Posted on January 31, 2016
Canine Anesthesia: Special Considerations Based on Breed, Size, and Conformation
While it’s true that Yorkshire Terriers and Great Danes are members of the same species, they are hardly alike when it comes to anesthesia. There is no one size fits all anesthetic protocol for dogs, and veterinarians must amend what they do based on their patient’s breed, size, and conformation.
Universal anesthetic strategies
While it’s important to individuate anesthetic protocols, there are some universal strategies that, with rare exception, should apply to all dogs undergoing anesthesia. They include the following:
– Performing pre-anesthetic physical examination and blood testing are important to minimize the potential for surprises after the patient is already under anesthesia. Blood test results ensure that the liver and kidneys (the organs responsible for clearing anesthetic drugs from the body) are functioning up to snuff.
– Placement of an intravenous catheter allows instant access to the bloodstream for administration of intravenous fluids as well as blood products and medications should an urgent need arise.
-Continuous monitoring of vital parameters including heart rate, respiratory rate, body temperature, blood pressure, and oxygenation provides early indicators of abnormalities. This allows for prompt and early intervention should a problem arise. The person monitoring anesthesia should ideally be dedicated to this task and this task alone.
Customized anesthetic strategies
Breed, size, and conformation all influence how veterinarians choose to safely transport their canine patients through general anesthesia. Specific examples are provided below.
Nothing challenges a successful anesthetic outcome quite like the conformational modifications associated with brachycephalic breeds. In Greek, “brachy” means short and “cephalic” refers to head. Over time, breeders have developed a number of “short-headed” (what I lovingly refer to as smoosh faced) breeds such as Pugs, Pekingese, Boston Terriers, Shih Tzus, and several varieties of Bulldogs.
Brachycephalic dogs are genetically programmed to have narrowing of the upper airway passages including the nostrils, nasal cavities, throat, trachea (windpipe), and larynx (the opening to the trachea). Additionally, brachycephalics often come with an elongated soft palate that hangs down over the larynx, blocking airflow in and out of the trachea. These inherited respiratory tract abnormalities are generically referred to as “brachycephalic syndrome.”
The elongated soft palate and narrowed larynx and trachea can complicate placement of an endotracheal tube, the breathing tube that is placed immediately after the dog has been anesthetized and then remains in the trachea during anesthesia. This tube provides oxygen and anesthetic gas to the dog. The size of the breathing tube that actually fits may be considerably smaller than is ideally suited to the size of the dog. This can compromise delivery of anesthesia and adequate exchange of respiratory gasses.
The elongated soft palate along with the “meatier” tissue in the throat of brachycephalic breeds make these dogs far more susceptible to airway obstruction and aspiration pneumonia (inhalation of vomited or regurgitated material into the lungs) during the recovery period from anesthesia. Either one of these events can be life threatening.
When working with a brachycephalic dog, it makes good sense to:
- Provide at least a few minutes of preoxygenation (oxygen delivery via a mask that fits over the face) before the dog is anesthetized. This will be beneficial if it takes longer than normal to place the endotracheal tube.
- Have several different size endotracheal tubes in the ready. Until the dog is anesthetized and the diameter of the larynx and trachea are assessed, the size of breathing tube that will fit is anyone’s guess.
- Leave the endotracheal tube in place as long as possible when the dog is recovering from anesthesia in order to help prevent airway obstruction and aspiration pneumonia.
- Be prepared to provide oxygen via mask after the endotracheal tube has been removed.
- Watch the dog like a hawk until recovery from anesthesia is 100 percent complete. Only a few seconds are required for a brachycephalic breed to get into serious trouble during the anesthetic recovery period.
Greyhounds and possibly other sighthounds
Greyhounds are known to have prolonged recoveries following anesthesia with thiopental, a drug that is no longer available in the United States. The prolonged recovery is caused by a deficiency of a specific liver enzyme responsible for metabolizing this drug for removal from the body. This same liver enzyme abnormality can cause Greyhounds to experience prolonged recovery periods following anesthesia with propofol, a drug commonly used in the United States (think Michael Jackson here). It is presumed, but not necessarily proven, that other sighthounds such as Afghans, Whippets, Deerhounds, Wolfhounds, and Borzois may share this anesthetic idiosyncrasy.
Many anesthetic drugs are “lipophilic”, meaning they are attracted to fat tissues. The very lean, muscular conformation of most sighthounds may limit normal uptake of these fat-seeking anesthetic drugs. Less drug taken up by the tissues means more drug in the bloodstream, and it is the amount in the bloodstream that dictates the level of anesthesia. This may help explain why lower anesthetic drug dosages are better tolerated by many sighthounds.
When working with a Greyhound or other sighthound, it makes good sense to:
- Administer an injectable anesthetic drug dose that is less than what would normally be used based on the dog’s body weight.
- Administer plenty of intravenous fluids before, during, and after anesthesia to help clear anesthetic drugs from the dog’s system.
- Be hyper-vigilant about monitoring anesthesia.
- Be prepared for prolonged anesthetic recovery times. Have appropriate staff available and schedule anesthetic procedures for earlier rather than later in the day.
Many herding breed puppies such as Collies, Australian Shepherds, Old English Sheepdogs, and Shetland Sheepdogs, are born with a mutation of the multidrug resistance (MDR1) gene. The MDR1 gene is responsible for effectively processing a number of drugs in the body. Mutation of this gene allows the abnormal accumulation of certain drugs within the central nervous system. The “poster child drug” that is problematic for dogs with the MDR1 mutation is ivermectin, a medication used to treat and prevent parasites. Acepromazine and butorphanol are two drugs commonly used in canine anesthetic protocols. They are reported to cause prolonged or excessive sedation in dogs with the MDR1 mutation.
When anesthetizing a herding breed dog, it makes good sense to:
- Find out if the dog has been tested for the MDR1 mutation. The results can help guide the anesthetic protocol.
- If using butorphanol and/or acepromazine, lower the dosage and proceed with caution.
Breeds susceptible to cardiomyopathy
Boxers, Doberman Pinschers, Irish Wolfhounds, Cocker Spaniels, and Great Danes are some of the breeds predisposed to cardiomyopathy, a disease of the heart muscle. For some dogs with cardiomyopathy, the very first evidence is an abnormal heart rhythm (arrhythmia) that is so mild it causes no overt symptoms. Anesthesia can cause this mild arrhythmia to become far more significant and potentially even life threatening.
When anesthetizing a dog that is a breed susceptible to cardiomyopathy, it makes good sense to:
- Run an electrocardiogram (ECG) to assess the heart rhythm as part of the preanesthetic screening process.
- Run a continuous ECG during anesthesia as well as throughout the recovery process.
- Have appropriate antiarrhythmic drugs in the ready, should a problem arise.
Toy and tiny breeds
Really small dogs can be challenging to safely anesthetize for a few reasons. It can be tough to successfully place an intravenous catheter in those tiny little legs. And if those tiny little legs are attached to a wiggler or a biter, the challenge becomes even greater.
Compared to their larger counterparts, little dogs are more susceptible to hypothermia (decrease in body temperature). Dropping a degree or two during anesthesia is normal, but, given the opportunity, tiny dogs will drop five degrees or more. This level of hypothermia can cause all sorts of other problems.
Additionally, small dogs are more prone to developing hypoglycemia (low blood sugar) while under anesthesia. This can result in weakness and neurological symptoms, from muscle tremors to seizures.
Lastly, when tiny patients are undergoing surgery, they are usually covered from head to toe with surgical drapes. This makes it difficult for the person who is monitoring anesthesia to gain access to their patient’s body parts to accomplish things such as adjusting monitoring probes, taking body temperature, and giving injections through the intravenous catheter.
When anesthetizing a tiny breed, it makes good sense to:
- Use sedation and/or local anesthesia for intravenous catheter placement.
- Use appropriate heating devices during anesthesia and the recovery period.
- Use warmed intravenous fluids rather than those that are cold or at room temperature.
- Consider the addition of dextrose (sugar) to the intravenous fluids.
- Monitor body temperature frequently.
- Monitor blood sugar levels before, during, and following anesthesia.
- Find creative ways to allow the person monitoring anesthesia to gain access to the patient under all those surgical drapes.
In general, anesthetic drug dosages are calculated based on the patient’s body weight. For giant breeds such as Great Danes, Mastiffs, and Wolfhounds, a drug dose based on body weight ends up being too much. This is because the way drugs are cleared from the body has more to do with the animal’s body surface area than its body weight. Giant breeds have a smaller surface area to body weight ratio compared to smaller dogs.
Additionally, the aging process in giant breed dogs is accelerated. Whereas a seven-year-old Sheltie is middle aged, a seven-year-old Saint Bernard has already reached senior citizen status, and seniors are at greater risk with general anesthesia.
When anesthetizing a giant breed of dog it makes good sense to:
- Begin with lower drug dosages than would be calculated based on body weight.
- Ensure adequate staff to safely move and position a very heavy anesthetized dog.
- Carefully consider the age of the dog when calculating drug dosages.
Do you recognize any anesthesia precautions that would apply to your dog? Has your dog ever experienced a complication from anesthesia?
Nancy Kay, DVM
Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
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