Animals should be fasted for 12 hours, with water being withdrawn 1-2 hours beforehand. Water deprivation in old animals can precipitate a uraemic crisis so overnight deprivation is not recommended if there is evidence of renal incompetence and an i.v. fluid line should be established before the administration of a sedative or anaesthetic.
All patients should be weighed before computation of drug dosages to avoid errors. If the animal is obese the lean body weight should be estimated.
Acepromazine can be given in low doses (0.02-0.05 mg/kg i.m.) but should not be used in the presence of cardiovascular disease such as endocardiosis and congestive heart failure, as it can cause rapid hypotension. Acepromazine should also be avoided in dogs prone to seizures, and it is contraindicated in the presence of renal impairment. Intravenous administration can cause profound hypotension and it should only be used by this route with great care. The author has witnessed a geriatric dog collapsing and dying immediately following the intravenous administration of acepromazine.
Xylazine and medetomidine should only be used with extreme care in geriatric canine and feline patients because they can cause deep and prolonged sedation with bradycardia and severe cardiovascular depression. Vomiting often follows their administration and this could result in aspiration in old animals with impaired laryngeal reflexes. Caution is needed when using these substances in the presence of pulmonary disease.
In dogs neuroleptanalgesics have the advantage that the narcotic component can be reversed by an antagonist. However they have proeffects on the cardiovascular and respiratory systems causing tachycardia or bradycardia, hypertension or hypotension, depressed respiration and cyanosis. After reversal some dogs relapse into a sedated state that may last for up to 36 hours.
These agents are contraindicated in the presence of impaired hepatic or renal function, should only be used at reduced dose rates (50% that recommended for adults) and with great care in elderly patients. When using narcotic analgesics or neuroleptanalgesics dogs should be premeditated with anticholinergics to avoid secondary bradycardia.
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 ().
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.
Inhalation anaesthetics are preferred to intravenous drugs for maintenance anaesthesia in geriatric patients because most of the agent is excreted unchanged via the lungs and recovery is not dependent upon drug metabolism.
Intermittent positive pressure ventilation (ventilator or manual) has advantages during prolonged surgery in the elderly to facilitate the maintenance of blood gases within normal limits. It also saves the patient’s body ‘work’, and offers a method for hastening the removal of anaesthetic agent in the presence of an overdose.
All general anaesthetics cause respiratory depression, and some impairment of respiratory function is likely to be present in this group of patients so oxygen flow rate should be maintained at 30-33% of inspired gas.
The most commonly used inhalation agents in small animal practice (halothane and isoflurane) can rapidly cause severe cardiovascular and respiratory depression if high concentrations are given. For this reason they should only be used with vaporizers specifically designed to be used with them. Isoflurane is preferred to halothane in old patients, because it does not potentiate the cardiovascular effects of catecholamines, cardiac output is well maintained at anaesthetic doses and it does not alter myocardial contractility, however it does cause a fall in total peripheral resiswhich can precipitate hypotension.
Recovery is prolonged with methoxyflurane, and it is contraindicated in the presence of renal or hepatic impairment. Nitrous oxide does not alter blood pressure by itself and helps reduce the dose of other inhalation agents. It should be avoided in patients with severe respiratory problems preoperatively, or if hypoxaemia develops during surgery.
If muscle relaxation is needed during surgery administration of a muscle relaxant is preferred to increasing the depth of anaesthesia. Non-depolarising relaxants must be reversed by an anticholinesterase so atropine is given first to block the unwanted muscarinic effects (such as bradycardia) of the reversal agent.
Pancuronium is a medium-duration relaxant (lasting 30-45 minutes). It may cause tachycardia in cats and dogs and is contraindicated in the presence of hepatic or renal impairment and obesity. Vecuronium causes minimal cardiovascular effects and has a duration of action of about 30 minutes. Atracurium has a similar duration of action, also has minimal vagolytic or sympatholytic properties and it can be administered to animals with hepatic or renal failure.
Depolarising muscle relaxants are easier to use as they don’t need to be reversed. In dogs suxamethonium at a dose rate of 0.3 mg/kg given intravenously will provide complete paralysis for 20 minutes. Supplements of 0.1 mg/kg can be given to prolong its effects. It is contra-indicated in the presence of hepatic impairment. In cats 1.5 mg/kg produces paralysis for 5 minutes.
Intravenous fluids should be administered throughout anaesthesia at a rate of 10 mI/kg per hour. The rate should be reduced in animals with congestive heart failure or anuric renal failure, and monitoring central venous pressure is advisable. Urine output should be maintained at more than 0.3 ml/kg per hour in the dog.
The need for nutritional support of patients with catabolic disease is becoming increasingly important. Anorectic animals or those with recognised catabolic disease such as congestive heart failure or cancer, should be provided with adequate nutrition before elective surgery either by force feeding or tube feeding. Nasogastric tubes (Fr6) can be passed easily in conscious dogs and cats, and they offer a simple, effective way of providing nourishment to geriatric patients.
In some cases postoperative malnutrition can be predicted, and early placement of nasogastric (or naso-oesophageal), pharyngostomy, gastorstomy or jejunostomy tubes is advantageous. Diets for tube feeding should be high in energy density, provide adequate nitrogen in the form of protein, and be administered in a liquid formulation to facilitate passage down the tube.
Use of a heated pad, circulating hot water blanket or insulated blankets help minimise heat loss and prevent the development of hypothermia during anaesthesia. A fall in body temperature can greatly prolong the recovery period.
Throughout anaesthesia it has been recommended that the following parameters should be monitored ():
- (1) heart rate and rhythm
- (2) respiratory rate and character
- (3) pulse rate and quality
- (4) mucous membrane colour
- (5) capillary filling rate
- (6) ECG
- (7) rectal temperature.
In some cases the following should also be monitored:
- (1) arterial blood pressure (direct or indirect)
- (2) urinary output
- (3) haematocrit
- (4) total serum protein
- (5) blood gases.
During recovery the environmental temperature should be kept warm, and fluid therapy and oxygen administration should be continued until the animal is conscious.
Endotracheal tubes should only be removed once laryngeal reflexes have returned. Antiemetic drugs may be indicated in patients exhibiting nausea or retching.