Induction Induction

Inhalation can be used, but may induce excitation if the animal is not sedated and the associated catecholamine release can cause cardiac disturbances. Isoflurane has advantages over halothane because it is relatively insoluble in blood, causes rapid induction and does not potentiate the effects of adrenaline on the heart.

Some old animals, particularly those with catabolic diseases such as chronic heart failure, hyperthyroidism, sepsis and cancer may have a fall in body fat content with a concurrent decrease in body muscle mass and these changes can influence the distribution of i.v. anaesthetic agents increasing their efficacy and prolonging their duration of action. Reduced hepatic function may also prolong their duration of action and delay recovery. In general, intravenous anaesthetic doses should be reduced in elderly patients.

Older animals should be given only 50% of the dose of thiopentone sodium required by young adults, i.e. give 2-3 mg/kg i.v. over 10 seconds followed by small incremental doses given to effect over about 1 minute. A 2.5% solution is suitable for healthy, large dogs, but a 1.25% solution is recommended for cats, small dogs and debilitated cases. Give just sufficient to induce narcosis then mask/intubate. Premedication may be needed to reduce excitation during induction and recovery. Thiopentone is not good for anaesthetic maintenance because of tissue saturation and the prolonged recovery period that results. Concurrent administration of chloramphenicol, streptomycin or kanamycin can also prolong the recovery period.

Greyhounds and other dogs with little body fat may take 24 hours to recover from the effects of thiopentone, and methohexitone or propofol may be more appropriate. Methohexitone is shorter acting and the recovery period less than with thiopentone but it needs to be used with care as it can cause severe respiratory depression if administered too rapidly, and greater excitement or inadequate relaxation if administered too slowly.

Propofol has an action similar to thiopentone and it has the advantage that it can be used without premedication, but it does cause slightly greater cardiovascular depression. It has a less cumulative effect than thiopentone resulting in a more rapid recovery.

Pentobarbitone is rarely used in clinical practice nowadays. It is metabolised slowly and may cause profound respiratory depression. It is contraindicated in the presence of hepatic impairment and recovery time is prolonged with hypothermia being a postoperative complication.

Alphadolone and alphaxalone combination is commonly used to provide anaesthesia in cats. Some cats may develop respiratory embarrassment following rapid induction and the presence of underlying lung pathology may predispose to this. Slow administration of the anaesthetic is recommended to minimise the occurrence of such incidents.

Ketamine can be used as a sole anaesthetic in the cat, but in dogs it should only be used following premedication with xylazine. When used by itself muscle relaxation is poor, necessitating the administration of xylazine or a benzodiazepine such as diazepam. In cats vomiting often occurs during induction, and recovery from ketamine/xylazine anaesthesia can be prolonged for up to 8 hours in the presence of hypothermia. Xylazine also induces bradycardia and the administration of atropine immediately after the xylazine injection is recommended to counter this effect.

Induction with ketamine following premedication with acepromazine and subsequent maintenance with halothane or nitrous oxide alone or in combination has been recommended for cats with hyperthyroidism (Thoday 1990).

Respiratory depression is a consequence of all forms of anaesthesia and preoxygenation for 2-3 minutes before and during induction will help to prevent hypoxaemia.

The animal should be fully intubated following induction, to avoid excessive dead space.