The stages of Anesthesia

Question: Dr. Mike,

Can you explain the stages of anesthesia?

Answer: J-

Anesthesia is sometimes divided into stages and some of the stages are further divided into planes.

Stage 1 anesthesia is the period between administration of an anesthetic and loss of consciousness. In people there is reported to be some disorientation prior to the loss of consciousness but in pets this is rarely obvious.

Stage 2 anesthesia is the period after loss of conciousness. In this stage there is sometimes uncontrolled movement, delirium, breath holding, irregular respiration, whining or howling and dilation of the pupils. This stage can be worrisome and most anesthetic protocols seek to keep this period as short as possible without endangering the patient.

Stage 3 anesthesia is the level at which surgery can be performed. The transition from Stage 2 to Stage three is usually denoted by the return of regular respiration, constriction of the pupils and the stopping of involuntary motion or vocalization by the patient. Stage 3 anesthesia is divided into four planes:

In Plane 1 Stage 3 anesthesia the patient still has blink reflexes and swallowing reflexes but has regular respiration with good chest motion. This stage would be considered "light" for surgical anesthesia.

In Plane 2 Stage 3 anesthesia the patient loses the blink reflexes, the pupils become fixed in one position (usually central) and respiration is still regular with good use of the chest muscles and diaphragm. This is the plane at which most veterinarians are comfortable performing surgery.

In Plane 3 Stage 3 anesthesia, the patient starts to lose the ability to use the chest muscles and abdominal muscles for respiratory efforts, so breathing becomes shallow and assisted ventilation is best when the patient must be maintained at or near this level in order to allow control of pain in surgeries that are unusually painful (such as extensive abdominal exploratories).

In Plane 4 Stage 3 anesthesia, the patient does not use the chest muscles and abdominal muscles at all, which means that all respiratory effort is produced by the diaphragm. This plane of anesthesia is very close to the point where the patient will stop breathing entirely and it should be avoided.

Stage 4 anesthesia is basically an anesthetic crisis. It is the time between respiratory arrest and death from circulatory collapse. Assisted ventilation is absolutely essential in this stage, as well as support for the circulatory system through IV fluid administration and medications to stimulate respiration and the cardiovascular system. Hopefully, this stage of anesthesia is never encountered in practice.

Mike Richards, DVM

Anesthesia risk and Pugs

Question: Hi Dr. Mike,

I have a five month old black pug who is the light of my life. We recently lost our cocker to cancer, and I am quite worried about having my pug boy neutered. I have read so many things about the bad reaction that they can have to anesthesia. I trust my vets completely, but since Buddy died, I am quite nervous about something happening to this puppy. Can you speak to pugs and anesthesia, please. Should I have him neutered at six months or wait until he gets older?

Thank you.
Lynn and Freddy the Pug

Answer: Lynn-

We have worked with two pug breeders over the years in our practice and so we have anesthetized a number of pugs. We have not had an anesthetic death in this breed that I can remember. Like virtually all vets, we are careful to try to get an endotracheal tube in quickly, to get control of the airway and we leave it in until pugs are clearly swallowing on their own. While this is also standard procedure for all breeds, we are always a little more aware of the potential for problems with short nosed breeds. We do often use a preanesthetic sedative and just use anesthetic gas for anesthesia,  with no induction agent (quick acting anesthetic injection), but only if the puppy is cooperative. This makes anesthetic recovery faster, which is nice in these breeds.

There is no way to assure that an anesthetic problem or death does not occur. In a recent study done in Canada, the risk of anesthetic death in dogs was about 1 in 900.  While this is a small risk, it is a real one. So you have to weigh the benefits of the surgery against the risks and then do what seems to be best. The major advantages to neutering are elimination of the risk for testicular cancer, a decrease in prostate problems, reductions in urine marking behavior, reduced aggression and less roaming and fighting in dogs that live outside.  The major disadvantage is the anesthetic risk associated with the procedure.

It is possible to neuter later in life if testicular cancer or prostate problems occur and to wait and see if urine marking behavior or aggression are a problem prior to deciding if neutering may be beneficial. On the other hand, I do think that the recovery from the neutering procedure is easier for younger dogs (less than 8 to 10 months) so when neutering is a planned procedure I do prefer to do it at a younger age.

The decision to pursue elective surgery is always a hard one for people who have had a dog die from anesthesia or who know they are prone to feeling guilty for some time if they make the decision to proceed with surgery and something goes wrong.  It helps a lot if there is a good reason to do surgery (if one or more of the expected benefits seems important) when making this decision. The fact that there is societal pressure to neuter should not be a part of this particular decision, in my opinion, because you do have a strong personal reason to be cautious.  However, if you do decide not to neuter, it is important that you provide responsible ownership and not allow unintended breeding, roaming, etc., although it sounds like you would do that based on your note.

Even though I do think that for most dogs, the benefits of neutering outweigh the risks, this is a personal decision and you need to do what you think best, since this is an elective surgery.

Mike Richards, DVM


Q: I am a new subscriber to your newsletter.  I have been doing some research and coming up with little to no answers. All I have to go on is the effects on a person.  I would like to submit the following questions to Dr. Mike to see what he has to say:

1.  How does anesthesia effect a dog?
2. Could a dog being placed under anesthesia three times in less than 12 days cause permanent damage?
3. How much time should the dog be given to recover from (1) anesthesia before
the next exposure?

A note to Dr. Mike:  the above question refers to a dog with depression, drooling, and lack of appetite.  Weight is good (above norm), vitals are good, no temperature.

Thank you - Deb

A: I am currently putting together information on anesthesia for the VetInfo Digest and the website. I will try to cover how anesthesia affects dogs in some detail in that information so it would be good to watch for it, too. In the meantime, I'll try to answer your specific questions as best I can.

One of the problems with discussing anesthesia is that anesthetic protocols vary widely. So widely that it is unusual for me to talk to another vet and discover that we use exactly the same anesthetic procedures for routine procedures. On top of that, we use different anesthetic procedures for different patients based on pre-existing conditions, the type of surgery we plan to do and factors such as patient age or the financial situation of the owner.

It is not often that multiple anesthetic procedures are necessary in a short period of time in a pet but it does happen. We had a cat in our hospital last year with injuries requiring twice daily treatment for eight days. It was not possible to work on this cat without anesthesia. So we anesthetized him twice a day for eight days. He did fine. We were using isoflurane gas for these procedures and it is absorbed in smaller concentrations than most anesthetics and is cleared quickly from the pet's system. It is not too unusual for us to use an anesthetic agent to enable us to take X-rays of a pet with a fracture, allow it to wake up and then do surgery on the fracture later the same day or the next day. I have treated two cases of strychnine poisoning in my career. We kept one of the dogs anesthetized for three days and the other one anesthetized for seven days. This was not our original intention but we were using pentobarital to control the seizures and it has a very prolonged action after the second or third time it is given to maintain anesthesia and it was necessary to give it several times to these dogs in order to counteract the effects of the strychnine. Both dogs lived and suffered no untoward effects that I could determine except that one of them appeared to develop an extreme dislike for me. I do not consider pentobarbital to be a very safe anesthetic and would not recommend this sort of use for ordinary problems.

If I had a patient that I had anesthetized several times in a recent period who had problems that I thought might be due to anesthesia I think I would be more worried about having made an anesthetic error in one of the individual procedures than worried about having anesthetized the patient too many times. In many cases in which anesthesia is used a number of other medications are used at the same time. The last cat that we treated that died during an anesthetic procedure had a severe flea problem. We sprayed the cat to kill the fleas after anesthetizing it because we were sincerely concerned that the fleas would contaminate the surgical site. We reviewed our anesthetic procedure very carefully after the death of this cat. I really thought that the anesthetic protocol and monitoring were satisfactory and have wondered to this day if we caused the cat's death by using a flea spray that it may have reacted adversely to while under
anesthesia when we wouldn't see the signs of the reaction as easily. Other medications can impact on anesthetic procedures, including antibiotics, sedatives, seizure control medications and pain-relievers. It can be very difficult to determine the exact cause of an anesthetic crisis even when all medications used are known. In this case, I can't give much advice at all because I don't know what anesthetic agents may have been used. If you know what anesthetic protocol was used, it would help a lot in providing a more specific answer.

I am assuming that the depression, drooling and lack of appetite have occurred after the anesthesia. It would be very very helpful to know what clinical signs prompted the surgical procedures, whether pre-anesthetic or post-anesthetic lab work was done and what treatments have been used to attempt to alleviate these problems. If I had to guess off the top of my
head what was happening I'd be worried about pancreatitis (sometimes surgical manipulation during abdominal surgeries leads to pancreatitis after the surgery), liver disorders, kidney disorders and an undiagnosed primary condition that could produce these symptoms.

If you can remember or obtain the anesthetic protocols and summarize the conditions leading to the decision to anesthetize, medications used in conjunction with anesthesia, lab work results and the progression of the signs (did they show up after the first anesthetic procedure, the second or
the third, etc.) I would be glad to review this information and see if it is possible to help you understand what is happening. Without being able to see and evaluate your dog's condition it is not going to be possible to help much with the diagnosis of the problem but it should be possible to at least give you more specific information on expected clearance times and why a particular anesthetic protocol may have been selected and its effect on the overall situation.

Mike Richards, DVM

Heart rate under Anesthesia

Q: What are the normal heart rates for a canine while under anesthesia?


A: I know that this sounds like a cop-out, but it depends on what anesthesia is being used, as well as the usual factors like size of the dog, physical conditioning of the dog and other variables like that. The normal heartrate for dogs is probably between 80 and 120 (some little breeds seem to go higher than this to me and many dogs in my office manage faster heartrates before settling down some during the exam). I get upset about heartrates lower than 60 but other vets have different "comfort" limits. Some anesthetic agents suppress heartrate and others have little effect. If I induced and maintained anesthesia on isoflurane gas I would expect a more normal heartrate than I would while using a barbiturate anesthetic. There are medications that increase heartrate, counteracting the anesthetic effect. This is why atropine and glycopyrrolate are often used in
conjunction with anesthesia.

Sorry for the long answer to a seemingly simple question.

Mike Richards, DVM


Michael Richards, D.V.M. co-owns a small animal general veterinary practice in rural tidewater Virginia. Dr. Richards graduated from Iowa State University's College of Veterinary Medicine in 1979, and has been in private practice ever since. Dr. Richards has been the director of the PetCare Forum...