Peer Reviewed

Anesthesiology

Purr-fect Feline Anesthesia

March 1, 2016 |

Issue: March/April 2016

Heidi Reuss-Lamky

LVT, VTS (Anesthesia & Analgesia, Surgery), FFCP

Heidi Reuss-Lamky graduated from Michigan State University’s Veterinary Technology Program in 1984. She has extensive experience in general practice, and since 1993 has devoted her technical expertise to the surgical department of specialty hospitals. She has been affiliated with Oakland Veterinary Referral Services in Bloomfield Hills, Michigan, since 2006.

Heidi became certified through the Academy of Veterinary Technicians in Anesthesia and Analgesia in 2003, and sat on the credentials committee from 2005 to 2009. She served in the president’s role on the Executive Board of the Michigan Association of Veterinary Technicians from 2007 to 2009. She was a charter member of the Academy of Veterinary Surgical Technicians and currently sits on the executive board. She has a special interest in veterinary behavior medicine and earned her Fear Free Certification in November 2017.

Heidi is an ardent advocate for the veterinary technology profession, and serves as a consultant for many allied veterinary industries. She is on the National Association of Veterinary Technicians in America Committee on Veterinary Technician Specialties, and also serves on the editorial review board for Today’s Veterinary Nurse Journal.

Heidi is a prolific author and lecturer, presenting anesthesia, surgical nursing, and Fear Free-related topics at veterinary meetings worldwide. She most recently published Chapter 8, “Waste Anesthetic Gas Collection and Consequences,” in Veterinary Anesthetic and Monitoring Equipment, edited by Kristen Cooley and Rebecca Johnson. She was also honored to receive the 2013 NAVC Dr. Jack L. Mara Memorial Lecturer award.

Heidi currently shares her home with her husband, Bryan, and 3 dogs, 2 cats, an African grey parrot and pond koi. In her spare time she enjoys horseback riding, gardening, travel, lecturing, and quenching her never-ending thirst for knowledge.

Read Articles Written by Heidi Reuss-Lamky

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) supraglottic 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.

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.

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.

Pulse oximetry provides continuous and noninvasive heart rate monitoring and an estimate of arterial hemoglobin saturation (SpO₂). Often, after extended periods of use, the clip-on tongue sensor causes blanching of the local blood supply, impeding accurate SpO₂ 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 (ETCO₂) monitoring measures the level of carbon dioxide in the expired gases from the alveoli. An abrupt decrease in ETCO₂ can be an early and reliable indication of impending cardiovascular collapse or cardiac arrest. Consequently, ETCO₂ 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 ETCO₂ 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 ETCO₂ sample collection tubing, it may be advantageous to eliminate excessive dead space by using a special ETCO₂ adapter, which is a substitute for the connection of the ET to the wye hose interface (FIGURE 4). Alternatively, sidestream ETCO₂ 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 ETCO₂ values should be maintained between 35 and 45 mm Hg.¹ It is prudent to avoid hyperventilation, which can result in ETCO₂ 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 anesthesia⁸ 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.