Carolina H. Riccó Pereira and Phillip Lerche Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio, USA, The selection of a particular anesthetic plan is predicated upon the patient’s physical status and temperament, the type of procedure for which anesthesia is being considered, anticipated degree of perioperative pain, the familiarity of the anesthetist with the anesthetic drugs, the type of facility and equipment available, the personnel available for assistance, and the cost of anesthetic drugs [1,2]. There is no single best method for anesthetizing dogs or cats, and familiarity with just one anesthetic technique at best limits a veterinarian’s ability to perform the myriad of surgical and diagnostic procedures commonly performed in a modern veterinary practice and at worst results in unnecessary risk to the patient. A debilitated dog or cat undergoing extensive repair of a fractured limb will require a different anesthetic regimen than one undergoing routine neutering, one requiring short‐term restraint for radiography, or a geriatric patient requiring extensive dental surgery. General anesthesia is characterized by muscle relaxation, unconsciousness, amnesia, and analgesia. It is difficult for a single drug to provide all of these elements without causing significant disturbances to patient homeostasis. Inhalant anesthetics come closest to satisfying all of these conditions, but even they are more useful when co‐administered with anesthetic adjunctive drugs such as sedatives, opioids, local anesthetics, or hypnotics. As a general rule, when formulating an anesthetic plan, it is best to consider using relatively low doses of several different drugs rather than a large dose of a single drug. For example, apnea resulting from a large bolus of propofol can be eliminated, or its duration shortened, by prior administration of opioids, acepromazine, benzodiazepines, or α2‐adrenergic receptor agonists, which allows administration of a lower propofol dose [3]. The opioid drugs, while important components of modern anesthetic regimens, by themselves do not produce general anesthesia [4]. Muscle rigidity, salivation, and long recoveries associated with large dosages of ketamine can be lessened when the latter is combined in reduced doses with opioids, α2‐adrenergic receptor agonists, and hypnotic drugs such as benzodiazepines [5]. Any plan for procedural sedation or general anesthesia must include a provision to control pain if it is present or anticipated. A good analgesic regimen should include drugs sufficient to ensure analgesia during and after the procedure. The one aspect that should not vary among anesthetic procedures is the degree of vigilance associated with monitoring an anesthetized dog or cat. Early warning of impending anesthetic difficulty is the single most important factor responsible for decreasing anesthetic‐related morbidity and mortality. Consideration should also be given to the use of anesthetic checklists to improve patient outcomes [1]. Recording a thorough history and conducting the physical and laboratory evaluation are the most important components of a preanesthetic evaluation. Even young, seemingly healthy, animals presenting for routine procedures, such as neutering, require both. These animals may have never been examined previously by a veterinarian, and congenital disorders, severe parasitism, or heartworm disease may be discovered. Review of patient signalment is a key preanesthetic consideration (Box 59.1). Anesthesiologists are often questioned about “sensitivity to anesthesia” in a variety of dog and cat breeds. Although several breed‐associated anesthesia concerns (e.g., predisposition to hereditary diseases that increase anesthetic risk) have been documented, all breeds have been successfully anesthetized by using standard anesthetic regimens, and most reports of “sensitivities” are anecdotal. An assessment of the validity of information relating to anesthesia in dogs on the World Wide Web reported that information was often incomplete, and that breed sensitivities mentioned were not supported by scientific evidence [6]. One well‐documented breed‐associated anesthetic concern is the altered pharmacokinetics of barbiturates and propofol in certain sighthounds, which is associated with a mutation in cytochrome P450 CYB2B11 enzyme expression [7–9]. Another is brachycephalic breeds and their associated airway anatomic malformations. This presents a concern for ensuring a patent airway throughout the perianesthetic period. There is also evidence that these breeds possess greater resting vagal tone, heightening the concern for anesthetic‐induced bradycardia [10]. Anesthetic risk has been shown to increase in these breeds as body weight decreases and as duration of anesthetic increases, with the incidence of intra‐ and postoperative complications being 1.6 and 4.3 times greater than in non‐brachycephalic dogs, respectively [11]. Since toy breeds have a greater surface area to body mass ratio and have a relatively greater metabolic rate, they require careful attention to maintenance of body heat and blood glucose concentrations. Additionally, they require a relatively greater dose of drugs on a per‐kilogram basis than larger dogs. Reduced doses of some drugs (acepromazine, butorphanol, maropitant, and ondansetron) should be used in breeds that are known to have the ABCB1 gene (formerly known as “MDR1”) [12]. Further information about canine breed‐specific anesthetic considerations is available in Chapter 50. Generally, there are no sex‐related differences in the response to anesthesia. However, a history of the estrus cycle will often identify recent estrus and thus alert the clinician to the concerns associated with an enlarged and vascularized uterus. This would potentially cause concern regarding blood loss during an ovariohysterectomy. Additionally, the owner of an intact female animal should be queried about the possibility of their animal being pregnant because the stress of surgery and anesthesia may adversely affect the fetus(es), and pregnancy can affect multiple body systems (see Chapter 45). Age is an important anesthetic consideration. Generally, the very young (less than 8 weeks old) and the aged (more than 80% of their expected lifespan) do not metabolize anesthetic drugs as rapidly as healthy adult animals [13]. Healthy geriatric patients may only require 25–50% of the dose of sedatives, hypnotics, tranquilizers, and opioids given to comparable young, healthy animals. In addition to questions concerning organ system function (Box 59.1), the owner should also be asked about any previous anesthetic episodes, past and present illnesses, and past and current medication history, including history of heartworm prophylaxis [14]. The time elapsed since the last feeding should be noted. The preanesthetic physical examination should be thorough, with all body systems considered (Box 59.2). Any abnormality discovered by physical examination or suggested by the medical history should be followed with appropriate laboratory or other suitable diagnostic testing. The assessment of an animal’s temperament is critical. Vicious or aggressive animals will require a different approach to anesthesia than quiet, cooperative individuals. There is no objective evidence supporting age‐based or American Society of Anesthesiologists (ASA) physical‐status‐based (see below) minimum laboratory evaluation requirements [1]. Regardless, the minimum preanesthetic laboratory data suggested for young, healthy dogs are hematocrit and plasma protein. These tests are easy, quick, and inexpensive to perform. Hematocrit is an indicator of hemoglobin concentration, which directly relates to the ability of the blood to transport oxygen to tissues. As a general rule, a hematocrit of less than 20% indicates the need for perioperative administration of blood. Hemoglobin concentration (g/dL) can be approximated by dividing the hematocrit by three. For elective procedures in middle‐aged to older animals, or animals treated chronically with medications that could alter liver or renal function (e.g., non‐steroidal anti‐inflammatory drugs, phenobarbital, or antineoplastic chemotherapeutics), a complete blood count, urinalysis, and biochemistry profile is recommended. Other laboratory tests should be performed (e.g., thoracic radiographs and/or echocardiography, abdominal radiographs, and/or ultrasound) if the history or physical examination suggests specific organ system disease. A minimum laboratory database prior to emergency anesthesia for a debilitated dog or cat should include packed cell volume, total protein, and electrolytes (sodium, potassium, and chloride). Table 59.1 Physical status classification of veterinary patients. ASA, American Society of Anesthesiologists. a Procedures performed under emergency conditions are denoted by placing an E after the physical status number. Many factors (e.g., age, breed, concurrent disease, surgical procedure, surgeon skill, and available equipment) contribute to the overall anesthetic risk for a given patient. One risk factor is the physical status of the patient. A convenient system of status classification for veterinary patients has been adapted from the ASA [1]. In general, physical status I and II patients appear to be at less risk for anesthetic complications. Physical statuses III–V are usually at greater anesthetic risk. However, this is not to imply that category I and II patients are at no risk from unanticipated anesthetic mishaps (Table 59.1). Healthy dogs and cats should be fasted for 4–6 h prior to being anesthetized, if possible [1]. Water can be allowed until just prior to anesthesia for most procedures. Dogs and cats less than 8 weeks old should not be fasted longer than 1–2 h because they are at a greater risk for perianesthetic hypoglycemia. They should receive dextrose‐containing intravenous (IV) fluids during any prolonged anesthesia (longer than 15 min) and/or serial blood glucose measurements until fully recovered. When possible, life‐threatening physiological disturbances should be medically addressed prior to anesthesia (Box 59.3). This may, however, not always be possible, and anesthesia should never be delayed if immediate surgical or medical intervention is needed to save the patient’s life. Several aspects should be considered when formulating an anesthetic plan (Box 59.4). In general, techniques for procedural sedation or general anesthesia rely primarily on local anesthesia, injectable anesthesia, or inhalation anesthesia. Regardless, these primary techniques are typically supplemented with some degree of drug‐induced sedation and systemically administered analgesics. Techniques frequently overlap. For example, inhalant anesthesia is usually initiated with injectable anesthetics. Local anesthetic nerve blocks are typically performed under general anesthesia or sedation. The remainder of the discussion regarding the choice of anesthetic and analgesic drugs assumes that the reader has reviewed and is familiar with the clinical pharmacology of the various anesthetic drugs as presented elsewhere in this text. Although the drug combinations described are suitable for a variety of patients, the reader should refer to the appropriate sections of this text or consult a veterinary anesthesiologist if questions remain about how to anesthetize and monitor specific patients. Drug availability can be an issue, so it is best to become familiar with a variety of techniques. By themselves, sedative–opioid combinations are suitable for short‐term restraint for minimally invasive procedures or those procedures not requiring general anesthesia, such as radiography or physical examination in an intractable animal (Tables 59.2 and 59.3). An advantage is that one or both of these components are reversible, allowing a rapid return to preanesthetic mentation and function (Table 59.4). Any of the short‐term injectable drugs discussed below can be added to the sedative–opioid regimen when complete immobilization or general anesthesia is necessary. If potentially painful surgical procedures are attempted under heavy sedation (e.g., laceration repair following dexmedetomidine–butorphanol), a local anesthetic should be incorporated in the plan to reduce the risk of animal arousal due to surgical stimulation. Table 59.2 Sedatives and tranquilizers for procedural sedation or premedication. IM, intramuscular; IV, intravenous; SC, subcutaneous. a Generally the low end of the dosage range is used IV and in sick or debilitated patients. Several drugs are available for procedural sedation or short‐term anesthesia (Tables 59.2, 59.3, 59.5, 59.6, and 59.7
59
Dogs and Cats
Introduction
Preanesthetic considerations
Signalment
History
Physical examination
Laboratory evaluation
ASA physical status a
Patient description
I
Normal healthy patient
II
Non‐incapacitating systemic disease (e.g., obesity, mild dehydration, and simple fractures)
III
Severe systemic disease not incapacitating (e.g., compensated renal insufficiency, stable congestive heart failure, controlled diabetes mellitus, or cesarean section)
IV
Severe systemic disease that is a constant threat to life (e.g., gastric dilation and volvulus)
V
Moribund, not expected to live 24 h irrespective of intervention (e.g., severe uncompensated systemic disturbance)
Physical status
Patient preparation
Fasting
Patient stabilization
Anesthetic and analgesic plan
Short‐term anesthesia (less than 15 min)
Drug
Dosage (mg/kg) a
Comments
Dexmedetomidine
Dogs, 0.002–0.02 IV, IM
Cats, 0.003–0.04 IV, IM
Moderate to deep sedation
Duration 60–180 min
Medetomidine–vatinoxan (Zenalpha®)
Dogs, 0.01–0.03 IM
Moderate to deep sedation
Duration 30–90 min
Acepromazine
Dogs, 0.02–0.1 IV, IM, SC
(3 mg maximum)
Cats, 0.02–0.1 IV, IM
(1 mg maximum)
Mild to moderate sedation
Duration 30–90 min
Midazolam
Dogs, 0.1–0.3 IV, IM
(7 mg maximum)
Cats, 0.1–0.2 IV, IM
Minimal sedation
Most useful when combined with other sedatives, opioids, or ketamine
Can cause excitation when given as the sole agent
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