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.
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.
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.
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.
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
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