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.