Maintenance Maintenance

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.