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Cat Dental Health & Tooth Care : Pre-Anesthesia Blood Work for Cats

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Spay/Neuter Patient Care: Anesthesia - Feline

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The word anesthesia comes from the Greek meaning "lack of sensation". Anesthesia is accomplished by administering drugs that depress nerve function. With general anesthesia, the patient is made unconscious for a short period. During this unconscious state, there is muscular relaxation and a complete loss of pain sensation.

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Adapted with permission from the Michigan Association of Veterinary Technicians, Vet Tech Insider, December 2015.

Anesthetizing cats can pose unique challenges for veterinary technicians. Not only are cats problematic to monitor under anesthesia, but their small size, interesting metabolism, variable temperament, and predisposition to particular health ailments can also complicate anesthetic administration and monitoring. Advanced preparation, skills, and knowledge allow astute technicians to anticipate patient requirements under a variety of circumstances, thereby improving the odds of a successful anesthetic procedure.

PATIENT CONSIDERATIONS

Preoperative Assessment

Thorough patient assessment is paramount. Patient signalment and history typically include patient name, species, breed, age, weight, and sex (intact, neutered, or pregnant), as well as discovery of recent health issues, current medications, and details surrounding the reason for presentation. ¹ Other important factors may include diet and housing conditions (indoor versus outdoor), preventive health status (e.g., date of most recent vaccine administration, fecal examination, feline leukemia virus test), and prior anesthetic episodes.

Whenever possible, a good physical examination is essential; all abnormalities should be further investigated. Preoperative blood analysis, radiography, and other diagnostic tests, such as blood pressure measurement, electrocardiography (ECG), echocardiography, and/or abdominal ultrasonography, may be necessary to completely define common feline health problems, such as obesity, diabetes, hyperthyroidism, renal insufficiency, hepatic lipidosis, hypertrophic cardiomyopathy, and asthma.

Minimizing Stress

Fear-free feline handling techniques are recommended by and available from the American Association of Feline Practitioners ( catvets.com). Although not always easy, every attempt should be made to minimize stress in cats to avoid the release of catecholamines in the perioperative period. The use of pheromone products (e.g., Feliway diffusers and sprays; ceva.us) in the examination room, feline wards, and on employees' hands and clothing and creation of dedicated feline wards help create a quiet, warm, clean, stress-free environment. ¹ Moreover, tender, loving care, when combined with pheromones (especially during mealtimes), can greatly reduce anxiety and promote food intake. ²

It is imperative to minimize stress whenever handling feline patients, using feline-friendly techniques when possible. Mildly stressed cats may be amendable to the use of "clipnosis" cat clips and/or air muzzles. However, sometimes the safest option for both patient and handler entails the use of chemical restraint and/or general anesthesia.

One sedation technique described for average-sized cats (3-4 kg) is administration of ketamine (1 mL PO) into the cat's mouth via a urinary catheter attached to the ketamine syringe. 1,3 The ketamine is rapidly squirted into the mouth when the cat attempts to bite the catheter. This technique results in a salivating, slower-moving, recumbent, and somewhat catatonic patient.

PREOPERATIVE CONSIDERATIONS

Numerous factors should be considered before anesthetizing cats. Severity of preexisting health problems guides the anesthetist in the choice of premedications, crystalloid fluid type and rate, induction agents, inhalant, and analgesic. For example, a typical feline hypertrophic cardiomyopathy patient requires careful titration of low-sodium intravenous fluid therapy (range, 2-5 mL/kg/hr) ⁴ to prevent fluid overload and pulmonary edema, while a patient with renal disease may benefit from preoperative diuresis, with particular attention to maintaining blood pressure, and avoidance of nonsteroidal anti-inflammatory drugs (NSAIDs).

The primary objective of premedication is to reduce the overall dosage of induction and maintenance agents while providing preemptive analgesia and anxiolytic effects. To this end, sedatives like acepromazine and/or midazolam can be combined with alpha-2 agonists or partial or pure mu opioid agonists for analgesia. Other preoperative considerations include the type of procedure being performed (elective versus emergent, short versus prolonged), patient factors that can adversely affect surgical positioning (e.g., large abdominal masses or gross obesity that compromises breathing), and expected level of discomfort associated with the procedure.

MONITORING AND EQUIPMENT CONSIDERATIONS

Ensure that emergency drug dosages have been calculated, surgical supplies are ready, and additional trained staff members are available to help should a crisis situation arise.

Critical Patients

Feline patients in shock may present with bradycardia, hypothermia, and hypotension. Because it is speculated that adrenergic receptors are down-regulated in hypothermic cats, aggressive warming measures and small volume resuscitation are advised (to a systolic blood pressure up to 60 mm Hg) to prevent fluid overload once the patient becomes normothermic. Therefore, intravenous fluid doses should always be administered "to effect." ⁵ Volume overload is more widespread in small patients and may manifest in anesthetized patients as serous nasal discharge, chemosis, pulmonary crackles, and, in severe cases, frothy fluid emanating from the endotracheal tube (ET).

Alternative therapies for hypotensive patients include the use of hetastarch or positive inotropic drugs (e.g., dopamine, dobutamine). When blood loss exceeds 20% of the total blood volume (or if the packed cell volume is <20% and total solids is <4 g/dL), ³ a whole blood transfusion should be considered. The patient should be blood typed in advance, as fatal reactions may occur when feline patients are administered incompatible blood products. The most common sign of a transfusion reaction in an anesthetized patient is hypotension.

Anesthetic Administration

Preoxygenation should be considered before induction whenever a difficult intubation is anticipated (such as when intubating feline patients), but it is considered particularly valuable for brachycephalic patients, patients with lung disease (e.g., asthma, heart disease), and patients with reduced functional residual capacity (e.g., pregnant females). Preoxygenating via flow-by, face mask (if tolerated), or induction chamber/oxygen cage for ≥5 minutes increases the reservoir of the lungs and replaces the air in the lungs with 100% oxygen. In the event of airway obstruction, difficult intubation, or apnea, preoxygenation permits a lapse of 3 to 4 minutes before the patient becomes hypoxic, compared with the 90 seconds it takes a nonpreoxygenated patient breathing room air to become hypoxic. ¹

Patient induction can be achieved via induction chamber, mask, or chemical routes. A mask or induction chamber should be considered only if an injectable agent is not an option. Chamber and mask induction are least desirable for several reasons:

Monitoring patients in an induction chamber is more difficult.

Staff members are exposed to high levels of waste anesthetic gases.

Excessive stress can be induced by pungent odors and a prolonged induction period.

Stress-induced cardiac arrhythmias can be severe, causing increased morbidity and mortality.

Postoperative analgesia is not provided by inhalants alone.

If chamber induction cannot be avoided, provide oxygen while allowing fractious cats to become calm before administering the inhalant. During chamber induction, monitor the patient frequently to evaluate level of sedation, and transfer the cat to a mask as soon as possible to better assess anesthetic depth. ³

A laryngoscope eases visualization of the feline laryngeal area during intubation. If necessary, use of a stylet can keep the ET rigid during the intubation process. However, laryngeal tissue trauma and tracheal tears may occur if the end of the stylet extends past the end of the tube or if the ET cuff is overinflated. Never force intubation.

If laryngeal spasm impedes intubation, lidocaine spray, 1% or 2% lidocaine solution drawn into a syringe, or a cotton swab impregnated with 2% lidocaine gel can be applied to desensitize the arytenoids. ¹ It is important to note that the use of benzocaine sprays to assist with intubation in cats can cause acute death due to the development of methemoglobinemia. Using a v-gel (docsinnovent.com) supraglottid tube (which covers the larynx) can provide an acceptable airway access alternative for some anesthetic procedures in cats (FIGURE 1).

Nonrebreathing circuits (e.g., Bain, Jackson-Reese) are typically used for maintenance of anesthesia in small patients (<7 kg; FIGURE 2). This type of circuit is advantageous in small patients because it decreases resistance to breathing. In addition, most nonrebreathing apparatuses are simple, inexpensive, and lightweight. The disadvantages of nonrebreathing circuits include dependence on high oxygen flow rates (200-500 mL/kg/min), increased cost of anesthetic agent(s) and oxygen, accelerated onset of hypothermia due to high oxygen flow rates, and potential for barotrauma if the pop-off valve is accidentally left closed.

Patient Monitoring

Pulse palpation is useful in evaluating heart rate (unless an arrhythmia is present) and is determined by the difference between the systolic and diastolic phase. The normal heart rate for anesthetized cats should range from 100 to 220 beats per minute. Bradycardia in cats should be avoided, as it can result in reduced cardiac output and hypotension. Similarly, tachycardia does not allow adequate time for cardiac filling, which also leads to decreased cardiac output and hypotension. Tachycardia can also increase the oxygen demand of the myocardium, inducing arrhythmias.

BOX 1 Methods for Measuring Blood Pressure

Two methods can be used to measure blood pressure indirectly-Doppler blood flow measurement and oscillometry.

Doppler methods use a "return-to-flow" principle to detect systolic blood pressure. Use of a Doppler device can prove advantageous to obtain periodic readings in anesthetized cats if oscillometric devices fail to record blood pressure readings. In cats, it is hypothesized that the resultant reading probably represents the mean arterial pressure (MAP). 1,7,8 As such, a correction factor of 14 mm Hg is added to the obtained reading to more accurately reflect the actual feline femoral systolic pressure. ⁸ Several readings should be obtained in a conscious patient and the results averaged.

Oscillometric methods detect intracuff changes caused by the pulse wave and calculate heart rate and systolic, diastolic, and MAP measurements. The author has had good experiences measuring blood pressure in cats while using a petMAP device (petmap.com), and satisfactory results with other oscillometric devices by placing the cuff around the proximal tail or over the distal humeral area, proximal to the elbow. Patient movement, smaller patient size (<5 kg), hypothermia or vasoconstriction, or patients with short legs or excessive skin all adversely affect results. ⁹

Regardless of the method used, selection of the correct-sized blood pressure cuff is imperative to obtain the most accurate results. Although, in general, the width of the cuff should extend 40% around the circumference of the limb, in cats it is acceptable to use a cuff that is only 30% of the limb's circumference. 9,10 Acceptable cuff locations include the forelimbs, tail, and hindlimbs; the areas proximal to the carpus and tarsus work best, but the ventral tail can also work well. ¹⁰ The cuff should be snug, but not too tight. Selection of an inappropriate cuff size is the most common source of error.

ECG monitoring is common during general anesthesia for small animal patients. It is important to ensure good contact of leads to skin by using ECG paste or alcohol when placing ECG leads. In addition, avoid wetting large areas of the skin and allowing the leads to come into direct contact with the table. Exact lead locations are not as important as ensuring that all waves are present (even if they are inverted). Because the ECG tracing does not provide information about chamber size or how efficiently the heart is ejecting blood, ECG should be used strictly for detection of dysrhythmias during the perianesthetic period. 1,6

Blood pressure monitoring is important during general anesthesia because all patients experience some degree of hypotension during general anesthesia. Blood pressure is determined by cardiac output and total peripheral resistance. Total peripheral resistance is defined as the resistance to blood flow created by the peripheral arterial system and capillary beds. Cardiac output is determined by a combination of the heart rate and stroke volume. Normal arterial blood pressure values for cats are a systolic measurement of 120 to 170 mm Hg and a diastolic measurement of 70 to 120 mm Hg. See BOX 1 for further information on blood pressure measurement.

Pulse oximetry provides continuous and noninvasive heart rate monitoring and an estimate of arterial hemoglobin saturation (SpO ₂). Often, after extended periods of use, the clip-on tongue sensor causes blanching of the local blood supply, impeding accurate SpO ₂ readings. The author has had success disarming the spring assembly of the clip and creating a modified clip that merely holds the probe ends opposed. This modification results in a clip incapable of producing pressure on the local blood supply (FIGURE 3).

End-tidal carbon dioxide (ETCO ₂) monitoring measures the level of carbon dioxide in the expired gases from the alveoli. An abrupt decrease in ETCO ₂ can be an early and reliable indication of impending cardiovascular collapse or cardiac arrest. Consequently, ETCO ₂ can be used to assess effectiveness of cardiopulmonary cerebral resuscitation techniques because delivery of carbon dioxide from the lungs requires blood flow, cellular metabolism, and alveolar ventilation.

Capnometers and capnographs monitor ETCO ₂ by evaluating samples of exhaled gases taken from the anesthetic circuit via an adapter placed on the end of the patient's ET. ⁸ This adapter must be placed precisely at the end of the patient's nose to eliminate excessive dead space and prevent rebreathing of carbon dioxide. Cutting the ET to a shorter length allows this placement, with the tube cuff situated immediately distal to the larynx, but no further than the thoracic inlet. 1,11

In smaller patients with sidestream ETCO ₂ sample collection tubing, it may be advantageous to eliminate excessive dead space by using a special ETCO ₂ adapter, which is a substitute for the connection of the ET to the wye hose interface (FIGURE 4). Alternatively, sidestream ETCO ₂ sample collection tubing can be attached to a 22-gauge needle that has been inserted directly into the lumen of the proximal section of the patient's ET.

In the absence of intracranial pathology, normal ETCO ₂ values should be maintained between 35 and 45 mm Hg. ¹ It is prudent to avoid hyperventilation, which can result in ETCO ₂ values <25 mm Hg. Values in this range may reflect decreased cerebral blood flow and oxygen delivery to the brain.

ANESTHETIC COMPLICATIONS

Hypothermia

Hypothermia is one of the most common anesthetic complications. Almost all anesthetized or sedated patients lose body heat under general anesthesia, but small patients are at the greatest risk, largely because of their small body surface-to-mass ratio. Hypothermia is exacerbated during prolonged surgical procedures, especially those that expose body cavities or use cold irrigation solutions.

Hypothermia itself is considered a form of general anesthesia because it increases the solubility of inhalants in the body, effectively increasing the dose delivered. Critically ill or otherwise compromised patients may face challenges if their core body temperatures decrease by as little as 2°F. ¹ Obviously, prevention is key when addressing hypothermia, and rewarming should be considered if patient temperature drops to ≤97.6°F. ¹²

Warmed irrigation fluids can be used to help restore core body temperature. There are also several ways to maintain an envelope of warm air around perioperative patients. Convection-type warm air devices, such as BAIR Huggers ( 3m.com) and electrically conductive fabric warmers, such as the HotDog Warmer ( augustinebiomedical.com) are the most effective, followed by warming units such as carbon-based conductive polymers ( inditherm.co.uk) and circulating warm water blankets. 1,11

At least 60% of the body surface area must be in contact with the external heat source for rewarming efforts to be most effective (FIGURE 5). ¹¹ If latex gloves or bottles of warm water are used for smaller patients, they must be initially warmed to a temperature of ≤107°F, not placed in direct contact with the patient, and removed once they cool to the temperature of the patient because, at that point, they contribute to heat loss rather than heat gain. 1,11

Commercially available wire electric heating pads and heat lamps have been associated with uneven heating, thermal injury, and/or electrocution and should be avoided. Furthermore, use thermometers carefully, as they may not always provide accurate results when used close to the surgical site.

Bradycardia and Tachycardia

Bradycardia is common in patients undergoing general anesthesia ⁸ and is defined as a heart rate of <100 beats per minute in cats. There are numerous causes of bradycardia, including drug side effects (e.g., opioids), excessive vagal tone, hypertension, hyperkalemia (e.g., in blocked cats), uremia, hypothermia, increased intracranial pressure (e.g., head trauma), profound hypoxemia, and deep-level inhalants.

Tachycardia is defined as >200 beats per minute in cats. ⁹ Causes of tachycardia include drugs (e.g., ketamine; anticholinergics; positive inotropes, such as dopamine; sympathomimetics, such as epinephrine), pain (e.g., inadequate plane of anesthesia), hyperthermia, anaphylactic reactions, hypovolemia, early-stage hypercarbia, and numerous disease states (e.g., hyperthyroidism, heart failure, central nervous system disease, anemia, hypokalemia).

Induction agents (e.g., barbiturates, alpha-2 agonists) and disease processes (e.g., splenic disease) may predispose patients to cardiac arrhythmias. ⁹

ANALGESIC OPTIONS

Signs of pain in cats can be quite variable and, at times, subtle-ranging from purring to self-mutilation or from fractious behavior and hissing to being withdrawn and quiet. Confounding interpretation of pain in cats are inconsistent observations of heart and respiratory rates, systolic blood pressure, and temperature. A better indication of adequate pain management in cats may be evidenced by the return of normal behaviors, such as grooming and attention seeking, and return of appetite. ²

In the Hospital

Although historically somewhat controversial, pure mu-agonist opioid analgesics (e.g., morphine, methadone, hydromorphone, fentanyl) have been used with great success in cats as they provide the most reliable form of analgesia for severe pain in cats. Paradoxical excitement (typically observed at higher opioid doses) can be avoided by addition of a sedative agent, such as acepromazine.

Mild to moderate pain may be treated with butorphanol or, preferably, buprenorphine. Alternatively, low doses of alpha-2 adrenergic agonists (e.g., dexmedetomidine, xylazine) may be combined with opioids in otherwise healthy patients. ³ Ketamine (an N-methyl-D-aspartate [NMDA] receptor antagonist) may be administered via constant-rate infusion in conjunction with opioids for adjunctive analgesic effects. 1,2

Local anesthetics, such as lidocaine and bupivacaine, may be employed for local, incisional, and ring (for onychectomy) blocks. Epidural analgesia, pain catheters, and transdermal fentanyl and lidocaine are other pain management techniques that can be used in the perioperative period.

At Home

Pain management may be indicated after the patient has been discharged from the hospital. The pH of the feline oral cavity allows buprenorphine to be absorbed sublingually, making this drug a good analgesic choice for acute or long-term use. Simbadol ( zoetisus.com) is a recently approved injectable buprenorphine formulation that lasts 24 hours in cats. NSAIDs are another excellent analgesic option: robenacoxib (Onsior, elanco.com) can be used in healthy cats for up to 3 treatment days. Meloxicam is currently approved for one dose in the United States but has been used for extended periods of time in other countries, using a gradually tapering dosage. 1,2,13

CONCLUSION

Although anesthetizing cats can be somewhat difficult and challenging, it can be a very rewarding experience with a positive outcome.

Want to learn more about anesthesia and analgesia in feline patients? Check out " Feline Anesthesia & Analgesia: Recent Developments" on TodaysVeterinaryPractice.com.

Purr-fect Feline Anesthesia

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Purr-fect Feline Anesthesia | Today's Veterinary Nurse

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CE TEST To take the CE test for credit, click here. This test is open until March 2018 Adapted with permission from the Michigan Association of Veterinary Technicians, Vet Tech Insider, December 2015. Anesthetizing cats can pose unique challenges for veterinary technicians.

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Guidelines for Anesthesia and Analgesia in Cats

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Guidelines for Anesthesia and Analgesia in Cats

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Guidelines on Anesthesia and Analgesia in Cats - ULAM Guidelines and SOPs - Michigan Medicine Confluence

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All surgical procedures, anesthetics, analgesics, antibiotics or other medications used on animals must be approved by the IACUC, described in the animal use protocol and performed by personnel listed on the protocol and appropriately trained for the surgical procedure.

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History

The effects and elimination of anaesthetic agents may be altered by specific diseases and anaesthesia may result in a long-term adverse effect on organ function that was already compromised. Drugs administered to control fear or anxiety, such as amitriptyline, paroxetine, sertraline, fluoxetine, or selegiline may have interaction with anaesthetic agents. Although related complications are not documented in cats, there are potential problems with these agents and they have been discussed in the previous chapter. Concurrent administration of drugs to control hypertension may potentiate hypotension during anaesthesia. Frequent measurement of blood glucose perioperatively is important for cats with diabetes and some cats will require infusion of glucose-containing fluid. Knowledge of previous adverse responses to anaesthesia, such as excitement, hypotension, or difficult recovery, is useful so that the current protocol can be modified accordingly.

Physical examination

The physical characteristics of the patient may immediately suggest a potential problem, such as brachycephalic syndrome in Persian and Exotic shorthair cats. With severe brachycephalia, the upper canine teeth rotate from the normal nearly vertically aligned position to pronounced dorsorotation or nearly horizontal position. The greater the degree of brachycephalia and dorsorotation, the narrower the nasal cavity, nasal airways and nares ( Schlueter et al., 2009). In the previous study, hypoplasia of the trachea was not present in the brachycephalic cats studied. Brachycephalic cats must not be left alone after premedication for fear of airway obstruction. Obese cats have impaired ventilation and are predisposed to hypoxaemia during sedation and anaesthesia. Dose rates of anaesthetic agents should be calculated on the ideal weights of these patients to avoid overdosage. Very thin cats are at increased risk for hypothermia and warming methods must be used throughout the anaesthetic episode, particularly during the time between induction of anaesthesia and start of surgery, and continued into recovery.

As in other species, age affects aspects of anaesthesia in feline patients. Although the demarcations between life stages are arbitrary, the AAFP and AAHA (American Animal Hospital Association) have determined feline life-stages as follows ( Hoyumpa Vogt et al., 2010):

Senior and geriatric cats will have decreased requirements for anaesthetic agents and are more likely than younger animals to develop hypoventilation, hypoxaemia, and hypotension during anaesthesia.

Auscultation of a heart murmur on routine physical examination should be further investigated. Cardiac pathology was identified using echocardiography in 53% of apparently healthy cats with a murmur ( Nakamura et al., 2011). A heart murmur may be auscultated in animals without underlying structural heart disease, and in cats that are hypovolaemic, anaemic or febrile, or may be absent despite the presence of significant heart disease such as hypertrophic cardiomyopathy (HCM). Nevertheless, the presence of cardiac disease such as cardiomyopathy, mitral insufficiency, or a patent ductus arteriosus (PDA) requires modification of the anaesthetic protocol to avoid an adverse outcome. The lungs should be auscultated for abnormal or absent air sounds that suggest the presence of pulmonary disease or diaphragmatic rupture. Some breeds are associated with increased incidence of certain diseases, such as the Maine Coon and cardiac disease.

The range of normal values for physiological parameters is wide and the measurements obtained may be influenced by the surroundings and method of monitoring ( Table 16.1). Respiratory rates (RR), heart rates (HR), and Doppler systolic arterial pressures (SAP) have been found to be slightly higher when obtained in the hospital compared with the home environment although, even at home, some cats struggle during measurements ( Quimby et al., 2011). Cats ≥11 years of age were found to have significantly higher SAP, diastolic arterial pressures (DAP) and mean arterial pressures (MAP) than younger cats ( Bodey & Sansom, 1998) and, in a later study, the increases in blood pressures were progressive with increasing age ( Sansom et al., 2004). Increased systolic arterial pressures of 160-179 mmHg and DAP 100-119 mmHg carry a moderate risk for organ damage, and SAP ≥180 mmHg with DAP ≥120 mmHg as carrying a severe risk for organ damage ( Brown et al., 2007). Cardiac abnormalities are frequent in cats with hypertension. Hypertension is present in 19-65% of cats with chronic kidney disease, although cats with extreme renal compromise may have lower pressures and cats with chronic kidney disease and hypertension may not have azotaemia ( Bodey & Sansom, 1998; Brown et al., 2007). Cats with hyperthyroidism may or may not be hypertensive but hypertension has been measured in 50-100% of cats with primary hyperaldosteronism. Hypertension is considered pathological in cats when ocular pathology is also present. Hypokalaemia and muscle weakness are also features of primary hyperaldosteronism. Measurement of blood pressure is advisable in older cats as part of the preanaesthetic evaluation. The cat must be completely relaxed, preferably in its cage, and the final values recorded should be the average of several measurements.

Diagnostic tests

Further information may be desired based on the patient's history and physical examination. Requirement for preoperative laboratory data is based on the patient's age, pre-existing disease, and the type and complexity of the planned procedure. Since organ malfunction increases with increasing age, haematological and biochemical measurements should be determined in mature and older animals and the results may provide warning of a problem that requires specific anaesthetic management. Laboratory tests should be performed on all cats with comorbidities. Preoperative test results from healthy or sick cats scheduled for complicated surgical procedures will provide baseline values for postoperative evaluation. Measurement of haematocrit, total protein, and blood urea nitrogen (BUN) is recommended in any dog or cat that has not had laboratory tests within 2-4 weeks of anaesthesia. The clinician has an obligation to determine clinical relevance of an abnormal result and need for further investigation ( Epstein et al., 2005; Klein & Arrowsmith, 2010). Administration of sedation or anaesthesia before collection of blood will alter the results ( Frankel & Hawkey, 1980; Dhumeaux et al., 2012; Reynolds et al., 2012). Even a small IV dose of ketamine, 2 mg/kg, with diazepam, 0.1 mg/kg, decreased red and white blood cell and platelet counts, albumin and triglycerides ( Reynolds et al., 2012). Some laboratory reference values may be different for various breeds. An investigation of 525 Birman, Chartreux, Maine Coon, and Persian cats in France identified clinically relevant differences for creatinine, glucose, and total protein ( Reynolds et al., 2010). The Birman cats were found to have higher creatinine concentrations than cats of the other breeds, and plasma glucose concentrations were higher in Chartreux and Maine Coon cats.

Impact of disease

The presence of abnormalities may indicate the need to adjust choice of agents and dose rates or anaesthetic management to improve safety. Cats with kidney disease or urethral obstruction have reduced urine flow and prolonged recovery from anaesthesia will result from use of agents that rely on renal excretion for elimination, specifically ketamine and tiletamine-zolazepam. Azotaemia decreases anaesthetic requirement, therefore, lower doses should be used and the drugs titrated to effect. Cats with chronic kidney disease should be administered balanced electrolyte solution IV during anaesthesia and even preoperatively and through the recovery period to encourage diuresis. The cats should be monitored after anaesthesia until confirmation of urine voiding.

Medications for cardiac disease should be continued on the morning of the day of surgery. Some medications used to treat hypertension increase the risk for hypotension during inhalation anaesthesia and preparations for treatment should be available. Increased HR may adversely impact cardiac function in cats with HCM. Atropine or glycopyrrolate should not be used for premedication in these patients but reserved for therapeutic use if necessary. Vasoconstriction will decrease cardiac output in the presence of mitral insufficiency. Medetomidine and dexmedetomidine or acepromazine should be used cautiously to avoid exacerbation of hypertension or hypotension, respectively. Fluid overloading should be avoided in cats with severe cardiac disease.

Consideration of the results from the physical examination and diagnostic tests should culminate in an overall assessment of the health status of the patient. Commonly, the patient is then assigned a category of 1-5, with 1 representing a healthy animal and 5 moribund, and with the suffix E in situations of emergency anaesthesia ( Chapter 1; ASA categories). This is a useful exercise because categories 3-5 have been associated with increased incidence of complications. Potential complications of the proposed medical or surgical procedure are not included with ASA status, therefore, the assigned category does not represent the total risk of anaesthesia and surgery.

Vaccination

The effect of anaesthesia on serological responses to vaccination at the time of neutering has been of concern. One investigation of serological responses to vaccinations before, during or after neutering kittens at 7, 8, or 9 weeks of age measured antibody titres that were comparable to kittens that were not anaesthetized, suggesting that anaesthesia and surgery did not have a significant impact ( Reese et al., 2008).

Handling

The ISFM and AAFP guidelines for handling cats encourages the veterinary team to 'think like a cat' ( Rodan et al., 2011). Since cats often respond to confrontation by avoidance or hiding, allowing a cat to feel hidden using towels or carriers may facilitate handling. Cats should be handled gently (unless unavoidable) using slow steady movements, a quiet voice and avoiding direct eye contact. Spraying synthetic feline facial pheromone in advance into the cage and on towels used for handling may reduce anxiety and fear aggression. Use of pheromones was found to add an additional calming effect in cats given acepromazine and to a lesser extent when cats were not given acepromazine, but had no appreciable effect on the ease of IV catheter insertion (Kronen et al., 2006). When approaching a cat in a cage, the opening should not be completely blocked (which the cat perceives as a threat) and the cat should be allowed to approach the handler. If the cat is reluctant to approach, the top of some carriers can be removed to gain access to the cat. The ISFM/AAFP guidelines recommend that the carrier should not be tipped up and the cat shaken out but to reach in and support the caudal end of the cat and back legs to encourage it to move forward. Sliding a towel around or over the cat before laying on hands may sufficiently relax the cat to facilitate easy removal from the cage. Most cats prefer touch on their heads and neck than other parts of the body. Signs of anxiety or fear include ears that are lowered and swiveled out, head drawn into the body, back slightly hunched, tail moved in to cover the feet, and sweaty paws. Immobility ('freezing') may also indicate anxiety.

'Scruffing' is a term for a variety of holds on the skin of the cat's neck. Opinions vary about use of scruffing ( Rodan et al., 2011). Some clinicians will never use scruffing, others use scruffing only if necessary to protect a cat or person from injury, while others believe that it is acceptable for short procedures or to prevent escape. A gentle hold is sufficient for most pets. Chemical restraint may be needed when an aggressive response is anticipated. Preanaesthetic medication reduces patient struggling during insertion of an IV catheter before general anaesthesia.

Nets of the clam-shell design with small holes are available commercially to capture cats reluctant to be handled ( Fig. 16.1). Any part of the cat is then available for IM injection. Also available are clear plastic boxes with one side that can be moved to immobilize the cat for IM injection.

Agents

α₂-Agonist sedatives

Experimental and clinical studies confirm that medetomidine and dexmedetomidine reliably produce sedation and analgesia in cats in 15-20 minutes after IM administration, although pre-injection excitement or aggression may result in sedation of less intensity. Dose rates based on body surface area are available in the product package, indicating that the µg/kg dose rate decreases as the animal's body weight increases. The duration of sedation is dose dependent. Equivalent sedative doses for dexmedetomidine are approximately one-half those for medetomidine. Analgesia produced by dexmedetomidine, 0.04 mg/kg (40 µg/kg), was reported to be equivalent to that provided by medetomidine, 0.08 mg/kg (80 µg/kg), IM ( Slingsby & Taylor, 2008).

Dexmedetomidine given at doses varying from 0.015 to 0.04 mg/kg (15-40 µg/kg), IM induced lateral recumbency in cats in approximately 20-30 minutes ( Monteiro et al., 2009; Slingsby et al., 2010). Maximum sedation without responsiveness lasted about 30 minutes from the highest dose and the sedative effects began to wane at 60 minutes after administration ( Slingsby et al., 2010). Although times vary somewhat between reports, in general, the duration of maximum sedation was similar for all the doses but the total duration of sedation tended to be longer the higher the dose rate. Experimental tests of analgesia indicated that dexmedetomidine does not produce analgesia at low dose rates but that analgesia is present when the dose of dexmedetomidine is increased to 0.04 mg/kg, IM. Further, the intensity and duration of analgesia from dexmedetomidine, 0.04 mg/kg, was less than that provided by buprenorphine, 0.02 mg/kg ( Slingsby et al., 2010). Dexmedetomidine, 0.04 mg/kg, administered by the buccal transmucosal (TM) route achieved the same effect as from IM administration ( Slingsby & Taylor, 2009). Onset of sedation was on average 30-35 minutes and duration an average of 98 minutes from both IM and TM routes. Medetomidine, at a lower dose of 0.02 mg/kg (20 µg/kg), produced sedation lasting 30-60 minutes in experimental cats ( Lamont et al., 2001).

The cardiovascular effects of medetomidine and dexmedetomidine are similar for all dose rates. After administration of medetomidine, 0.02 mg/kg, HR were decreased by 40% from the preadministration values and, although systemic vascular resistance (SVR) increased significantly at 15 minutes (vasoconstriction), MAP was unchanged. Cardiac output (CO) was decreased by 63% at 15 minutes, an effect due to a combination of decreased HR and decreased stroke index, the latter presumed due to the increase in afterload. No significant changes in blood gas values were noted. Similarly, HR decreased after administration of dexmedetomidine and SAP as measured by Doppler non-invasive blood pressure measurement (NIBP) was unchanged for 60 minutes (Monteiro et al., 2009). Prior administration of atropine, 0.05 mg/kg, IM significantly increased HR and SAP for 45 minutes, although not to the high levels recorded in dogs. Concurrent administration of atropine with these sedatives is not usually recommended.

The sedative effects following administration of xylazine, 1 mg/kg, or romifidine, 0.2 mg/kg, IM are similar although muscle relaxation and analgesia were less with romifidine and were not increased significantly by increasing dose rate ( Selmi et al., 2004). Medetomidine and dexmedetomidine are generally chosen over romifidine for use in cats.

α ₂-Agonist sedatives are commonly administered in combination with an opioid. The addition of butorphanol, 0.2 mg/kg, IM to a low dose of dexmedetomidine, 0.01 mg/kg, significantly increased sedation and muscle relaxation over administration of dexmedetomidine alone ( Selmi et al., 2003). The combination of buprenorphine, 0.01 mg/kg, with dexmedetomidine, 0.02 mg/kg, IM produced a similar duration and degree of maximum sedation as dexmedetomidine, 0.04 mg/kg, and a greater duration of analgesia ( Slingsby et al., 2010). A combination of midazolam, 0.4 mg/kg, butorphanol, 0.4 mg/kg, and dexmedetomidine, 0.005 mg/kg (5 µg/kg), IM produced deep and long-lasting sedation ( Biermann et al., 2012). With this combination, HR decreased, CO decreased by approximately 50%, and blood pressure was unchanged.

The addition of ketamine to the combination of medetomidine or dexmedetomidine and butorphanol (popularly known as 'kitty magic') can produce sedation at low doses and anaesthesia at higher doses. The combination of 0.1 mL each of medetomidine (1 mg/mL) or dexmedetomidine (0.5 mg/mL), butorphanol (10 mg/mL), and ketamine (100 mg/mL) mixed in one syringe and administered IM will induce heavy sedation within 5 minutes ( Ko et al., 2009). These volumes are calculated for cats weighing 4.5 kg and are equivalent to dexmedetomidine, 0.011 mg/kg, butorphanol, 0.22 mg/kg, and ketamine, 2.2 mg/kg. More or less of the total volume may be administered depending on the desired effect. The volume of each drug may be increased to 0.2 mL/4.5 kg or 0.13 mL/3 kg to induce general anaesthesia for castration and to 0.3 mL/4.5 kg or 0.2 mL/3 kg to produce 40 minutes of anaesthesia suitable for ovariohysterectomy. Administration of this drug combination IV is generally at half the IM dose. Substitution with buprenorphine for butorphanol is not as satisfactory for injectable anaesthesia because the onset time for buprenorphine is long, but is suitable for premedication to inhalation anaesthesia.

The sedative effects of medetomidine and dexmedetomidine may be antagonized by administration of atipamezole IM. The optimal recommended dose for atipamezole is 2.5× the dose of medetomidine, which equals half the volume of medetomidine previously administered, or up to 5× the dose of dexmedetomidine for deeply sedated cats. Allowance should be made for the time elapsed since administration of the α ₂-sedative, as less atipamezole will be necessary over time. Overantagonism can lead to the cat becoming excessively alert. Antagonism of the sedative will remove analgesia and reveal excitatory effects of ketamine, when present. In an emergency, administration of atipamezole, 5-20 µg/kg has been recommended slowly IV (off label) over several minutes. Atipamezole is not FDA licensed for use in cats in the USA.

Vomiting has been recorded in cats after administration of xylazine, medetomidine, dexmedetomidine, and romifidine in most ( Lamont et al., 2001; Selmi et al., 2004; Slingsby & Taylor, 2009; Slingsby et al., 2010; Biermann et al., 2012) but not all reports ( Selmi et al., 2003). The cats are usually still standing at this point which decreases the risk of aspiration.

Concerns with use of α ₂-agonists centre on their sedative and cardiovascular effects. When any of these agents is used for premedication, the dose of subsequently administered anaesthetic agent may be decreased by half or even more, especially when an opioid or a low dose of ketamine has been included. α ₂-Sedatives profoundly decrease CO and this must be taken into consideration when selecting an anaesthetic protocol for old or sick cats and cats with cardiac disease. Use of medetomidine or dexmedetomidine is not recommended in very young kittens.

Benzodiazepines

Diazepam and midazolam are rarely used alone as they produce minimal sedative effects and may cause agitation or restlessness in healthy patients. Benzodiazepines are commonly administered as adjunct agents to opioids or anaesthetic agents to increase sedation or to block excitatory effects of ketamine. Midazolam, 0.1-0.5 mg/kg, may be administered IM or IV and diazepam, 0.1-0.5 mg/kg, may be administered IV. Oral forms of diazepam are not recommended because of the risk of hepatotoxicity.

Opioids

Factors to be considered when choosing an opioid are the preferred route of administration, speed of onset of effect, duration of effect, efficacy of analgesia in relation to the species and procedure to be performed, and adverse effects.

The speed of onset is of significance depending on the time available between administration and the start of anaesthesia or painful process, with IV and IM administration as the most reliable routes. Onset after IV administration is rapid for butorphanol, hydromorphone, oxymorphone, fentanyl, and methadone. Onset of analgesia following administration of buprenorphine, 0.02 mg/kg, was 15 minutes after IV administration in one study (Steagall et al., 2008a), 35 minutes for both IV and IM routes in another ( Slingsby & Taylor, 2008), and 4 hours after IM administration in another ( Robertson et al., 2003). Onset of effect after IM administration is relatively rapid with butorphanol and hydromorphone at 15-20 minutes ( Lascelles & Robertson, 2004; Wells et al., 2008) and peak effect of hydromorphone, 0.1 mg/kg, was measured at 30 minutes ( Robertson et al., 2009). Onset after IM injection of meperidine (pethidine) 5 mg/kg, was approximately 20 minutes with analgesia persisting for 60 minutes ( Millette et al., 2008).

Methadone, 0.3 mg/kg, IV quickly causes sedation that lasts 30 minutes. Antinociception was detected 5 minutes after IV administration, was most intense for 60 minutes and a significant effect persisted for 4 hours ( Ferreira et al., 2011). In a clinical investigation of cats undergoing ovariohysterectomy, premedication with methadone, 0.6 mg/kg, IM provided pain relief for 4 hours after surgery ( Rohrer-Bley et al., 2004). The duration of analgesia from hydromorphone, 0.1 mg/kg, against experimental nociception was twice as long after IM than IV administration, 5.7 hours compared with 2.7 hours, respectively, and this dose rate produced significantly more analgesia than half that dose rate ( Lascelles & Robertson, 2004; Wegner & Robertson, 2007). Thermal antinociception was recorded at 4-6 hours after IM injection of morphine, 0.2 mg/kg ( Robertson et al., 2003). Morphine may not be as analgesic in cats as in other species because the glucuronidation pathway is less efficient in cats, preventing formation of a morphine active metabolite ( Taylor et al., 2001). Evaluations of the duration of antinociception from butorphanol, 0.1-0.8 mg/kg, IM vary from 5 to 480 minutes ( Robertson et al., 2003; Wells et al., 2008). A clinical perception is that the duration of analgesia from butorphanol is up to 2 hours and that while analgesia may be satisfactory for soft tissue surgery, butorphanol does not provide sufficient analgesia for orthopaedic procedures. No sedation should be expected after administration of buprenorphine alone. Duration of analgesia from buprenorphine, 0.02 mg/kg, was recorded as 7-8 hours ( Slingsby & Taylor, 2008).

Care must be taken when mixing opioids in the same patient as µ agonists, buprenorphine, and butorphanol possess different receptor affinities that may result in diminished analgesia. Administration of hydromorphone and buprenorphine together resulted in minimal analgesia for the first 2 hours followed by onset of analgesia ( Lascelles & Robertson, 2004) and, in another investigation, simultaneous administration of butorphanol and buprenorphine resulted in some cats without analgesia ( Johnson et al., 2007).

Tramadol, 1 mg/kg, SC was an inefficient analgesic agent in an experimental investigation ( Steagall et al., 2008b) whereas 2 mg/kg, administered SC 1 hour before ovariohysterectomy provided adequate analgesia postoperatively in 50% of cats ( Brondani et al., 2009). Cats given both tramadol and a non-steroidal anti-inflammatory drug (NSAID) did not need rescue analgesia and did not develop hyperalgesia. A longer elimination half-life for tramadol was measured in cats than in dogs ( Pypendop & Ilkiw, 2007).

Routes of administration other than IV and IM are not reliable for all opioids. Subcutaneous injection of hydromorphone has a slower onset of peak effect, a shorter duration of antinociception and is associated with more undesirable side effects ( Robertson et al., 2009). Buprenorphine administered SC or by buccal transmucosal (TM) routes produced significantly less analgesia than from IV or IM injection ( Slingsby & Taylor, 2008; Giordano et al., 2010), although one investigation using thermal nociception found the efficacy and duration of analgesia from buprenorphine similar after TM and IV administration ( Robertson et al., 2005). TM administration is accomplished by squirting the drug from a 1 mL syringe inserted between the teeth and the cheek. Interestingly, administration was found to be significantly easier (without the cat turning its head away) using preservative-free buprenorphine than buprenorphine from a multidose bottle ( Bortolami et al., 2012b). The authors proposed that the smell or taste of the chlorocresol preservative in the product intended for multiple injections could be responsible for the aversive behaviour and that the cat's response to the preservative might result in less precise administration or increased swallowing of buprenorphine. Plasma concentrations of butorphanol after TM administration remained below the concentrations anticipated for effective antinociception ( Wells et al., 2008). Butorphanol is not as lipophilic as buprenorphine and the authors proposed that the cats' oral pH was not high enough to promote maximal absorption. TM administration of methadone resulted in a lower peak plasma concentration than after IV even though the TM dose was double the IV ( Ferreira et al., 2011).

Naloxone is an opioid antagonist and will reverse opioid-induced sedation, analgesia and hyperthermia. Naloxone, 0.01-0.02 mg/kg, is administered IM or slowly IV.

Transdermal (TD) fentanyl

Most investigations have described the effects of a reservoir system fentanyl patch using one 25 µg/h patch per cat (see previous chapter for precautions for use and dangers for human toxicity). Despite considerable variability between cats, the increase in plasma concentration of fentanyl is quicker in cats than in dogs reaching a presumed analgesic threshold by 12-18 hours ( Lee et al., 2000; Egger et al., 2003). Maximum plasma concentration was measured at 44 hours after application in one group of cats and mean values remained constant for 4 days ( Lee et al., 2000). In contrast to dogs, plasma concentrations of fentanyl decreased more slowly in cats after the patches were removed, presumably due to a slow depletion of a cutaneous depot of fentanyl. Differences in skin structure may partly explain interspecies variability in serum or plasma fentanyl concentrations achieved following application or removal of a patch. Arbitrarily decreasing the surface area of the patch by removing part of the adhesive layer was not recommended for the reservoir type patch as that action would not correspond to a predictably linear decrease in fentanyl transfer (Lee et al., 2000). Nonetheless, comparison of full or partial exposure in cats weighing 1.3-4.3 kg anaesthetized for ovariohysterectomy revealed significantly higher plasma concentrations in cats with full exposure ( Davidson et al., 2004). The authors proposed a 38% decrease in calculated delivery rate of fentanyl resulting from a 50% decrease in surface area. Dysphoria was observed in several cats that were fully exposed to the patch membrane. Application of a 25 µg/h patch to cats weighing 2.2-5 kg scheduled for anaesthesia and onychectomy resulted in a similar range of plasma fentanyl concentrations without adverse effects ( Gellasch et al., 2002). In a study comparing serum cortisol concentrations in conscious cats and during anaesthesia with or without surgery, application of a TD fentanyl patch 12 hours before anaesthesia was associated with significantly decreased cortisol during and after surgery, supporting the authors' conclusion that the TD patch diminished pain or stress ( Glerum et al., 2001). After application of a fentanyl patch, cats' behaviours should be observed for signs of overdosage, such as dysphoria, lethargy, inappetence. Cats should also be monitored for signs of pain. Adequate analgesia should not be assumed as plasma fentanyl concentrations may be low or undetectable in some cats after application of a patch ( Lee et al., 2000).

TD fentanyl results in a small (18%) but significant decrease in isoflurane requirement in experimental cats ( Yackey et al., 2004). A decrease in serum fentanyl concentrations after induction of anaesthesia has been measured in cats that developed hypothermia (35°C, 95°F), presumably due to decreased cutaneous blood flow from vasoconstriction ( Pettifer & Hosgood, 2003). The mean fentanyl concentrations of all the cats were within values believed to be associated with analgesia so that the decrease may be of clinical relevance only in some cats. A further concern is that plasma fentanyl concentrations may increase when a cat is positioned with the fentanyl patch (and skin) in direct contact with a heating pad or heated cage floor.

Side effects of opioid administration

Side effects of opioids in cats are excessive salivation, vomiting, euphoria or dysphoria, and increased body temperature, with some variations between agents. Signs of euphoria (behaviour including more than usual meowing, purring, rubbing, rolling, and kneading with forepaws) was present for 2-6 hours after IV methadone and 6-12 hours after TM administration ( Ferreira et al., 2011). Euphoria has been observed after administration of other opioids, lasting from 30 minutes with butorphanol to 24 hours with buprenorphine ( Gellasch et al., 2002; Robertson et al., 2003; Steagall et al., 2008a; Bortolami et al., 2012a). Euphoria has also been observed after administration of tramadol ( Pypendop & Ilkiw, 2007; Steagall et al., 2008b). Dysphoria (staring into space, wary of people, pacing, jumping at the cage walls) has been observed in some cats given oxymorphone, hydromorphone, fentanyl, or tramadol. Mydriasis is induced in cats by µ opioids, butorphanol, and buprenorphine, and dilated pupils may persist for 6-8 hours or even 2-3 days. Cats may vomit or excessively salivate after administration of morphine, hydromorphone, methadone, and tramadol. The addition of acepromazine may decrease the incidence of vomiting. Cats given butorphanol, buprenorphine, or fentanyl are not likely to vomit.

Increased body temperature frequently occurs when cats have been given an opioid. Cats with hyperthermia may pant or behave no differently from the expected effects of the drug(s) administered. Body temperature was monitored for 24 hours in experimental cats with a wireless thermistor implanted within the abdomen that were given different opioids and combinations ( Posner et al., 2010). Moderate hyperthermia was defined as 40.1°C (104°F). Hydromorphone, morphine, buprenorphine and butorphanol produced mild to moderate increases in body temperature. Various doses of hydromorphone increased cats' temperatures to similar extents with a peak around 1.5-2 hours after administration and a return to baseline by about 5 hours. The maximum temperature recorded for any cat in this study was 40.7°C (105.3°F) but temperatures up to 42.5°C (108.5°F) have been reported ( Niedfeldt & Robertson, 2006). Greater increases in temperature have been noted after opioid administration in conjunction with inhalation anaesthesia compared with opioid administration without general anaesthesia. It has also been noted that cats that were coldest during anaesthesia developed the highest peak temperatures during recovery ( Posner et al., 2010). Increased body temperatures were also recorded for several hours after anaesthesia in cats with fentanyl patches applied for premedication ( Lee et al., 2000; Glerum et al., 2001). Mild to moderate increases in temperature should resolve without treatment. Cooling measures should be instituted to treat severe hyperthermia and injection of naloxone may further decrease body temperature.

Non-steroidal anti-inflammatory agents

An NSAID will contribute to the comfort of a cat after a surgical procedure. Recommended dose rates for use in cats are given in Table 16.3, however, not all NSAIDs are licensed in all countries and the clinician should refer to local information and regulations ( Sparkes et al., 2010). Single doses administered for treatment of acute pain may have a duration of 18-20 hours. Although the risk of acute renal failure is low in healthy cats, the ISFM and AAFP panel recommendations list increased risk of adverse renal effects associated with hypotension during anaesthesia, cats that are dehydrated or hypovolaemic, older cats, cats with concurrent cardiovascular, renal or hepatic disease, and concurrent administration of ACE inhibitors, diuretics, and beta-blocking drugs. NSAIDs should be used with extreme caution if at all in cats with severe liver dysfunction or hypoalbuminaemia.

Ovariohysterectomy, castration

Pain scoring has identified that cats exhibit signs attributable to pain after ovariohysterectomy and castration, and pain has been present in some cats for at least 24 hours ( Balmer et al., 1998; Slingsby & Waterman-Pearson, 1998, 2002; Glerum et al., 2001; Al-Gizawiy & Rudé, 2004; Rohrer-Bley et al., 2004; Gassel et al., 2005; Grint et al., 2006; Tobias et al., 2006; Benito-de-la-Víbora et al., 2008; Brondani et al., 2009; Steagall et al., 2009b; Giordano et al., 2010; Murison & Martinez Taboada, 2010). A comparison of the flank and midline surgical approaches confirmed that wound tenderness was greater following a flank incision (Grint et al., 2006), as might be expected from muscle involvement in the flank incision. An inexperienced surgeon may cause more tissue trauma than an experienced surgeon, however, a study evaluating postoperative behaviours after ovariohysterectomy in dogs found no significant association with experience of the surgeon or duration of surgery ( Wagner et al., 2008).

Frequently, in clinical practice, a sedative such as acepromazine, medetomidine or dexmedetomidine, an opioid and an NSAID are administered in conjunction with anaesthetic agent(s) in order to achieve both intra- and postoperative analgesia ( Bortolami et al., 2012a; Mathis et al., 2012). Anaesthesia for ovariohysterectomy may be maintained with an inhalant agent preceded by premedication and induction of anaesthesia with ketamine, ketamine with diazepam or midazolam, tiletamine-zolazepam, thiopental, propofol, or alfaxalone. Anaesthetic protocols using only injectable agents are commonly used when surgery time is likely to be less than 30 minutes ( Table 16.4). Published investigations involving cats presented for ovariohysterectomy or castration should be evaluated cautiously because the anaesthetic protocols may have been limited combinations to facilitate study of a single analgesic drug. It is important when selecting analgesic drugs to consider their onset times to avoid start of surgery before onset of analgesia. For example, administration of buprenorphine or an NSAID at induction of anaesthesia does not provide sufficient time for onset of analgesia before the surgical procedure and, in some cases, recovery from anaesthesia. The durations of analgesia of the agents chosen must also be known for scheduling of the redosing interval. Butorphanol and pethidine (meperidine) have short durations of action of about 2 hours and a single administration of these drugs for premedication will be insufficient for postoperative analgesia ( Balmer et al., 1998; Slingsby & Waterman-Pearson, 1998). Methadone, hydromorphone, oxymorphone, and buprenorphine may provide analgesia for 4-6 hours. Published research indicates that administration of only an NSAID for analgesia does not provide sufficient analgesia after ovariohysterectomy ( Brondani et al., 2009; Murison & Martinez Taboada, 2010) although, in some studies, cats receiving an NSAID had better total pain scores than those receiving an opioid only ( Slingsby & Waterman-Pearson, 1998; Gassel et al., 2005). Inclusion of an NSAID was found to decrease incisional hyperalgesia ( Al-Gizawiy & Rudé, 2004; Benito-de-la-Víbora et al., 2008) and decrease pain scores when administered in combination with an opioid ( Brondani et al., 2009; Steagall et al., 2009b).

Rescue (interventional) analgesia is usually in the form of an opioid at the same or reduced dose that is used for premedication, and is commonly administered IM for rapid onset of analgesia. An NSAID (SC administration) may be included if one has not already been administered. Examples of drugs used for rescue analgesia in cats after ovariohysterectomy are:

Management of anaesthesia in high volume practices and animal shelter programmes may of necessity be more regimented than tailored to the individual patient, and preanaesthetic evaluation of feral cats may not be possible. Guidelines for spay-neuter programmes published by the Association of Shelter Veterinarians are detailed and contain many suggestions for good patient care ( Looney et al., 2008). The guidelines stress that a standard volume dose rate of anaesthetic agents for all cats should be avoided and suggest that doses either be calculated on a per kg basis or calculated for cats in several weight ranges, for example <1 kg, 1-2 kg, and 2-4 kg, and the volume of each drug be written out in a chart for easy reference. Anaesthetic protocols should provide analgesia. Reversible drugs are useful, especially in feral cats, but recognizing that reversal of medetomidine or dexmedetomidine removes some of the analgesia. In the context of shelter medicine, an animal with a mild infection or non-infectious medical condition may be anaesthetized because of the likelihood that the animal will not be available for neutering at a later date ( Looney et al., 2008). The presence of disease may be responsible for deaths during or following anaesthesia ( Gerdin et al., 2011).

Feral cats

Feral cats are defined as free-roaming cats without owners. A feral cat is caught in a special trap that can be adjusted to immobilize the cat for IM injection. The anaesthetic agent combination must reliably induce anaesthesia in unsocialized cats without causing overdosage. A small number of cats may need supplementation with additional anaesthetic agent to provide a sufficient depth of anaesthesia for ovariohysterectomy or castration. In one series of 101 feral cats ( Harrison et al., 2011), the weight of each cat was determined by subtracting the known weight of the trap from the combined total weight. Medetomidine, 0.1 mg/kg (100 µg/kg), ketamine, 10 mg/kg, and buprenorphine, 0.01 mg/kg, were administered into the paralumbar muscles. If the cat was still responsive after the first injection, additional medetomidine, 0.02 mg/kg, was administered IM. Ketamine, 2.5 mg/kg, was injected IM if anaesthesia was still insufficient. Atipamezole, 0.125 mg/kg, SC was administered at the completion of surgery and the cats were released the next day. Eleven cats required additional medetomidine ± ketamine and 11 cats required supplemental isoflurane, half of which were after 45 minutes of satisfactory anaesthesia. The initial injection induced recumbency in 5 ± 5 minutes and the median time from injection to start of surgery was 23 minutes. Monitoring the cats immediately after induction and during anaesthesia is important and support treatment must be available. In the previous investigation, significant decreases in SpO ₂ were observed in cats in the first few minutes of anaesthesia, one cat was apnoeic after induction of anaesthesia, one cat was cyanotic and responded sluggishly to O ₂ supplementation, one cat had bradycardia (<60 beats/min), and three cats were hypotensive (Doppler SAP <90 mmHg).

An older review of 7501 feral cats anaesthetized for ovariohysterectomy (59%) or castration (41%) utilized a combination of tiletamine-zolazepam, ketamine, and xylazine (TKX) ( Williams et al., 2002). A vial of 500 mg of lyophilized tiletamine-zolazepam was reconstituted with 4 mL of ketamine (100 mg/mL) and 1 mL of 10% xylazine (100 mg/mL) so that each mL of solution contained 50 mg tiletamine, 50 mg zolazepam, 80 mg ketamine, and 20 mg xylazine. The average dose per cat was 0.25 mL of the combination injected IM. Using the average weight of 3 kg from 194 cats, the dose rates can be calculated as tiletamine-zolazepam, 8.3 mg/kg, ketamine, 7 mg/kg, and xylazine 1.7 mg/kg. The standard dose of 0.25 mL was adjusted according to the estimated size of the cat and additional small doses were administered when the first dose was insufficient. A single dose provided adequate anaesthesia for 79.5% of cats. Yohimbine, 0.5 mg for adults and 0.3 mg for kittens, was injected at the end of surgery to reverse the xylazine. Twenty-six cats (0.35%) died before hospital discharge, of which 17 were considered to be anaesthetic deaths. In another report, the same anaesthetic drug combination (0.25 mL IM) was administered to 22 male and 67 female feral cats, and a single injection provided sufficient anaesthesia in 92% of all cats ( Cistola et al., 2004). The mean time to start of ovariohysterectomy was 28 minutes and surgery time was 11 minutes. All cats survived to discharge but hypoxaemia, detected by pulse oximetry, was prevalent and some cats became hypotensive. These abnormalities are potential causes of anaesthetic death but all the cats in this study survived without treatment. The authors stressed the difficulty of routinely supplying supplemental O₂ in a high volume Trap-Neuter-Return programme when, for example, 12 cats are anaesthetized simultaneously. The authors also noted that MAP increased at the start of surgery and questioned the adequacy of the combination for providing postoperative analgesia.

When dealing with a small number of feral cats in a practice environment, addition of pulse oximetry and Doppler or oscillometric arterial pressure monitoring, and supplementation with O ₂ when needed, is feasible and advisable.

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The effects and elimination of anaesthetic agents may be altered by specific diseases and anaesthesia may result in a long-term adverse effect on organ function that was already compromised. Drugs administered to control fear or anxiety, such as amitriptyline, paroxetine, sertraline, fluoxetine, or selegiline may have interaction with anaesthetic agents.

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