Induction And Maintenance Of Anesthesia

By | 2011-07-16

Injectable agents

The advantages of the injectable anesthetics are that they are often easy to administer, they frequently involve minimal stress and they prevent the problems encountered with breath-holding when using gaseous induction techniques. Disadvantages include the problem of reversal for some agents, the often varying responses depending on the individual animal and the frequent respiratory depression, hypoxia and hypotensive effects they produce.

Propofol

Propofol has some use in small mammals. It may be used in most species of small mammal at a dose of 10 mg / kg after the use of a pre-anesthetic medication such as acepromazine, but requires intravenous or intraosseous access. Its effects in rabbits are broadly similar to those seen in cats and dogs although breath-holding is very common and may cause problems where intubation is not possible.

Ketamine

Mustelids

Ketamine may be used alone for chemical restraint in ferrets at doses of 10-20mg / kg but, as with cats and dogs, the muscle relaxation is poor and salivation occurs. More often ketamine is combined with other drugs such as the alpha-2 agonists, xylazine and medetomidine. In ferrets 10-30 mg / kg ketamine may be used with 1-2 mg / kg xylazine, preferably giving the xylazine 5-10 min before the ketamine.

Ketamine has been used with medetomidine and butorphanol in ferrets in the same manner as so-called ‘triple anesthesia’ in cats with dosages of 5 mg / kg (ketamine), 0.08 mg / kg (medetomidine) and 0.4 mg / kg (butorphanol). Reversal is achieved with atipamezole at 1 mg / kg.

Lagomorphs

In rabbits ketamine has a very rapid renal excretion and so is short acting. It may be used at a dose of 20-35 mg / kg in conjunction with medetomidine at 0.3-0.5 mg / kg or with xylazine at 5 mg / kg using lower doses for debilitated animals. The advantages are a quick and stress-free anesthetic, but the combination will cause ‘blueing’ of the membranes and make detection of hypoxia difficult. True cyanosis also occurs. Respiratory depression during longer procedures may become a problem and intubation is often advised. If used, medetomidine may be reversed using atipamezole at 1 mg / kg.

It should be noted that multiple anesthetics using a combination of ketamine and xylazine have resulted in myocardial necrosis in rabbits.

A triple combination can also be used in rabbits with 0.2 mg / kg medetomidine, 10 mg / kg ketamine and 0.5 mg / kg butorphanol given subcutaneously which will provide approximately 20min anesthesia. Alternatively, the intravenous route may be used when a lower dose of the three drugs is required: 0.05 mg / kg medetomidine, 5 mg / kg ketamine and 0.5 mg / kg butorphanol. This gives a more rapid induction, but shorter duration of anesthesia, although it may be used as an induction combination prior to intubation and maintenance on gaseous anesthesia. Again reversal with atipamezole will hasten recovery.

Muridae / Cricetidae

Ketamine can be used at 90 mg / kg in combination with xylazine at 5 mg / kg intramuscularly or intraperitoneally in rats, with mice and hamsters requiring 100-150 mg / kg of ketamine and 5 mg / kg xylazine. These combinations provide 30min or so of anesthesia. In gerbils the dose of xylazine may be reduced to 2-3 mg / kg as they appear more sensitive to the hypotensive effects of the alpha-2 agonists drugs, with ketamine doses reduced to 50 mg / kg.

Ketamine may also be used in combination with medetomidine at doses of 0.5 mg / kg in these species. The advantages of the alpha-2 agonists are that they produce good analgesia (which ketamine does not) and that they may be quickly reversed with atipamezole at 1 mg / kg. Their disadvantages include their severe hypotensive effects, and that once administered any injectable anesthetic is always more difficult to control than a gaseous one. They also increase diuresis and may exacerbate renal and, of course, cardiovascular dysfunction.

Hystricomorphs

Ketamine at 20-40 mg / kg may be used in conjunction with xylazine at 3-5 mg / kg in guinea pigs and chinchillas to produce a light plane of anesthesia. Ketamine at 40mg / kg may also be used with medetomidine at 0.5mg / kg for guinea pigs, or ketamine at 30 mg / kg with medetomidine at 0.3 mg / kg for chinchillas. Reversal with 1 mg / kg atipamezole may be performed. Both of these may be improved after an acepromazine pre-anesthetic medication of 0.25 mg / kg.

Alternatively for chinchillas a ketamine (40 mg / kg) and acepromazine (0.5 mg / kg) combination can be used. Induction with these drugs takes 5-10 min and typically lasts for 45-60 min, but recovery may take 2-5 hours. Reducing the dose of acepromazine / ketamine and using a reversible alpha-2 agonist in addition may be beneficial, but should be weighed against the greater hypotensive effects of the alpha-2 agonist drugs.

This author prefers a combination of acepromazine, medetomidine and ketamine for minor procedures such as radiography, dental work and straightforward extractions in chinchillas. Acepromazine is given subcutaneously (0.1-0.2 mg / kg) 10-15 min before administering a combination of 1-5 mg / kg ketamine and 0.01-0.05 mg / kg medetomidine, using the lower end of the range for very debilitated patients.

Fentanyl / fluanisone (Hypnorm®)

This drug combination is a neuroleptanalgesic licensed for use in rats, mice, rabbits and guinea pigs in the UK. Fentanyl is an opioid derivative and fluanisone is a neuroleptic.

Lagomorphs

Fentanyl / fluanisone may be used as sedation only on its own at a dose of 0.5 ml / kg intramuscularly. This produces sedation and immobilisation for 30-60 min according to the data sheets, but its analgesic effect due to the opioid derivative fentanyl will persist for some time after. It may be reversed with 0.5mg / kg butorphanol intravenously or 0.05mg / kg buprenorphine, both of which will counteract the fentanyl and its analgesia and substitute their own pain relief.

Alternatively, to provide greater anesthetic depth, fentanyl / fluanisone may be combined with diazepam (0.3 ml Hypnorm® to 2mg / kg diazepam) and administered intraperitoneally or intravenously (but drawn up in separate syringes as they do not mix), or with midazolam (0.3 ml Hypnorm® to 2mg / kg midazolam) administered intramuscularly or intraperitoneally in the same syringe. Hypnorm® also may be given intramuscularly followed 15min later by midazolam intravenously into the lateral ear vein. These two combinations provide good analgesia and muscle relaxation with a duration of anesthesia of 20-40 min. Again fentanyl may be reversed with buprenorphine or butorphanol given intravenously, or in emergencies naloxone at 0.1 mg / kg intramuscularly or intravenously may be used, but this provides no substitute analgesia.

Fentanyl / fluanisone combinations are well tolerated in most rabbits, but they can produce respiratory depression and hypoxia, which can lead to cardiac arrhythmias and even arrest.

Muridae

Fentanyl / fluanisone may be used as sedation only on its own at a dose of 0.01ml / 30g body weight in mice and 0.4 ml / kg in rats. Again this produces sedation and immobilisation for 30-60 min and may be reversed with buprenorphine or butorphanol as above.

Alternatively it may be combined with diazepam (mice 0.01ml / 30g Hypnorm® with 5mg / kg diazepam intraperitoneally; rats 0.3 ml / kg Hypnorm® with 2.5 mg / kg diazepam intraperitoneally) where the diazepam and Hypnorm® are drawn up in separate syringes as they do not mix, or with midazolam. Midazolam is miscible with Hypnorm® and for rodents the recommendation is that each drug is mixed with an equal volume of sterile water first and then mixed together. Of this stock solution, mice receive 10ml / kg and rats 2.7ml / kg as a single intraperitoneal injection. These two combinations provide anesthesia for a period of 20-40 min.

Hystricomorphs

Hypnorm® may be used for sedation only on its own at a dose of 1 ml / kg intramuscularly. This may be problematic in guinea pigs as large volumes are required and Hypnorm® is an irritant and may cause lameness when the whole dose is placed in one spot: multiple sites are therefore preferred. Alternatively it may be combined as above with diazepam (1 ml / kg Hypnorm® and 2.5 mg / kg diazepam each drawn up in separate syringes) and administered intraperitoneally, or with midazolam by making the stock solution as described for Muridae, and then administering 8 ml / kg of this solution intraperitoneally. Hypnorm® may be reversed with the partial opioid agonists buprenorphine and butorphanol or with the full antagonist naloxone.

Volatile agents

Volatile agents have the advantage over injectable agents in that it is easier to alter the depth of the anesthetic quickly. The recovery times are often much shorter than with injectable anesthetics and frequently their side effects are less, particularly with isoflurane and sevoflurane. However, their disadvantages include the fact that there is a drying effect on the airways of the patient when using inhalational anesthetic agents that can cause dehydration during long procedures. They also create problems if used as an induction agent, as many species will breath-hold during this procedure.

Halothane

This has been used in all small mammals, however its margin of safety is less than that of isoflurane. Induction concentrations should not exceed 3% and anesthesia can be maintained with 1.5%. Disadvantages include possible cardiac arrhythmias, particularly in lagomorphs who are some of the main culprits of breath-holding. These may lead to apnoea and cardiac arrest. Use of halothane is best after pre-anesthetic medication with acepromazine rather than Hypnorm® as the latter agent requires extensive hepatic metabolism as does halothane.

Isoflurane

This is now becoming the most widespread volatile agent used for induction and maintenance of general anesthesia in small mammals as well as in dogs and cats. A pre-anesthetic medication incorporating an analgesic is usually administered as isoflurane has no post-anesthetic analgesic qualities and it is irritant to the mucous membranes of many animals. Inspired concentrations required for induction of anesthesia vary from 2.5-4%. Breath-holding still occurs, but the practice of supplying 100% oxygen to the patient for 2min prior to anesthetic administration helps minimise hypoxia. After this pre-oxygenation, gradually introduce the isoflurane, first 0.5% for 2min, then assuming regular breathing, increase to 1% for 2min and so on until anesthetic levels are reached allowing a smooth induction. Surgical anesthesia can usually be maintained at 1.5-2.5% assuming adequate analgesia; the MAC for this agent is 2.05% in rabbits. One of the main advantages of isoflurane is its improved safety profile in debilitated patient as <0.3% of the gas is metabolised hepatically, the rest merely being exhaled for recovery to occur. Recovery following use of this agent is rapid.

Sevoflurane

This is not currently licensed for use in small mammals in the UK unlike isoflurane, but its wide safety margins and the fact that it appears less noxious to small mammals and so induces less breath-holding when used as an induction agent, have made it an ideal anesthetic choice. Induction with sevoflurane still causes some breath-holding in rabbits and guinea pigs if used at 8%, therefore this author prefers to use an induction level of 4%. Inspired concentrations somewhere around 2-3% in 100% oxygen are required to maintain anesthesia: sevoflurane has an MAC of 3.7% in rabbits.

Aspects of maintaining gaseous anesthesia

As with all gaseous anesthetics, placing an endotracheal tube after induction of anesthesia is recommended whenever possible. This is relatively straightforward in rabbits using a number 1 Wisconsin flat bladed paediatric laryngoscope and a 2-3 mm tube. The aid of a rabbit mouth gag helps visibility, however the procedure becomes a specialised one for rodents such as rats and mice where rigid guide tubes / wires and smaller scopes are used to guide the tube into the larynx. In these species therefore, face masks are more commonly used. In rabbits and ferrets the use of lidocaine spray on the larynx helps to reduce laryngospasm and aids intubation. Ferret intubation is straightforward and resembles the procedure for a cat.

Intermittent positive pressure ventilation

This may be necessary in some individuals who breath-hold during induction. If intubation is not possible then three options are available.

(1) Ensure a tight-fitting face mask and have an Ayres-T piece / Mapleson C / modified Bain circuit with half litre bag attached which can be used to attempt ventilation.

(2) Place a nasopharyngeal tube through the medial meatus of the nose, into the pharyngeal area and insufflate oxygen (41 / min is required to combat the resistance of the small diameter tubing).

(3) Perform an emergency tracheotomy with a 25 / 27 gauge needle attached to the oxygen outlet.

Anaesthetic breathing systems

Most of the small mammals described here are <2kg in weight. For this reason an Ayres T-piece, a modified Bain or Mapleson C circuit are the best anesthetic systems to use to minimise dead space. For larger rabbits, an Ayres T-piece is usually sufficient.