Sunday, October 17, 2010

Why veterinary anesthesia is a thrill a minute

Last week I had my anesthesia and analgesia final exam. Some people seem to think that anesthesia is boring. (The Amateur Transplants refute that fallacy pretty well.) Anesthesia is in fact pretty complicated. I sort of wished that the choices you have to make for each anesthesia case could be written out in a huge decision tree, but I didn’t have the energy to do it myself. Roughly, here’s how it goes.


First, you might want to make the animal sleepy so it doesn’t try to bite you, kick you, or run away from you. The choice of sedative or tranquilizer here is dependent on a lot of factors. How old or young is the animal? (Foals may not need any premedication at all.) How sick is it? (Healthy animals benefit from different premedications than critically ill ones.) Does it have heart disease? (Many sedative/anesthetic drugs depress the function of heart muscle and could kill an animal with heart disease.) And, of course, what species is it? For this exam, we learned details about the kinds of drugs that were best for dogs, cats, horses, goats, sheep, cows, llamas, and alpcas. We did a whirlwind tour of issues in rabbits, ferrets, rats and mice, birds, lizards, and snakes.


Once the animal is premedicated and drowsy (or if it is naturally unlikely to protest), the next step is to induce anesthesia. Many of the same questions of drugs come up, and many of the same drugs are used for this stage of the process, just in different amounts or given by different methods. It is nice to have the animal mellow for this stage, because commonly the next set of drugs is given intravenously. It is best to put in an IV catheter to do this. A catheter allows easy access later. This is useful to give more induction agent, as they are sometimes given in bursts; to give a reversal agent if necessary, which you may have to do in a hurry if there are problems; or to give other drugs while the animal is anesthetized, such as antibiotics or pain medication, without having to find a vein again.


Once anesthesia has been induced, it has to be maintained. You don’t just knock an animal out and hope it stays out for as long as you need — well, actually, sometimes you do. The question is how long you need to keep the animal under, how painful the procedure is going to be, and sometimes whether you have the equipment for your preferred maintenance method. If you are in the field with a horse that can’t be brought in to a hospital, you will probably not choose to intubate the horse and maintain it with an inhalant anesthetic. For a short procedure, one dose of injected anesthetic might be sufficient, or anesthesia could be maintained by an IV drip. On the other hand, for most surgeries on dogs in a  veterinary hospital you will choose to employ an inhalant anesthetic.

How are you going to get the inhalant gas into the animal? Most commonly you will intubate (put a tube down the animal’s throat). In some cases, you will not be able to do that, and will put a mask on the animal’s face instead. We don’t like masks because they are liable to leak and you don’t want anyone but the animal to be getting that gas — the veterinarian would be breathing small amounts of anesthetic gas during every surgery, which is not a good thing. But sometimes intubation isn’t possible, particularly for very small animals. Interestingly, we were taught that you do intubate snakes, and that in fact you do so before inducing anesthesia — you sedate them and then put the tube in while they are still awake.

Recovery and post-operative analgesia

When the animal wakes up, there are another host of questions. Some animals are more liable to regurgitate than others. Regurgitation is a Bad Thing (it can lead to inhaled stomach contents and aspiration pneumonia, which may be fatal), and so extubation may be done using different techniques to avoid it, depending on the species.

Post-operative pain control is also extremely varied among species. As a general rule of thumb, you go for NSAIDs in horses and cows, but opioids in dogs and cats. Opioids can reduce gut motility, which is bad in animals with big complicated stomachs like horses and cows. (However, we do still use them in those animals.) On the other hand, NSAIDs can cause kidney issues in dogs and cats, while horses and cows handle those drugs really well.

NSAIDs are particularly interesting in veterinary medicine. In human medicine, there was an attempt to find COX2-selective NSAIDs, which would inhibit only the COX2 form of the enzyme (which is involved in pain) and not the COX1 form (which is essential to normal stomach wall function, among other things), in the hopes of finding NSAIDs which handled pain without causing stomach ulcers. Unfortunately, COX2-selective NSAIDs have turned out to be problematic in human medicine, causing an increased risk for heart attack, among other things. These side effects have not manifested in veterinary patients on COX2-selective NSAIDs, however, and most of the NSAIDs that we give dogs are COX2-selective. Interestingly, the COX2 selectivity of any particular NSAID is different in different species. In other words, a drug that selectively inhibits COX2 in a dog may inhibit COX1 and COX2 equally in a human.

Which leads me to my public service announcement of the day: acetaminophen (Tylenol) can kill a cat, and ibuprofen (Advil) can destroy a dog’s kidneys. Always ask your veterinarian before giving your pet any form of pain relief. There’s lots out there for them, but the exact drugs that are appropriate differ hugely between species.

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