Genetic differences and anatomic factors can contribute to variability and complications during anesthetic events.
Certain breed differences can lead to greater risks for airway obstruction, increased responsiveness to anesthetic drugs, and delayed recovery, all of which can result in increased anesthesia-related morbidity and mortality. Individual genetic variability can trigger unexpected and adverse responses to anesthetic drugs, which need to be identified by good recordkeeping and consistent patient monitoring. Although genetic differences are typically held responsible for prolonged recoveries and increased drug responsiveness, true genetic sensitivity has been demonstrated in only a handful of breeds, including the greyhound and the collie.
Because many canine breeds can suffer from cardiac disease, both acquired and congenital, it is important to know which patients are likely to be affected before the anesthetic protocol is planned. If cardiac disease is suspected, a full cardiac workup with a veterinary cardiologist is recommended.
Brachycephalic breeds have anatomic considerations that may affect anesthetic outcome. Most brachycephalic breeds suffer from brachycephalic airway syndrome (BAS), which is characterized by stenotic nares, elongated soft palate, everted laryngeal saccules, and hypoplastic trachea.
Many brachycephalic dogs respond well to acepromazine in conjunction with an opioid; however, the sedative dose should be half of that used for nonbrachycephalic dogs. Full mu-opioid agonists can be used but because they may cause excessive respiratory depression, a reversal agent should be available. Dexmedetomidine should be avoided because of the presence of high vagal tone in these breeds. Anticholinergics, such as glycopyrrolate, may be used to decrease airway secretions and counteract high vagal tone.Affected dogs have narrower upper airways than do dogs with normal anatomic features.1-3 Because in brachycephalic breeds additional airway contraction can occur with stress (ie, increased respiratory effort, turbulent flow), clinicians need to be prepared for possible upper airway obstruction. Furthermore, brachycephalic dogs must be monitored closely after premedication, throughout anesthesia and the postoperative period, and after extubation. An oxygen source and endotracheal tube should be readily available.
Preoxygenation is recommended before dogs with BAS are induced.2-4 Propofol or a similar short-acting drug should be used for induction and intubation should be completed as rapidly as possible. Mask inductions should be avoided,3,4 and smaller endotracheal tubes should be used.
Because brachycephalic breeds tend toward obesity, controlled or mechanical ventilation is often necessary. Most problems associated with mechanical ventilation occur during induction and recovery, so monitoring is particularly important.
Extubation should be postponed until the patient is bright, alert, swallowing—even chewing on the endotracheal tube.4 If extubation is attempted while the patient is sedated and groggy from anesthesia, there is increased risk for upper airway obstruction. If upper airway obstruction occurs, the patient should be reintubated.
Brachycephalic cats should be handled with care from premedication to recovery and treated in a manner similar to dogs.
Most breeds do not have true sensitivities to anesthestics, although sighthounds (particularly the greyhound) do have genetic factors that cause them to metabolize drugs differently. Therefore, before designing an anesthetic protocol, examination findings and blood work results need to be evaluated. Sighthounds have a higher packed cell volume and lower serum albumin concentration than do mixed breeds.2,3 Sighthounds also should be evaluated for cardiac abnormalities (eg, dilated cardiomyopathy).
Because sighthounds are high-energy animals, they may experience high levels of hospitalization stress. Acepromazine is recommended for preventing stress in healthy sighthounds, but some individuals may be more sensitive to its sedative effects, so a lower dose (0.02–0.03 mg/kg) is advised.4,5 In addition, sighthounds metabolize drugs more slowly as compared with the average dog. Thiobarbiturates should be avoided and other induction agents (eg, propofol, ketamine, etomidate) used instead.1-3 Of note, propofol is metabolized more slowly in these dogs because of reduced hepatic enzymatic activity, which may result in slower recovery.4,6,7
Sighthounds have a low percentage of body fat (17%) compared with the average dog (35%),6 which leaves them at risk for hypothermia during anesthetic procedures.2,3,6 Therefore, it is important to use hot water blankets and forced-air warmers during the perioperative period.
The effect of certain anesthetic drugs on herding dogs (eg, collie, border collie, Australian shepherd, Shetland sheepdog) is somewhat controversial. However, these breeds have a high prevalence (eg, up to 75% in collies in the United States) for genetic mutation in the ABCB1 (formerly MDR1) gene.8,9 ABCB1 encodes P-glycoprotein, an adenosine triphosphate–driven pump that is an integral component of the blood–brain barrier and provides protection from toxic drug accumulation in body fluids, such as cerebrospinal fluid (CSF).10 In the collie and other herding breeds, this genetic mutation results in a defective pump that allows a select group of drugs to accumulate within the brain.
Acepromazine and opioids, particularly butorphanol,9 are members of this drug group, and their accumulation in CSF may cause marked sedation and respiratory depression. When using these agents in susceptible breeds, the dose should be decreased by 25% and patients closely monitored for side effects.5,9
Because of their size, toy breeds can present unique challenges. Obtaining an accurate weight and using the appropriate dose of anesthetic drug are essential. Monitoring during surgery likewise is important. In small patients, Doppler blood pressure measurement has been more accurate than oscillometric monitoring; in addition, it provides an auditory sound to monitor heart rate and rhythm.
Toy breeds have a greater body surface area–to–body mass ratio and higher metabolic rate, which can lead to lower body temperatures and hypoglycemia.1 It is important to support normal body temperature during anesthesia, monitor blood glucose levels, and apply adequate supplementation as indicated.3
Giant breeds often respond profoundly to normal therapeutic doses of sedatives, such as acepromazine. In this patient population, it is important to either reduce the dose of acepromazine (0.01–0.025 mg/kg) or calculate the dose based on lean body mass or surface area and not the actual body weight.2,3
Genetic variation is also noted in Doberman pinschers. In addition to a predilection for developing dilated cardiomyopathy, these dogs can have von Willebrand disease, which impairs normal clotting. It is important to evaluate the coagulation status of these patients before surgery.
If von Willebrand disease is suspected, desmopressin (also known as DDAVP) given before surgery promotes von Willebrand factor secretion from endothelial storage sites.3
In this patient population, the use of NSAIDs is somewhat controversial and other analgesic options should be explored. If NSAIDs must be administered, preference should be given to cyclooxygenase (COX)-2–selective drugs.
Boxers of UK Lineage
On rare occasions, individual variability can result in a subpopulation within a breed that responds differently to anesthetics. An example can be seen in boxers from the UK. In this subpopulation, acepromazine often causes severe bradycardia, hypotension, and collapse, so a reduced dose of acepromazine (0.01–0.025 mg/kg) is recommended.2,4
Because there are no published reports describing similar effects in US-bred boxers, standard doses of acepromazine are typically routine in the US subpopulation.
Although it is important to be aware of breed-related anesthetic differences, the primary consideration is the individual patient and tailoring the anesthetic protocol accordingly. With proper perioperative workup and appropriate patient monitoring, safe and successful sedation and anesthesia can be performed in any breed of dog or cat. Patient monitoring should begin with premedication and end only after the patient has been extubated and is normothermic, stable, and alert.
STEPHANIE KREIN, DVM, is an anesthesia resident at Tufts Cummings University School of Veterinary Medicine, where her primary focus includes cardiovascular and respiratory physiology in both small and large animals. Prior to becoming an anesthesia resident, she practiced as an emergency veterinarian near Cape Cod, Massachusetts.
LOIS A. WETMORE, DVM, ScD, DACVA, is an assistant professor in the department of clinical sciences at Tufts Cummings University School of Veterinary Medicine, where her focus is anesthesiology and pain management. Her research interests include polymorphisms in opioid receptors, the effects of MDR1 mutations on oral morphine absorptions, and the metabolism of NSAIDs in dogs. Dr. Wetmore received her DVM from Colorado State University and ScD from Harvard School of Public Health.
- Dogs and cats. Bednarski RM. In Tranquilli WJ, Thurmon JC, Grimm KA (eds): Plumb and Jones’ Veterinary Anesthesia and Analgesia, ed 4—Ames, IA: Blackwell Publishing, 2007, pp 705-706.
- Anaesthesia of the dog. In Hall LW, Clarke KW, Trim CM (eds): Veterinary Anaesthesia, ed 10—London: WB Saunders, 2001, pp 393-394.
- Canine breed-specific problems. Cuvelliez S, Ronenay Y. In Greene SA (ed): Veterinary Anesthesia and Pain Management Secrets—Philadelphia: Hanley & Belfus, 2002, pp 233-237.
- The anesthetic period: Predictable problems. Modifications for breeds. In Sawyer DC: The Practice of Small Animal Anesthesia—Philadelphia: WB Saunders, 1982, pp 186-187.
- Plumb’s Veterinary Drug Handbook, ed 7. Plumb DC—Hoboken, NJ: Wiley Blackwell, 2011.
- Anesthesia of the sighthound. Court MH. Clin Tech Small Anim Pract 14:38-43, 1999.
- Evidence for propofol hydroxylation by cytochrome P4502B11 in canine liver microsomes, breed and gender differences. Hay Kraus BL. Xenobiotica 30:575-588, 2000.
- Breed distribution of the ABCB1-1Δ (multidrug sensitivity) polymorphism among dogs undergoing ABCB1 genotyping. Mealey KL, Meurs KM. JAVMA 293:921-924, 2008.
- Affected breeds. Washington State University veterinary clinical pharmacology laboratory; http://vetmed.wsu.edu/depts-vcpl/breeds.aspx; accessed January 2012.
- A review on the impact of P-glycoprotein on the penetration of drugs into the brain. Focus on psychotropic drugs. Linnet K, Broeng Ejsing T. Euro Neuropsychopharmacol 18:157-169, 2008.
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