Philip R Judge BVSc MVS PG Cert Vet Stud MACVSc (Vet Emergency and Critical Care; Medicine of Dogs)
Cats receiving anaesthesia are documented to have higher mortality rates and dogs. There may be many reasons for this disparity, including patient physiology, small patient size, difficulties in patient monitoring, differences in drug metabolism, the presence of underlying undiagnosed covert disease (such as hypertrophic cardiomyopathy), and the magnitude of peri-anaesthetic anxiety, among other factors.
These differences require that veterinarians and nurses approach anaesthesia in the cat with a different mindset to anaesthesia in the dog, so that appropriate monitoring and support measures are applied, to minimise anaesthetic risk.
Cats vs. Dogs: Key Differences
1. Fasting before anaesthesia
- Traditional fasting times of 6-12 hours are frequently recommended but are not evidence-based. Shorter fasting times have been associated with less gastro-oesophageal reflux in dogs, but results are conflicting among different studies.
- Gastric emptying time:
- The gastric emptying in the dog and cat usually begins within 15 minutes of ingestion of food and should be complete within 1-4 hours in the dog, and as little as 30-120 minutes in the cat.
- The shorter gastric emptying time in cats may imply the requirement for a shorter fasting period prior to anaesthesia. However, other factors may influence the likelihood of gastroesophageal reflux in the cat, including:
- Anaesthetic and pre-anaesthetic drugs may influence the incidence of gastroesophageal reflux e.g. morphine, methadone and dexmedetomidine/medetomidine/xylazine all can increase the risk of gastroesophageal reflux in the cat
- Stress, meal size and low dietary moisture can slow gastric emptying rate in cats, increasing the risk of gastroesophageal reflux under anaesthesia
- There are currently no evidence-based recommendations for fasting in the cat prior to anaesthesia
- Because of the risk of delayed gastric emptying due to a range of factors, the anaesthetic team should be prepared for vomiting or gastroesophageal reflux in any patient
- At the clinician discretion, shorter fasting times of 3-4 hours may be advocated, as long as the final pre-anaesthetic meal is a small wet food feeding
- Reduction of stress appears to offer potential benefits in normalization of gastric emptying. To this end, pre-clinic visit administration of gabapentin (50-100 mg PO) may be recommended, provided there are no contraindications
- Water should be made available until the time of premedication
2. Pre-Anaesthetic Assessment
- Pre-anaesthetic testing should include thorough patient history review, physical examination, and laboratory and imaging assessments as appropriate
- Many cats can be stressed at the time of physical examination and assessment, and this may be reflected in abnormal physical and laboratory findings, making interpretation of abnormal findings challenging. For example, clinical and laboratory findings that may result from either stress or underlying pathology may include
- Elevated blood and urine glucose levels
- The presence of cardiac gallop or murmurs
- The presence of tachycardia or bradycardia
- The presence of hypertension
- Testing for hyperthyroidism in middle-aged to older cats can be valuable. Hyperthyroidism affects up to 10% of cats aged 10 years and older, and can result in hypertension, hypertrophic cardiomyopathy, and cardiac arrhythmias
- Abnormal cardiac findings on physical examination should be evaluated. Cats with cardiomyopathy are frequently asymptomatic. Cats with clinical examination findings of gallop or murmur are recommended to have echocardiography performed, as well as NT-ProBNP screening to facilitate diagnosis of cardiac disease.
3. Anaesthetic Equipment
- Because of their small size and small lung capacity, cats are uniquely susceptible to deleterious side effects of inappropriate or malfunctioning anaesthetic equipment
- Optimal anaesthetic equipment for cats includes
- Anaesthetic machine with precision out of circuit vaporiser
- A non-rebreathing anaesthetic circuit with pressure manometer and safety pop-off relief valve. Alternatively, in cats larger than 3 kg, a paediatric rebreathing circuit with low-resistance tubing, and lightweight plastic rebreathing valves and minimal dead space may also be considered
- Standard anaesthetic equipment including supplies for intravenous access, endotracheal intubation, and monitoring equipment (ECG, pulse oximeter, blood pressure (doppler), thermometer, oesophageal stethoscope, capnography etc.
- The risk of high airway pressures, and associated barotrauma and lung damage is high in cats, because of their small lung volume (400 ml in a 3-5 kg cat), coupled with the high flow rates of oxygen required in non-rebreathing anaesthetic circuits (>200 ml/kg/minute; minimum flow rate 500 ml/minute). These two factors can lead to damage to a cats’ airways in as little as 30 seconds, if the circuit is accidentally closed. As a result, essential safety features in feline anaesthesia should include:
- In-circuit manometer
- Safety pop-off valve and/or a high-pressure alarm, placed in the expiratory limb of the non-rebreathing circuit, that emits a loud noise if circuit pressure become excessive.
- The oxygen flush valve should never be used when a cat is connected to an anaesthetic circuit, as the flow rate can be as high as 40 litres/minute and can result in significant lung and airway damage.
- Minimising anaesthetic circuit dead space is crucial in the cat, owing to their small size, which can lead to carbon dioxide rebreathing, reduced tidal volume as well as respiratory fatigue. Dead space should ideally be less than 2-3 ml/kg (less than 20% of tidal volume). Where possible, use of low-dead space endotracheal tube adaptors fir attachment to side-stream sampling lines should be used. Important sources of dead-space include:
- The patient end of breathing circuits
- Side-stream capnograph adaptors
- In-line capnographs – note that in-line capnographs increase circuit resistance also.
- Elbow adaptors – Note that a single elbow adaptor can add up to 8 ml of dead space
- Minimise circuit resistance – to reduce the effort required for the cat to breathe.
- In-line capnographs increase anaesthetic circuit resistance.
- The diameter of any capnograph adaptor should always exceed the internal diameter of the patient endotracheal tube to reduce circuit resistance
4. Anaesthetic Circuits and Oxygen Flow Rates
- The small patient size, and the use of non-rebreathing anaesthetic circuits means oxygen flow rates and rebreathing bag sizes are quite different to those used in rebreathing systems for dogs over 7 kg.
- Oxygen flow rates:
- Non-rebreathing circuits – Recommended oxygen flow rate > 200 ml/kg/minute with a minimum flow rate of 500 ml/minute
- Rebreathing circuits –
- First 15 minutes: 2-3 litres/minute
- Maintenance: 0.5 litre/minute
- Rebreathing bag size
- 0-5kg: 500 ml
- >5kg: 1 litre
5. Anaesthetic Circuits and Oxygen Flow Rates
- Kittens up to 5 months of age have variable (reduced) ability to metabolise drugs via the hepatic P450 enzyme systems, leading to profound or prolonged effect of sedative and anaesthetic medications metabolised in the liver.
- Senior/geriatric cats older than 10 years of age:
- Have a higher risk of anaesthetic-related death (independent of ASA status)
- Are more sensitive to the respiratory and cardiovascular effects of sedative and anaesthetic drugs
- Have limited compensatory responses to homeostatic changes e.g. cardiac output etc.
6. Blood Volume
- The blood volume of the cat is smaller than that of the dog, as a percentage of bodyweight.
- Fluid therapy boluses should be dosed at 5-7 ml/kg, given over 10 minutes
- Anaesthetic fluid rates for maintenance fluid therapy are 2.5-3 ml/kg/hr
- Pulmonary oedema may develop with prolonged fluid therapy at rates higher than maintenance rates, including traditional “surgical fluid rates” of 5 ml/kg/hr and above.
7. Emergency and critical illness anaesthesia
- Cats have unique physiology – particularly notable when they suffer shock, hypovolaemia, inanition, and cardiac, respiratory and abdominal disease
- Clinical signs of a cat in shock are
- Cats have limited ability to contract their spleen during haemorrhage. Splenic volume is also smaller than in the dog (1.5 ml/kg vs. 3.5 ml/kg)
- Lack of nutritional support in acute illness can result in rapid development of a catabolic state, leading to development of hepatic lipidosis, as well as thiamine deficiency
- Cats presenting with seizures require thorough evaluation for secondary (intra-cranial and extracranial diseases) illness, as idiopathic epilepsy is uncommon in the cat
- Acute heart failure is characterised most frequently by respiratory distress, +/- dysfunction of a limb (due to thromboembolism), but can also be characterised by abdominal organ dysfunction and abdominal pain
- Acute abdominal disease may result in abdominal pain in less than 35% of cases, with some cats showing respiratory distress as the primary presenting sign – necessitating a thorough clinical evaluation of the patient using diagnostic imaging, blood tests/urinalysis etc.
8. Emergency and critical illness anaesthesia
- Monitoring the feline patient under anaesthesia can be more challenging than in the dog, owing to the small patient size, and the impacts on sensitivity of monitoring equipment.
- Optimum monitoring equipment for feline anaesthesia includes
- Clinical observation of physical parameters and anaesthetic depth assessment
- Heart rate – particularly valuable with oesophageal stethoscope
- Respiratory rate and effort – does not indicate effective ventilation or oxygenation. Tidal volume is difficult to assess in the feline patient due to small patient size – necessitating instrumentation such as pulse oximetry or blood gas analysis to verify oxygen saturation
- Pulse palpation
- Pulse oximetry – reliable when signal quality if good and pulse rate accurate
- ECG – small patient size often results in poor ECG signal
- Capnography – helpful in determining if alterations in fresh gas flow are required. Inspired carbon dioxide in a non-rebreathing circuit should not exceed 5 mm Hg.
- Blood pressure
- Ideal cuff width affects accuracy of reading. Optimum cuff width is 40% of the limb circumference
- Doppler blood pressure is more reliable than oscillometric blood pressure devices
- Normal systolic blood pressure in the cat is 100-120 mm Hg; hypotension is classified as systolic arterial pressure <90 mm Hg
9. Laryngeal spasm
- Cats have a very sensitive laryngospasm reflex in response to laryngeal stimulation or irritation, which can result in difficult endotracheal intubation.
- Placement of 1-2 drops of 2% lidocaine (20 mg/ml) on the arytenoid cartilages, 60-90 seconds prior to intubation attempts will reduce the risk of severe laryngospasm during intubation attempts
10. Airway pressures
- Endotracheal cuff inflation should be at the lowest volume and pressure required.
- Inflate the endotracheal tube cuff using 0.5 ml increments of air until no leak can be heart when the rebreathing bag is squeezed to a pressure of no greater than 16-18 cm water
- Endotracheal cuff pressure measurement devices are also available e.g. Tru-Cuff syringes. Optimum cuff pressure is 20-24 cm water pressure in the cat.
Feline anaesthesia presents several challenges that are distinct from canine anaesthesia. Whilst this list is not exhaustive, it provides the basis for consideration of several key areas that require conscious thought and evaluation when performing anaesthesia in the cat. It is important to tailor each anaesthetic to the individual patient, and to optimise patient monitoring from the pre-anaesthetic period, through to full patient recovery – with knowledge that the highest period of anaesthetic mortality in cats is in the anaesthetic recovery period (up to 60%) – irrespective of ASA patient risk score.
References and Suggested Reading
- Grubb, Tamara, Jennifer Sager, James S. Gaynor, Elizabeth Montgomery, Judith A. Parker, Heidi Shafford, and Caitlin Tearney. “2020 AAHA anesthesia and monitoring guidelines for dogs and cats.” Journal of the American Animal Hospital Association 56, no. 2 (2020): 59-82.
- Davis, Harold, Tracey Jensen, Anthony Johnson, Pamela Knowles, Robert Meyer, Renee Rucinsky, and Heidi Shafford. “2013 AAHA/AAFP fluid therapy guidelines for dogs and cats.” Journal of the American Animal Hospital Association 49, no. 3 (2013): 149-159.
- Robertson, Sheilah A., Susan M. Gogolski, Peter Pascoe, Heidi L. Shafford, Jennifer Sager, and Gregg M. Griffenhagen. “AAFP feline anesthesia guidelines.” Journal of feline medicine and surgery 20, no. 7 (2018): 602-634.
- Steagall, Paulo V., Sheilah Robertson, Bradley Simon, Leon N. Warne, Yael Shilo-Benjamini, and Samantha Taylor. “2022 ISFM consensus guidelines on the management of acute pain in cats.” Journal of Feline Medicine and Surgery 24, no. 1 (2022): 4-30.
- Rodan, Ilona, Nathalie Dowgray, Hazel C. Carney, Ellen Carozza, Sarah LH Ellis, Sarah Heath, Lee Niel, Kelly St Denis, and Samantha Taylor. “2022 AAFP/ISFM cat friendly veterinary interaction guidelines: approach and handling techniques.” Journal of Feline Medicine and Surgery 24, no. 11 (2022): 1093-1132.
- Matthews, Nora S., Thomas J. Mohn, Mingyin Yang, Nathaniel Spofford, Alison Marsh, Karen Faunt, Elizabeth M. Lund, and Sandra L. Lefebvre. “Factors associated with anesthetic-related death in dogs and cats in primary care veterinary hospitals.” Journal of the American Veterinary Medical Association 250, no. 6 (2017): 655-665.
- Garcia, R. S., P. C. Belafsky, A. Della Maggiore, J. M. Osborn, B. H. Pypendop, T. Pierce, V. J. Walker, A. Fulton, and S. L. Marks. “Prevalence of gastroesophageal reflux in cats during anesthesia and effect of omeprazole on gastric pH.” Journal of veterinary internal medicine 31, no. 3 (2017): 734-742.