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Myths and misconceptions in anesthesia (Proceedings)

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Nov 01, 2009

This lecture is intended to review some of the common misconceptions or outdated thoughts that can be found in small animal anesthesia. Many myths or misconceptions stem from a truth that has been misapplied over the years, extended to more than just one circumstance, or told over so many times that the veracity of the idea becomes muddled. In addition, some information about certain drugs used in anesthesia is based on older literature where doses studied were very different from those currently recommended. Some material from this lecture is taken from a publication by Wagner et al in 2003, which is referenced at the end of these proceedings notes.

Misconception: Certain dog breeds are sensitive to anesthesia

Although many dog breed publications will report anesthetic sensitivity to certain drugs (e.g. ketamine, acepromazine) or to anesthesia in general, there are very few scientifically documented breed sensitivities to anesthesia. When owners or dog breeders present their animal and express concern about perceived anesthetic risk based on the animal's breed, it is important for the veterinarian to educate them to the general risks of anesthesia in all breeds/species and to reassure the client that the particular breed of the animal will be considered but that it is unlikely that there is a heightened risk based on the breed.

The one known and documented drug sensitivity based on breed is that of greyhounds having delayed recoveries after thiobarbiturates (thiopental). This delayed recovery is due in part to the typically lean body type of these dogs such that there is little adipose tissue for the drug to redistribute to; however, the primary reason for the delayed recovery is that this breed metabolizes the drug via a different hepatic pathway and drug metabolism occurs much more slowly. Most anesthesiologists extend this breed sensitivity to other sighthound breeds as well (whippets, Italian greyhounds, borzois, afgan hounds) even though these breeds have not been specifically studied.

Boxers with English genetic lines may be more sensitive to the hypotensive effects of acepromazine. Boxers that come from American genetics do not appear to have this sensitivity. When presented with a boxer, if I am unsure of the genetic line, I use acepromazine cautiously or at low doses, and I monitor blood pressure throughout the procedure.

There are certainly dog breeds that present certain anesthetic challenges or considerations. Brachycephalic breeds (bulldogs, boston terriers, Pekinese, etc) usually have the classic 4 anatomic features that can make intubation and airway management challenging: stenotic nares, elongated soft palate, everted laryngeal saccules, hypoplastic trachea. These cases need to be monitored whenever they are sedated, preoxygenation prior to intubation is warranted, endotracheal tube sizes may have to be adjusted down, and extubation should only be performed when the animal is able to actively and forcibly "spit out" the tube. Even after extubation the animal should be monitored, especially if sedatives or opioid analgesics are on board. Other dog breeds that impose certain challenges are Shar-pei's, in whom IV catheter placement can be challenging because of their skin folds and generally thick skin. Some dog breeds have genetic predisposition to develop certain diseases that can increase anesthetic risk, for example Dobermans can have dilated cardiomyopathy and/or Von Wildebrand factor deficiency, toy breeds can have portosystemic shunts, etc. The management details for these patients are beyond the scope of this lecture. The point is that their breed per se does not increase their anesthetic sensitivity; rather it is their breed-related systemic disease.

Myth: Pre-anesthetic medication complicates anesthesia by adding too many drugs and will delay recovery

There are many valid benefits to pre-anesthetic medication. These include sedation and anxiolysis, which decreases the animal's stress response and the stress of those personnel that have to restrain it, preemptive analgesia prior to a surgical procedure, significant decrease in induction and inhalant dose requirements, a smoother and more consistent anesthetic plane, and a smoother and less painful recovery. In our anesthesia department, most animals (dogs and cats) receive a sedative + an opioid for premedication, occasionally with an NSAID as well, prior to anesthetic induction. We administer the drugs IM (occasionally IV if a catheter is already in place), then 20 minutes later the animal is ready for induction. By incorporating premedication, we reduce induction dose requirements by 50% or more, depending on doses used and how sedate the animal becomes, and we reduce inhalant requirements similarly by >50%. This means that blood pressure, heart rate, and respiratory drive are all better maintained during surgery, because it is high doses of inhalants, not premedications, which depress cardiovascular and respiratory function the most dramatically.

It is true that some premedications (e.g. acepromazine, opioids, alpha-2 agonists if not reversed) may delay recovery, but this is generally a desirable quality. Anesthesia and surgery are exhausting events and a restful recovery, where the animal is kept quiet, warm and comfortable, are necessary components of a successful surgical outcome. If the animal needs to be discharged to its owner at the end of the day, the premedication should be planned such that doses are kept low for drugs that are not reversible, or reversible drugs can be used. The clinician should be mindful, however, of analgesic needs as well and not limit analgesic therapy simply for the sake of a quick recovery.