Anaesthesia for neutering kittens

Although anaesthesia of the very young carries a greater risk than that of healthy adults, there are some compelling reasons for neutering kittens between 2-4 months of age. With attention to specific potential problems in young kittens, anaesthesia may be carried out without serious harm, and the benefits realised. Feral kittens that are trapped are unlikely to be caught this way again and if not neutered before release will remain entire for the rest of their lives. Similarly, kittens collected and rehomed by welfare organisations may not be presented again, and should be neutered before rehoming if neutering is to be ensured. Healthy kittens of 2-4 months are robust creatures and should tolerate well performed anaesthesia as well as their human paediatric counterparts.

Potential problems in anaesthesia of young kittens

Size

Young kittens presented for neutering are small, many weighing around I kg or less. As a result, handling and placing an intravenous catheter may be difficult to perform and stressful to the kitten. Sedative and anaesthetic agents are most easily given by intramuscular or subcutaneous injection and by inhalation. Kittens must be weighed accurately to ensure correct drug dosage and diluted solutions used to ensure accurate dosing.

Hypothermia is another potential hazard, due to the high body surface area to volume ratio, limited body fat and immature thermoregulatory control. Considerable care must be taken to prevent heat loss whenever possible. This could include clipping the minimum amount of fur without compromising sepsis, careful use of surgical , scrub so that the kitten is kept as dry as possible, minimal exposure of abdominal contents to prevent excessive evaporation, careful insulation with bubble wrap or space blankets, and use of heated pads or, better still, hot air blowers such as the Bair Hugger®, during surgery. The kitten should also be kept warm until fully recovered, as further heat may be lost after anaesthesia before normal mobility is resumed.

Small size may also make surgery more difficult and lead to increased tissue trauma around the incision. However, the organs to be removed are also small, so this may be to the kitten's advantage as tissue damage should thus be minimised. Circulating blood volume is inevitably small, and haemorrhage must be prevented, as loss of more than a few millilitres will be harmful. However, since the tissues to be removed are not fully developed, the blood supply should also be limited and the risk of major haemorrhage be less than in the adult.

Immaturity

The cardiovascular and respiratory systems of the kitten are immature, resulting in less cardiopulmonary reserve in the face of anaesthetic-induced depression. It is important that normal heart and respiratory rates are maintained during anaesthesia as adequacy of both ventilation and cardiac output are dependent primarily on rate of breathing and heart beat rather than an increase in depth or stroke volume. Anticholinergic drugs should be immediately available to treat bradycardia, if they are not given routinely. Renal function is also immature in young kittens and the kidney does not yet have normal ability to concentrate urine. It is thus essential that kittens are not allowed to become dehydrated as they are unable to compensate.

A number of metabolic processes are immature in the kitten. Of particular relevance to anaesthesia are energy metabolism and the pathways through which drugs are broken down and excreted. Hypoglycaemia may develop during anaesthesia in young kittens and may cause prolonged recovery. Underdeveloped pathways of drug metabolism may slow the elimination of some anaesthetic drugs, also leading to prolonged recovery from anaesthesia. Drugs eliminated by routes not dependent on such pathways are thus preferable; inhalation anaesthetics are removed by exhalation and the effects of some drugs may be reversed by the use of antagonists.

Pain and behaviour

It is important that the kitten's experience of anaesthesia and surgery is not unpleasant, as this may be its first experience of contact with mankind. Calm but firm handling and good analgesia are essential in ensuring the process is not a nasty experience. It has long been believed that young animals do not experience pain in the same way as adults, making pain relief unnecessary. There is absolutely no evidence for this, although conduction of stimuli that are perceived as pain may take a few microseconds longer to reach the brain in a kitten than in an adult. Hence, analgesia is an essential component of the anaesthetic protocol for neutering kittens in the same way as it is required for older animals.

Care during anaesthesia

Anaesthesia for young kittens has all the same requirements as for adults, in that fundamental care of normal physiological function is essential. The ABC of resuscitation is the first requirement: Airway, Breathing and Circulation. These are monitored in the same way as in adults; airway and breathing by observation of thoracic movement and excursion of the breathing reservoir bag, and circulation by palpation of the pulse. Additional aids to monitoring, applicable to both sexes, but usually only required for neutering female kittens where the surgery is longer, are measurement of arterial pressure using Doppler flow detection, pulse oximetry, ECG and capnography. Doppler flow detectors are easily applied to the digital arteries on front or hind limbs and act as a pulse detector even without being used for measuring arterial blood pressure. Additional requirements for monitoring are body temperature, essential in such small animals as it must actively be maintained as close to normal as possible, at least above 36^oC. Since surgery for neutering is generally of only a few minutes duration, it is generally unnecessary routinely to infuse fluids, as would be normal for longer surgeries. However, isotonic fluids with glucose supplementation should be infused at 10 ml/kg/hr intravenously if anaesthesia if prolonged beyond 10-15 minutes or if there is any haemorrhage.

Anaesthetic protocols

Table 1: Some anaesthetic protocols suitable for neutering young kittens
Anaesthetic protocol	Comments
1 Midazolam 0.2 mg/kg & ketamine 10-20 mg/kg IM. Maintain anaesthesia with volatile agent in oxygen (±nitrous oxide) after intubation for females. Complete castration using mask administration in males	Relaxation may be inadequate for intubation or castration; deepen anaesthesia with isoflurane, sevoflurane or halothane in oxygen by mask. Recovery may be violent
*2 Midazolam O.2mg/kg&ketamine 5-10mg/kg&opioid IM** * any of buprenorphine 0.01 mg/kg or methadone 0. 1 -0.2 mg/kg or pethidine 5 mg/kg or butorphanol 0.4 mg/kg	Relaxation better than midazolam & ketamine alone, often sufficient for castration or intubation. Deepen/maintain anaesthesia with volatile agents as in 1
3 Medetomidine 0.08 mg/kg & ketamine 5 mg/kg & butorphanol 0.4 mg/kg IM (or SC) Supply oxygen by mask Reverse medetomidine with atipamezole 0.4 mg/kg	Good surgical conditions Potential for marked CV and respiratory depression - not ideal for very young kittens. If used, anaesthesia time must be short (< 15 min) and must be reversed with atipamezole Care to maintain body temperature Butorphanol short acting analgesic, post op further opioid ± NSAID essential (see Table 2)
*4 Medetomidine 0.01 mg/kg & ketamine 5 mg/kg & opioid IM** * any of buprenorphine 0.01 mg/kg or methadone 0.1-0.2 mg/kg or pethidine 5 mg/kg or butorphanol 0.4 mg/kg Reverse medetomidine with atipamezole 0.04 mg/kg	Uses lower dose of medetomidine, hence safer than 3, especially for very young kittens. Anaesthesia may not be sufficient for intubation or castration; deepen/maintain anaesthesia as in 1
5 Midazolam 0.2 mg/kg & ketamine 5-10 mg/kg & acepromazine 0.03-0.05 mg/kg IM	Relaxation better than midazolam & ketamine alone, often sufficient for castration or intubation. Deepen/maintain anaesthesia with volatile agents as in 1. Less suitable for younger kittens as acepromazine may have prolonged effect and contribute to hypothermia. NB no analgesic effect after anaesthesia - opioid ± NSAID essential (see Table 2)
6 Mask induction with halothane, isoflurane or sevoflurane in induction chamber or by mask Maintain anaesthesia with volatile agent after intubation (females) or with mask (males)	Sevoflurane likely to produce smoothest induction. Recovery rapid in all cases. NB no analgesic effect after anaesthesia - opioid ± NSAID essential (see Table 2)
7 Zolazepam & tiletamine (11 mg/kg of Zoletil®) or Telazol®	Not available in UK but used in USA for neutering male kittens. May be inadequate relaxation, recovery may be prolonged and not always smooth. NB no analgesic effect after anaesthesia - opioid ± NSAID essential (see Table 2)

Table 2: Analgesic methods suitable for neutering young kittens
1 Opioids Buprenorphine 0.01 mg/kg Methadone 0. 1 -0.2 mg/kg Pethicline 5 mg/kg Butorphanol 0.4 mg/kg	Any opioid used as part of injectable anaesthetic provides pre-emptive analgesia. NB Pethidine and butorphanol are very short acting. If used, more opioid should be given post operatively. Buprenorphine produces 8 or more hours analgesia and is recommended. Repeat dose once for overnight effect after surgery
2 NSAIDs Carprofen 4 mg/kg - best given pre op Ketoprofen 2.0 mg/kg - immediately post op Meloxicam 0.2 mg/kg - best given pre op	Single dose given SC will give 24 hour effect Analgesics are best given pre-operatively for pre-emptive effect Carprofen widely used pre-operatively. Ketoprofen may affect renal haemostasis and should be given post op. Meloxicam is now licensed for cats by the injectable route
Multimodal analgesia - opioid and NSAID Combine 1 & 2	Recommended method - better analgesia than either opioid or NSAID alone. Analgesic effect through different routes. Elimination from body through different routes - no potential for additive toxicity

Induction of anaesthesia in young kittens is normally brought about by the use of heavy premedication given by the intramuscular or subcutaneous route and supplemented by inhalation anaesthetics given in oxygen. The most suitable agents for initial injection are benzodiazepines, ketamine and the opioids, usually given in one of a range of combinations. This may provide sufficient anaesthesia for castration of male kittens but usually requires supplementation with volatile agents for females, as surgery is longer. In both cases, oxygen should be given even if volatile anaesthetics are not required. In male kittens it is sufficient to use a well fitting face mask attached to a T-piece or mini Bain circuit. Anaesthesia for female kittens is best continued after tracheal intubation to protect the airway, using one of the same non-rebreathing circuits. Intubation must be gentle, as the larynx and trachea are small and delicate, and a non-cuffed tube is sufficient in order to allow the largest possible internal diameter tube to be placed. Local anaesthetic spray on the larynx will facilitate intubation, but care must be taken not to exceed 10 mg/kg lidocaine. In feral kittens induction of anaesthesia is probably less stressful for all concerned if induced with a volatile anaesthetic agent in oxygen ducted into an airtight chamber. Anaesthesia can then be maintained via a mask or tube as after injectable premedication. If inhalation induction is used the kitten should also be given opioid analgesics before surgery to reduce anaesthetic requirements and provide post operative analgesia. (See Table 3)

Table 3: Anaesthetic agents

Benzodiazepines cause little cardiovascular or respiratory depression, and in very young kittens produce good sedation even when used alone. In older animals the effect is not so marked, and combination with other sedatives or analgesics is required. Benzodiazepines are synergistic with opioids and ketamine and use of such combinations is ideal for young kittens.

Opioids produce little sedation when used on their own, but provide excellent analgesia, little cardiovascular depression and reduce the required dose of other sedatives and anaesthetics. Opioid combinations are thus ideal for use in young kittens as this provides good sedation and induction of anaesthesia as well as contributing to post operative analgesia.

Ketamine is an injectable anaesthetic widely used in cats, which causes little cardiovascular or respiratory depression. On its own ketamine does not produce good relaxation although analgesia is good. In combination with other sedatives or analgesics muscle relaxation is produced and surgical conditions are generally good.

Phenothiazines, such as acepromazine may be used, but in very young kittens elimination from the body is slow and recovery may be prolonged and associated with serious hypothermia.

Alpha-2 adrenoceptor agonist agents such as xylazine and medetomidine may be used, but since these cause marked bradycardia and require hepatic metabolism they are less well tolerated by young kittens than by adult cats. Alpha-2 agents do, however, have the advantage that they can be reversed by antagonists such as atipamezole, and have been used successfully in kittens in this way.

Volatile anaesthetic agents can be used, after the injectable agents have sedated the kitten, to deepen anaesthesia enough to allow intubation in female kittens prior to maintenance of anaesthesia via the breathing circuit. Sedation with opioid, ketamine and benzodiazepine combinations may be sufficient to allow castration of male kittens, but where this is inadequate volatile agents may be given by mask to allow the procedure to be completed. Halothane has been used successfully for many years; however, isoflurane has a more rapid onset and little hepatic metabolism and is probably a better choice. Sevoflurane should prove to be even better as it is less irritant to the respiratory system and allows even smoother induction. Nitrous oxide may be used up to 67 per cent of the inspired gas to enhance the analgesic effect of the gas mixture.

Recovery

Monitoring should be continued in the recovery period until the kitten is fully conscious and able to maintain sternal recumbency without difficulty. It is essential that they are not allowed to lose heat during this period: temperature should be monitored regularly, and the kitten wrapped in space blankets or bubble wrap and actively warmed if the temperature falls below 36oC. Prolonged hypothermia will prolong recovery.

A second cause of prolonged recovery if hypoglycaemia. If recovery is slow, blood glucose should be measured using a rapid test strip on a single sample taken percutaneously, and if below 5 mEq/1 should be treated by oral glucose as long as the kitten is able to swallow. On the rare occasion the kitten remains unconscious an intravenous line must be placed, most easily in the jugular vein, and 5 per cent dextrose infused (10 ml/kg/hr).

Analgesia is best provided in the recovery period by multimodal methods. An opioid component of the original injection will provide pre-emptive analgesia and this is enhanced with a NSAID. Buprenorphine, 0.01 mg/kg, is the opioid of choice as this is highly effective in the cat and gives many hours post operative cover. Carprofen, 4 mg/kg, can be given subcutaneously before surgery starts and provides 24 hours analgesic cover. This approach is sufficient to provide good analgesic cover for both male and female kittens.

Dr Polly Taylor, Department of Clinical Veterinary Medicine, University of Cambridge (2002)
Published in FAB Journal 40, 22 - 24 and reproduced here by kind permission

Potential problems in anaesthesia of young kittens

Anaesthetic protocols

Recovery

Further reading