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Anesthetizing Cats: Why they aren’t just smaller dogs.

 

          

                            

 
Cats and dogs are two completely different packages, be it the way they communicate, their feeding habits or their behaviour to name a few. Our minds are guided to consider cats as just another breed of small dog, thanks to our curriculum in Vet School, Dosage regimens of many commercially available pet products and the similar mentality of most clients. Cats are the among the least domesticated mammalian pets and their ‘partly wild’ nature provides them the resilience to overcome some of the iatrogenic insults to a certain degree. Current estimates indicate that approximately 0.11% (1 in 895 anesthetics) of healthy cats die of an anesthetic-related death, which is more than twice as frequent as has been recently reported in dogs (0.05% or 1 in 1849)This might seem quite farfetched for those who rarely anesthetize healthy adults for routine and minor procedures because such cats rarely get into complications but here are some unique considerations to be made while anesthetizing cats to reduce the incidence of peri-operative and intra-operative morbidity and mortality.

Drug Metabolism in Cats

It has been well documented and known by most veterinarians that important metabolic differences exist between cats, dogs, and humans. The activity of UDP‐glucuronosyltransferase (UGT), an enzyme involved in the conjugation of many substrates, is much lower in cats than in dogs and humans as evidenced by a low rate of elimination of drugs like acetaminophen and a high potential for toxicity. In addition, they may also have reduced Phase I metabolism (oxidation, reduction, hydrolysis) compared to other species, due to lower activity of some hepatic cytochrome P450 enzymes. This may contribute to the differences in drug metabolism observed in cats, compared to dogs or humans.

In a study on the pharmacokinetics of Tramadol in the cat, the rate at which O‐desmethyltramadol (the primary metabolite of tramadol) was produced appeared to be like that of humans deficient in CYP2D6- an enzyme needed for its metabolism and which is naturally deficient in cats. These same humans were found to experience less pain relief from this drug than the normal ‘good metabolizers.’ This implies that the level of analgesia produced by tramadol is lesser in cats than in the average human or dog. Likewise, the total body clearance of propofol was reported to be at least twice as fast in dogs than in cats. It is however unclear why cats clear propofol much more slowly than dogs; in any case, the differences in metabolism of propofol may have clinical consequences, as increasing the duration of propofol infusion has been reported to significantly prolonged recovery in cats.

 Preanesthetic Patient assessment

Obtaining a complete patient history from an owner can be challenging when history regarding food and water intake, urinary and fecal output may be unknown in cats that spend more time outdoors in multi-cat households, or when the data provided by non-observant owners is unreliable and misleading. Cats often show no obvious clinical signs early in a disease process because the signs are subtle and often go unnoticed or are assumed to be normal. All cats should have a preanesthetic physical examination that evaluates their signalment, physiologic parameters and all body systems but we as veterinarians often face cats which are fearful, fractious, un-cooperative and difficult to handle. These reasons, however, stand invalid and preanesthetic patient assessment should not be compromised. Administration of 50–100 mg PO Gabapentin 2–3 h prior to visit by the owner or some kind of chemical immobilization calms fractious cats and allows for a complete physical examination. Cardiomyopathy is often a ‘silent’ disease in cats and further investigations must be made if suspected. 

 Use of Non‐steroidal anti‐inflammatory drugs (NSAIDs)

The use of NSAIDs in cats has been reviewed by numerous authors. Although these drugs are commonly used in cats for their anti‐inflammatory and analgesic properties, caution is recommended since glucuronide conjugation is a major metabolic pathway for most of these drugs. The incidence of adverse effects (renal injury, hepatotoxicity, gastrointestinal ulceration) in cats may therefore be higher than in some other species and will invariably be experienced by most practicing veterinarians. Nevertheless, there is favorable evidence for the short‐term, perioperative use of carprofen and meloxicam. The use of these drugs must be avoided in cats having a ASA risk score of not more than 1. The routine usage of NSAIDs as a post-operative analgesic should also be carefully considered as cats are more sensitive to the adverse effects of these drugs in the immediate post-operative period because of the possible anesthetic insult and surgical stress. Multimodal analgesic techniques and constant rate infusions may be considered intra-operatively with or without the use of NSAIDs. Pre-operatively and post-operatively, Opioid analgesics, sedatives and anesthetics with analgesic properties should be considered before dosing NSAIDs if at all they are used. For example, a cat anesthetized with an α-2 receptor agonist (which provides moderate analgesia) and Ketamine requires significantly low dosage of Meloxicam when compared to a cat anesthetized with a Benzodiazepine (which provides no analgesia) and Ketamine to provide the same level of analgesia.

 Endotracheal intubation

Tracheal intubation of cats is essential to the maintenance of a patent airway during anesthesia. However, it is technically a lot more challenging when compared to intubating dogs. A Confidential Enquiry into Perioperative Small Animal Fatalities found that endotracheal intubation increased the odds of anesthetic‐related death in cats by approximately two‐fold. Previous studies have also suggested that endotracheal intubation is associated with major complications in cat, reasons being the small size of the cat’s upper airway and tendency of larynx to spasm on mechanical stimulation of the soft palate, pharynx, larynx, and trachea. It is possible that improper intubation technique would be more likely to cause complications in cats than in dogs. It has been suggested to desensitize the larynx with a local anesthetic prior to intubation in cats in order to decrease the incidence of laryngeal spasm and ease intubation; nevertheless, it is possible that improper intubation technique would be more likely to cause complications in cats than in dogs. A study also reported that most anesthetic deaths occur soon after anesthesia ends (within the first 3 h) and are commonly caused by airway obstruction secondary to laryngeal spasms and hypothermia.


A 1 ml syringe being advanced over the top of the laryngeal opening and 0.2 ml of 2% lidocaine being dropped on to the top of the arytenoids. The cat should be put back on oxygen for 60–90 seconds (time for action) after which intubation is attempted. 


A feline-specific Supraglottic Airway Device (SGAD- v-gel; Docsinnovent®) is a suitable alternative to intubation. Studies show that the time to obtain a clinically acceptable capnograph reading was shorter when an SGAD was used compared with an Endotracheal Tube, fewer attempts were needed, and less propofol was required. The device is suitable for spontaneous ventilation and for controlled mechanical ventilation.



 

A supraglottic airway device (v-gel) specifically designed for the cat’s pharyngeal and laryngeal anatomy.




   



  The tip of the supraglottic airway device is lodged in the esophagus and the opening lies over the laryngeal opening.







Potency of Inhalant anesthetics

The anesthetic potency of inhalant anesthetics tends to be higher in cats than in many other species, including dogs and horses i.e. the dose of anesthetic required to attain or maintain a certain plane of anesthesia is much lesser in cats as characterized by their low minimum alveolar concentration (MAC values). Clinical experience of many authors also suggests that at similar anesthetic depth, blood pressure and cardiac indices tends to more depressed in cats than in dogs.

Fluid therapy and blood volume

Blood volume in cats has been reported to be 56–67 mL/kg in contrast to approximately 80-90 mL/kg in the dog. Hence, Cats may be at higher risk of fluid overload and/or excessive dilution of blood components than dogs, particularly when large amounts of fluids are rapidly administered, because their absolute blood volume is smaller The Confidential Enquiry into Perioperative Small Animal Fatalities suggested that the administration of intravenous fluids to cats during anesthesia resulted in an approximately four‐fold increase in the risk of death. Excessive fluid administration resulting in fluid overload is suspected to be at least partly responsible for these fatalities. Rapid fluid administration during inhalant anesthesia may also be more frequent in cats than in dogs, since blood pressure tends to be lower at similar anesthetic concentrations. Moreover, due to their small size and a relatively high incidence of undiagnosed (subclinical) cardiomyopathy, some cats are more prone to receiving excessive fluid volumes than are individuals of many larger species leading to fatal anesthetic complications. The recommended intraoperative fluid maintenance rate is 3 ml/kg/h of a balanced crystalloid solution in healthy adult cats undergoing routine procedures. That accounts to a maximum of 15-20 ml of crystalloids to a cat during a spay which feels miniscule when compared to the volume administered to a dog weighing the same. One study also pointed out that cats given fluids during anesthesia were nearly 4 times as likely to die as those that did not receive fluids.

Anesthesia circuits

Non-rebreathing circuits (NRC) are widely used in feline anesthesia because they offer less resistance than a rebreathing circuit, which is an important consideration in small patients. Rebreathing of CO2 is prevented by high oxygen flow rates; Investment in pediatric circuits and low dead space adapters is recommended if anesthesia in cats is maintained using gaseous anesthesia on a routine basis. Only non-rebreathing circuits should be used in cats with a body weight less than 3 kg.  and the oxygen flush valve should never be used when a cat is connected to an NRC. Pediatric rather than adult breathing hoses are recommended if and when using a rebreathing circuit (for cats >3kg) as they have lower equipment dead space. Excessive equipment dead space leads to rebreathing of CO2 and decreases the effective tidal volume available for gas exchange.

Monitoring during anesthesia

Cats are sensitive to the cardiovascular and respiratory depressant effects of inhalant agents; significant decreases in mean arterial pressures have been reported with isoflurane. Small patient size makes routine and otherwise easy procedures like palpation, auscultation, Pulse oximetry and ECG monitoring a bit more challenging. The normal heart rate in anesthetized cats is 100–180 bpm whereas it is 70-140 bpm in dogs.

Managing the Critical Cat

The average feline thorax can contain 300 ml of fluid before respiratory distress is seen- much more when compared to that of a dog. So, a cat with a history of say trauma or underlying subclinical heart disease may be eupneic but may collapse to even the ‘safest’ of pre-medicants or anesthetic agents. Cats have an extremely limited ability to contract their spleen to counter possible intra-operative hemorrhage. This warrants for stabilization of anemic the cat prior to the procedure if that is viable or to have back-up of a Donor in case a transfusion is necessary. Cats, unlike dogs can quickly develop a catabolic state, which predisposes them to the development of hepatic lipidosis because of lack of nutritional support. Therefore, both pre-operative and post-operative fasting must be as limited as what can be managed without compromising on anesthetic safety protocols.  Due to the cat’s inherent ability to mask disease, many cats that present as emergency cases are already in a critical condition and additional stress or mechanical/ chemical restraint can result in sudden death.

Hypothermia

Cats are susceptible to hypothermia due to their high surface area to body mass ratio. Hypothermia depresses immune function and has been linked to an increased incidence of wound infections and delayed healing time. Methods to maintain body temperature should be initiated in the patient-holding area before pre-medication and continued during anesthesia and recovery. Maintaining core body temperature using an active warming approach, such as forced warm air devices and medical grade electric and circulating warm water blankets, will be more effective than passive heat retaining methods such as blankets, towels and bubble wrap. Limiting the clipped area, using warmed scrub and lavage solutions, considering the type of anesthetic circuit used and keeping the cat dry are equally important.

 

Further reading

1)      AAFP Anesthesia Guidelines - https://doi.org/10.1177%2F1098612X18781391

2)      Lumb and Jones Veterinary Anesthesia and Analgesia - Textbook

3)      Feline Anesthesia and Pain Management – Textbook

4)      Comparative Anesthesia and Analgesia of Dogs and Cats Peter J. Pascoe and Bruno H. Pypendop - https://doi.org/10.1002/9781119421375.ch35

I am Dr. Varun Sastry, a small animal vet. I am an accomplished veterinary surgeon and with this blog, intend to use my experience to contribute to the profession and keep updated with the very latest in small animal practice. An enthusiast of 'Evidence-based Veterinary Medicine', graduated from Veterinary College, Hassan with a Bachelor's degree and from Veterinary College, Bangalore with a Master's degree in Surgery. I'm pursuing a Postgraduate Certificate in Small Animal Surgery (PG Cert.) offered by the university of Chester, UK.

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