December 8, 2014
Originally published in the August 2014 issue of Veterinary Practice News
The practice of mating individuals with desirable physical characteristics and/or the ability to excel at performing specific tasks has led to the development of over 170 recognized dog breeds in the United States.1 This large variety of dog breeds translates to a very diverse patient population for veterinarians; one may have to anesthetize a great Dane and Chihuahua in the same day. Owners and dog breeders will often express concern about their particular breeds being sensitive to anesthesia; however, there is only one breed–specific anesthetic sensitivity that is supported by scientific evidence: greyhounds.
Even though a particular breed may not have a true anesthetic sensitivity, that breed may still warrant specific anesthetic considerations.
By selecting for specific traits in dogs we have inadvertently or purposely created unique anatomic, physiologic and metabolic characteristics that may ultimately affect how a given dog is able respond to general anesthesia. An excellent example of this is English bulldog.
The greyhound is the only breed with a documented breed-specific sensitivity to particular anesthetic drugs. Greyhounds have prolonged recoveries following anesthesia with thiopental due to a deficiency in the hepatic enzyme responsible for the first step in the metabolism of that drug.7
Given that thiopental is no longer commercially available in the U.S., this will be of little clinical significance unless this drug makes its way back into the commercial market. Compared to mixed-breed dogs, greyhounds may also experience prolonged recoveries after anesthesia with propofol.8
Similar to the thiopental sensitivity, greyhounds’ sensitivity to propofol is due to a deficiency in the cytochrome p450 enzyme (CYP2B11) responsible for the metabolism of propofol.9
Greyhounds and other similarly lean-muscled breeds (i.e. Afghan, whippet, borzoi, etc.) may also appear more sensitive to lipophilic drugs (i.e., most anesthetic drugs) due to a lower volume of distribution. With a smaller volume of distribution, a given dose of a lipophilic drug will result in a higher plasma concentration and a potentially more significant clinical effect in very lean-bodied dogs compared to other dog breeds with typically more adipose tissue.
Anesthesia of small and toy breed dogs offers several unique challenges that are specifically related to the small size of these patients and not specifically to their breed.
Intravenous catheter placement can be difficult due to the small size of the patient’s blood vessels and the potentially challenging physical restraint. Small patients are easy to overpower, but one must provide the appropriate level of physical restraint without accidentally harming these fragile patients.
In addition to the appropriate choice of drugs for premedication, the use of EMLA cream can greatly facilitate intravenous catheter placement. EMLA is a topical anesthetic cream that can be applied over the proposed IV catheter sites at the time of premedication administration.
While the patient is given time to allow the sedation to take effect (~30 minutes) the local anesthetic will desensitize the skin over the IV catheter sites, thus eliminating or reducing some of the stimulus that may cause a patient to struggle during IV catheter placement.
Small breeds also have a high surface area to body mass ratio, making them especially susceptible to heat loss and hypothermia.
Radiant heat losses can be reduced by increasing the temperature in the operating suite. Convective heat loss can be minimized by keeping the patient covered with blankets. Conductive heat losses can be minimized by preheating the surgery table, using warm surgical preparation solutions, warm fluids for any flushing, and using warmed IV fluids.
Evaporative heat losses can be minimized by limiting the amount of time that body cavities are open and the use of a Bain non-rebreathing system can help minimize evaporative losses from the respiratory tract.
Anesthetic monitoring of small patients can also be challenging. The size of the patient can affect the accuracy of oscillometric blood pressure measurements2; it has been suggested that Doppler blood pressure measurement be used for patients less than 10 kg.3
When surgery requires that the patient be positioned at the far end of the table (away from the anesthetist), hands-on assessment can also be limited due to the small size of the patient. Under such circumstances a clear disposable surgery drape can allow visualization of the patient. The use of a mini extension set can also allow prompt IV drug delivery with minimal flushing when patient access is limited.
Drugs that are formulated in relatively high concentrations (i.e. dexmedetomidine, fentanyl, ketamine) can also be diluted out to ensure more accurate dose administration.
The anatomic abnormalities commonly encountered in the upper airways of brachycephalic breeds (bulldog, pug, etc.) warrant several considerations for both induction of and recovery from anesthesia.
Brachycephalic syndrome is characterized by stenotic nares, an elongated soft palate, everted laryngeal saccules and a hypoplastic trachea. These abnormal features can obscure visualization of the larynx and make endotracheal intubation challenging.
These patients should be monitored closely after premedication administration because excessive sedation and muscle relaxation could potentially result in the elongated soft palate occluding the upper airway.
It is also difficult to accurately guess the appropriate size of endotracheal tubes for these breeds, so a variety should be available at induction. Pre-oxygenation with an appropriately fitting mask with the oxygen flow at 4-5 L/minute for at least 3 minutes can provide the anesthetist with a longer window of time to intubate before the patient becomes hypoxic.
A goal for induction should be to avoid apnea since there is potential for a difficult or unsuccessful intubation. Administering propofol slowly and to effect may help reduce the likelihood of apnea on induction. During intubation, a tongue depressor can be used to dorsally displace the soft palate and provide visualization of the arytenoids and epiglottis.
Many of the same concerns regarding induction are applicable to recovery from anesthesia. Brachycephalic dogs should be extubated when they will no longer tolerate the endotracheal tube. Breathing through the endotracheal tube often provides less resistance to air flow than a brachycephalic dog’s upper airway, thus the work of breathing is reduced by the endotracheal tube.
Upon extubation the patient must be alert enough to provide the extra work required for breathing once the tube is removed. These patients should be monitored closely after extubation since any residual sedation and muscle relaxation could potentially result in hypoventilation, hypoxia and/or upper airway occlusion.
The anesthetist should be prepared to provide oxygen supplementation via a mask until the patient can maintain its hemoglobin saturation at 95 percent or greater without oxygen support.
Several dog breeds may merit careful consideration of certain drug dosages. Some commonly used veterinary drug references suggest that boxers and giant breed dogs may be especially sensitive to the effects of acepromazine.4
Giant breed dogs often do not require the same mg/kg drug dose as a small breed dog to achieve the same desired effect (i.e. sedation). This is because metabolic rate (i.e. drug metabolism) is more closely correlated to body surface area than body weight, and giant breed dogs have a smaller surface area to body weight ratio than smaller dog breeds.5
Thus it is likely that giant-breed dogs appear more sensitive because they are getting a larger dose of acepromazine relative to their body surface area than smaller dogs. One may want to err on the low end of the dose range or not exceed a maximum dosage regardless of body weight, as has been suggested,6 in order to avoid excessive and/or prolonged sedation when administering premedications to giant-breed dogs.
Giant breeds also tend to have shorter life expectancies than smaller dogs, meaning that they become geriatric (i.e. attaining 75 percent of their life expectancy) at a younger absolute age. Thus one should be especially conservative with drug doses in older giant-breed dogs.
There are anecdotal reports of European-breed boxers being more sensitive to the effects of acepromazine; however, there is currently no published clinical evidence to support a boxer sensitivity to acepromazine or a boxer-specific dose of acepromazine.
Until such information is available it may be prudent to avoid or use reduced doses of acepromazine in European-breed boxers and remember that one can always administer additional drug if needed.
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Another breed specific consideration related to drug choices and dosages is the potential for mutations in the multidrug resistance gene (MDR-1) in herding breeds (i.e. Collie, Australian shepherd, border collie, Shetland sheepdog, Old English sheepdog).
The MDR-1 gene encodes the information necessary for the transcription and translation of P-glycoprotein (P-gp), a transmembrane protein that plays an important role as a drug efflux transporter.
A mutation in the MDR-1 gene results in a defective transport mechanism that can ultimately lead to toxic levels of some drugs in the central nervous system. The most notable and confirmed of these drugs is ivermectin.
Dogs can have mutations in one (heterozygous) or both (homozygous) of their MDR-1 genes, with homozygous dogs completely lacking a functional P-gp and thus being more severely affected by drugs that are P-gp substrates. Laboratory tests are available to determine if a given dog has a mutation in the MDR-1 gene and whether that patient is homozygous or heterozygous.
Acepromazine and butorphanol are the only commonly used anesthetic drugs on the list of P-gp substrates,5,10 and there are anecdotal reports of prolonged or excessive sedation with the use of these drugs in dogs with MDR-1 mutations. There have been recommendations to avoid acepromazine and butorphanol in homozygous dogs and reduce the dose in heterozygous dogs,10 but at this time there is no published evidence of altered pharmacokinetics to support a specific dosing strategy.
In conclusion, by considering breed differences in size, anatomy and drug response, the practitioner can design an anesthetic plan with the lowest chances of complications or unwanted side effects.
When significant concerns or questions arise, a board-certified diplomate of the American College of Veterinary Anesthesia and Analgesia can always be consulted to help with planning of anesthesia and to address owner concerns
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