Anaesthesia and Surgical Protocols for kittens

(C)Susan Little, DVM; http://www.sniksnak.com/cathealth/earlyage.html

Kittens

Anaesthesia and surgery in the paediatric patient carry special considerations and the practitioner must be aware of, and respect, these factors. Kittens have a rate of oxygen consumption about 2-3 times that of the adult cat. The tidal volume of the kitten is about the same as that for adults, so the respiratory rate of kittens must be 2-3 times higher to meet oxygen demands. Since many anaesthetic drugs depress respiration, this is of major concern. Kittens should generally be allowed to breathe spontaneously under anaesthesia, but must be closely monitored for hypoventilation. As well, kittens will have more rapid induction and recovery from inhalation anaesthesia than adults.

There are significant differences in cardiovascular physiology in kittens. The kitten’s heart is less able to increase contractile force or stroke volume, so cardiac output is primarily rate dependent. This means that bradycardia (<150 bpm) in kittens can be a serious problem and should be corrected quickly. As well, hypotension is a greater problem in anaesthetized kittens than in adults. Kittens have immature sympathetic innervation so that parasympathetic effects dominate. This results in strong vagal tone. Many authors recommend atropine (0.02-0.04 mg/kg IM, SC) or glycopyrrolate (0.01 mg/kg IM, SC) as part of pre-medication. These drugs will also dry airway secretions, which can be beneficial.

Kittens are also less able to compensate for haemorrhage. Since haemoglobin concentration and PCV is lower in kittens, a small amount of blood loss during surgery may cause a clinical anaemia, especially for kittens between 2-8 weeks of age. Another important fact is that hepatic glycogen stores are minimal in kittens, so that fasting rapidly causes hypoglycaemia.

Young kittens are especially prone to hypothermia due to their larger surface area to volume ratio, an immature thermoregulatory system, reduced ability to shiver, and decreased subcutaneous fat. Heat loss can be exacerbated by evaporation of alcohol used in surgical preparations. Kittens undergoing ovariohysterectomy tend to have lower intraoperative body temperatures than those undergoing castration, probably due to exposure of the peritoneal cavity. Hypothermia can result in bradycardia, decreased cardiac output and hypotension. If drug metabolism is slowed by hypothermia, a prolonged recovery from anaesthesia will result.

Neonatal kittens have incomplete or absent enzyme systems necessary for drug metabolism. Renal function is immature, so that urine is not well concentrated and even mild dehydration is not well tolerated. Drugs that are metabolised in the liver may have a longer duration of effect, depending on the age of the kitten. Drugs that are highly protein-bound will have greater unbound fractions in the bloodstream due to low plasma albumin concentrations when they are administered IV. The result will be an apparent increase in sensitivity to the drug. Drugs which are not protein bound will have a larger volume of distribution, which may appear as an apparent resistance to the drug. Repeated IM dosing of drugs that require redistribution to muscle and fat will cause a prolongation of drug effect. In general, the dose of anaesthetic agents should be reduced in the neonatal kitten. Kittens should be weighed accurately and drug doses calculated carefully. Small volumes of IV drugs are better administered diluted.

Xylazine

In general, it is best to avoid phenothiazine tranquillisers in young kittens, especially those under 12 weeks of age. Prolonged CNS depression, as well as potentiation of hypotension and hypothermia, are potential downfalls of acepromazine. If acepromazine is used in kittens from 12-16 weeks of age, the dose should be reduced (0.025-0.05 mg/kg IM, SC). Likewise, xylazine is also best avoided in kittens. Xylazine is normally metabolised extensively by the liver, and it can cause marked bradycardia and depression of cardiac output. Xylazine in combination with acepromazine can cause prolonged sedation and profound cardiovascular depression in kittens.

Opioids

Opioids (meperidine, oxymorphone, fentanyl, butorphanol) are better choices for paediatric premedication. They are generally well tolerated by kittens, do not depress myocardial contractility, and provide good analgesia. They can produce bradycardia and so require atropine or glycopyrrolate. They may also produce respiratory depression that may require administration of oxygen. Even though opioids do undergo hepatic transformation, it is uncommon to see prolonged CNS depression in kittens. The drugs should be dosed lower than for adults (i.e. oxymorphone 0.05-1.0 mg/kg IM, SC). If necessary, the effects of opioids can be reversed with naloxone.

Ketamine

Low doses of ketamine (4-10 mg/kg IM) can be used before mask induction. However, since this drug tends to produce copious salivation, atropine (0.02 mg/kg IM, SC) or glycopyrrolate (0.011 mg/kg IM, SC) should be administered as well to prevent aspiration of material in the laryngeal area or plugging of an endotracheal tube. Ketamine produces pain on injection. Although it does not depress cardiovascular function, an apneustic breathing pattern sometimes develops. Since ketamine is largely excreted through the kidneys, it should not be used on kittens under 8 weeks of age or prolonged recovery will result. An analgesic should be used for ketamine alone does not provide adequate pain relief in cats. The combination of ketamine and xylazine should be avoided in kittens, for the stimulant effects of ketamine are not enough to counteract the cardiovascular depression caused by xylazine.

Zoletil

Tiletamine/zolazepam (Zoletil®) is a drug combination similar to ketamine/diazepam. It is administered at 11mg/kg IM to cats and produces pain on injection (probably due to the low pH of the solution). It provides smooth inductions and recoveries as well as skeletal muscle relaxation. Similar to ketamine, increased salivation and airway secretions are seen. The duration of effect is 15-40 minutes, depending on dose.

Barbiturates

Thiobarbiturates can be used with care in kittens over 12 weeks of age. One main drawback is that they require IV injection. They depend on redistribution to fat and muscle and so are not appropriate in younger kittens. Kittens must be weighed and the dose calculated accurately. Dilution of the drug to a 1-2% concentration can facilitate accurate dose measurement. Thiobarbiturates are largely protein bound, so a greater proportion of the drug is available to cross the blood-brain barrier in kittens. This means the dose of drug necessary for induction is lower in kittens than in adult cats. Repeat doses should not be given after induction since the small muscle mass is easily saturated with drug.

Inhalation anaesthetics

The inhalation anaesthetic of choice for kittens is isoflurane. It is less soluble in blood and tissues than halothane and so provides faster inductions and recoveries. Up to 25% of the halothane dose is metabolised in the liver, but isoflurane is largely breathed off and requires almost no hepatic metabolism. Isoflurane also maintains better cardiovascular function than halothane. Many kittens are amenable to premedication with oxymorphone and mask induction with isoflurane. It is important to remember that a non-rebreathing circuit (i.e. Bain circuit) must be used with kittens. An oxygen flow rate of 500 ml/kg/min is used. Anaesthetic monitoring of intubated patients can be accomplished using an oesophageal stethoscope. Rectal temperature should be checked as soon as surgery is completed.

Where endotracheal intubation is desired, either cuffed or non-cuffed Magill or Cole tubes (2-3.5 mm) should be used. Non-cuffed tubes allow for usage of a larger diameter tube, which may be less prone to obstruction with airway secretions. Endotracheal tubes under 3.0 mm in diameter are easily clogged by secretions and may require suctioning if anaesthesia lasts longer than about 30 minutes, but this is uncommon with early age altering surgeries. It is important to avoid causing laryngeal trauma during intubation. Endotracheal tubes that are too long for the patient may result in primary bronchial intubation. An alternative to intubation is a snug-fitting clear facemask, especially when supplemental anaesthesia is required for castrations.

Paediatric tissues are more susceptible to trauma during surgical handling. Avoid lacerating skin during pre-surgical clipping and minimize the amount of hair coat removed to help avoid hypothermia. Since even a small volume of blood loss is very important, meticulous hemostasis is necessary. Medium stainless steel hemostatic clips are suitable for use in both male and female kittens. They are especially valuable for castrations, since techniques where the spermatic cord is tied on itself, or the vas deferens and the spermatic artery are tied together, are not suitable in young kittens due to the fragility and small size of the spermatic artery. There is at least one report of a kitten haemorrhaging severely enough to cause death after this type of castration technique (Howe, 1997).

Any suture material used in males and females should be non-irritating and of small diameter (3-0 to 4-0 catgut or polyglactin 910 [Vicryl®]). Subcuticular tissues can be closed with nylon in a continuous intradermal pattern. When this is done, skin sutures are unnecessary. Alternatively, tissue adhesive can be used to avoid skin sutures, which may be chewed out by kittens.

Faggella and Aronsohn (JAVMA, 1993) evaluated different anaesthetic protocols in 96 kittens (48 males, 48 females). For males, they recommended tiletamine/zolazepam (11 mg/kg IM) since it provided good sedation and analgesia that was adequate in 85% of kittens. Supplemental isoflurane must be available if needed and can be administered via facemask. For females, they recommended midazolam (0.22 mg/kg IM) and ketamine (11 mg/kg IM) mixed in the same syringe. Female kittens were then intubated and maintained with isoflurane. Surgical prep was done using warmed chlorhexidine solution diluted with saline.

A report on 775 cats undergoing prepuberal gonadectomy at Texas A&M University (Howe, 1997) detailed different anaesthetic protocols. Male and female kittens were given the same anaesthetic protocol. All kittens under 24 weeks old were given acepromazine (0.055 mg/kg IM), butorphanol (0.22 mg/kg IM), glycopyrrolate (0.011 mg/kg IM) and ketamine (11 mg/kg IM). All were intubated and maintained on either halothane or isoflurane. Most kittens required a few minutes of gas anaesthesia via mask before intubation. The student surgeons in this report required less time to alter paediatric kittens than older kittens, so it is expected that experienced surgeons would require even less time.

Anaesthetic complication rates following early age altering reported in the literature vary. Faggella and Aronsohn’s study of 96 kittens reported no complications. Howe’s survey of kittens altered by fourth year students for humane organizations at Texas A&M University found that prepuberal gonadectomy did not increase morbidity or mortality during the intra- or postoperative period (7 days after surgery). Kittens altered at less than 12 weeks of age actually had a lower postoperative complication rate (6.5%) than those altered at greater than 23 weeks of age (10.8%).

Some guidelines for paediatric patients can facilitate safe and effective anaesthesia and surgery:

1. ensure kittens have a complete physical exam prior to surgery; are vaccinated and treated for parasites several days before; postpone surgery if any abnormalities are found (such as cryptorchidism)

2. weigh each kitten accurately (to the nearest 100g) to calculate drug doses

3. combat hypoglycaemia by withholding food for only 3-4 hours before surgery; feed a small meal within 1 hour of recovery from anaesthesia

4. keep kittens with their littermates before surgery in a warm, quiet environment to decrease excitement and stress; minimize handling; use IM injections rather than IV to minimize restraint and stress

4. treat kittens with a prolonged recovery or who will not eat after recovery with 50% dextrose given orally

6. combat hypothermia with insulation against cold surfaces (i.e. re-circulating warm water blankets) during induction and surgery; minimize hair coat clipping; avoid alcohol in surgical preps; warm surgical prep solutions; check rectal temperature at the end of surgery; postoperatively, use warm blankets, hot water bottles, or heat lamps (but monitor closely to avoid thermal injury)