet, which has not had reason to be anaesthetized for many, many years, might now experience less of the excitable stage she suffered through for an hour or more coming out of anesthesia in the past?

Answer: The answer is yes and no. I have had reason to be anaesthetized 3 times in my life. The first time I woke up from being anaesthetized I found myself strapped down to a table with a guy standing over me who said, "Boy, you really trashed this place." Apparently I was quite physically violent as I came out of anesthesia. This was about 30 years ago. About 2 or 3 years later I had reason to be anaesthetized again and woke up under the same circumstances, strapped down. This was before I was a veterinarian and before I knew anything about anesthesiology.

By the time I needed to be anaesthetized the third time I was certified as an anesthesiologist. When I went to the hospital this time I knew what drugs I wanted and spoke to the anesthesiologist before surgery. I was given the protocol I requested and woke up normally and peacefully. So, in answer to your question, to a degree how a patient responds to many drugs is a function of their endogenous makeup. I can say this with confidence because at the college it is not uncommon to have patients that are routinely anaesthetized. They almost always seem to have a certain make-up about the way they recover. If we see a rough recovery, yes, we will change drugs. Sometimes it matters, sometimes it doesnít.

So, getting back to your question, Yes, it might be possible that, following anesthesia, using medetomidine and not reversing it and letting the sedative and the analgesic properties of it go on for awhile would be beneficial. However, I would point out, that having a patient lying quietly in a cage, sedate, possibly not breathing as adequately as they might, and not being observed because everybody is running around trying to do many things at once, that is a liability.

On the other hand, this is a pretty safe drug, and it can be reversed. So, while Iím not a big proponent of real profound sedation going way beyond the end of the time it is necessary, on the other hand I want to do what I can for that patient. Having the patient in a cage beating itself up, whether because of pain or excitement, is obviously a situation I would like to avoid. There has to be something in the middle for such a patient.

Medetomidine is a drug I have used precisely for this purpose. We have had patients wake up and be very excited. We didnít even use medetomidine to start with in their regimen of getting anaesthetized and I have given them extremely low dosages of it, just enough for the patient to chill out. Given all the choices for ways of doing that with drugs I would take medetomidine as my first choice for doing it over the other mainstay of sedation in veterinary medicine which is acepromazine.

My opinion of acepromazine is that it is a drug that has been around in veterinary medicine forever. Because it is cheap and people know about it, it tends to get used and used and used. We donít use a whole lot of it at the college, most institutional veterinary practices do not. The reason being that we have other drugs that do the same thing that donít give us the same kind of side-effects that acepromazine does. If I am going to think sedation for a patient I think of medetomidine first unless the patient has some type of cardio-vascular problems in which case I would probably go with an opiate rather than acepromazine.

Question: Have you had patients that you canít get to sleep?

Answer: It is not so much a matter of patients that are "resistant" to anesthetic. Usually, when a patient gets excited, one of the very first things that happens is, because of what goes on internally, a lot of blood starts getting shunted out to the skeletal muscle. Normally our skeletal muscle and vasculature is not a high priority organ for blood flow, not nearly as high a priority as the brain, the heart, the kidneys, and other vital organs. But the pattern of blood flow distribution can be changed and during excitement a lot of blood gets shunted out to the skeletal muscle and that takes blood away from the brain.

Thatís why once the patient does get excited it takes a lot more drug to get him asleep. You keep giving drug and you keep giving drug and where is it going? Well, it is not going to the brain. It is going to the skeletal muscle and vasculature. To overcome all that you need to give a lot more drug.

Generally speaking once a patient gets really excited theyíll stay internally, if you will, excited for a period of time. There are changes that happen with regard to hormones and other neurotransmitters which are substances that run everything. Basically, that excitement phase, if it is profound and lasts for a long time, has to be overcome. It just takes that much longer to get them settled into an even plane of anesthesia

The other thing that can be done is to employ some regional analgesia as an epidural. This is where we actually inject local anesthetic drugs; we can also use opiates or medetomidine. A needle is used to inject the drug into the space surrounding the spinal cord. That is a technique that enables us to avoid the systemic whole body administration of drugs that potentially have cardio-vascular or respiratory depressant effects and still knock out the pain.

Trying to control surgical pain with the use of an epidural is another way to address the issue of how do you keep the bitch in a really light plane of anesthesia during a C-section and not get the pups real depressed. As in everything there are good things and bad things about that. If you do it just right, and everything works just right, there can be profound analgesia for quite awhile. (Dentists routinely do surgery on patients that are awake with the use of regional anesthetics.)

One problem is that a practitioner that doesn't do epidurals frequently is likely not skilled in the fine art of the proper technique epidurals require and then it is hard sometimes to make it work. It is something that people have to practice and get the feel of it. We teach that at the college, we do epidurals and spinal analgesia a lot for complicated orthopedic cases and so forth. It is a way of managing pain and is very effective when it is done right and it takes well.

The other problem with epidurals is that even if you put the right drug in the right place epidurals only work 80% of the time. This speaks to the background failure rate of 20% for epidurals and interestingly enough this is about the same failure rate as for acupuncture in the hands of a skilled acupuncturist and it probably has a lot to do with neurological function and things that are being investigated. So, that is the trade-off with epidurals. When they work well, it is another way of managing things.

Question: What about age? Considering they may weigh only a pound or so is there more risk in anesthesia for toy breed puppies?

Answer: Not really. The size implication is only a matter of certain kinds of techniques, especially related to endotrachial entubation and how easy or hard veins are to hit for managing a catheter. For most robust and hardy individuals (that is: without a fundamentally debilitating congenital disease, and in the absence of any complicating factor like pneumonia or other kinds of acquired or congenital problems), from the time they are a few months old into mid-life, there really isnít terribly any difference between anaesthetizing humans or a 3 month, vs. six month, vs. year-old, or five year old dog.

Question: I have always heard that sighthounds, because of low body fat, and Bulldogs because of their screwed up respiratory systems, require less anesthesia per body weight than other breeds. Is that true?

Answer: Kind of true. Nobody could possibly deny on the basis of just simple observation that certain breeds have different body morphology than others. The distribution of lean body mass to fat tissue is significantly different in the Greyhound than it is in Labs or other dogs of similar size but different conformation. Yes, it is true that that can have an effect on how we do things.

It is probably more important to consider it the other way, when we have an individual that is grossly overweight for their size. In this case we would try to make an estimate of what their real body mass is without the extra tissue. It turns out that a lot of the anesthetic attributes in sighthounds, particularly in Greyhounds, has a lot to do with the difference in how they metabolize certain drugs compared to other breeds rather than simply the body fat to body mass ratio. We might more importantly make different choices about which drugs to use rather than how much of something. The thing about Bulldogs is just that their upper airways are disproportionately small compared to their body size so we make adjustments in the endotrachial tube size.

In closing let me leave you with something I hope isnít too sobering a thought.

Basically, in anesthesia, when studies looking into problems associated with anesthetic events are done the one thing that is almost always implicated in negative events is some period of time when a patient doesnít have enough oxygen and their blood becomes hypoxemic.

There are five or six different reasons that a patient can become hypoxemic. Not the least of which is the inadvertent failure of a person in charge of their anesthetic period to ventilate them when they need to be ventilated. There is a lot about what goes wrong during anesthesia that can be traced back to human error as well as just the underlying disease and so forth.

There has been tremendous development and interest in minimizing that human error and providing those of us who practice anesthesiology with an opportunity to monitor ourselves as well as the patient. Most of the patient monitoring that we do now enables us to look at problems as they are developing way before they become serious consequences. What you really should be concerned about is whether or not the people who are administering anesthesia are paying close attention and have the experience to pull it off. 99.9% of the time that is the way it is.

The incidence of negative events in human anesthesia is about 1 in 100,000 cases. That is simply that the person gets anaesthetized and ends up with some persistent problem in their health just because they happened to get anaesthetized that day and not a consequence of the disease process or the surgery that was going on. That is the background incidence rate of the chance of having a problem just because of anesthesia. Everything about modern day anesthesiology works toward absolutely minimizing the amount of risk. And that is true of veterinary anesthesia as well. We have really good drugs these days, smart people, concerned clients, and good practitioners, so the background rate of problems in anesthesia, whether in private practice or institutional practice is extremely low.

 

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