NOAH Compendium of Animal Medicines: Isocare - Contra-indications, warnings, etc

Isocare

Contra-indications, warnings, etc

Contra-indications

Isoflurane should not be administered to animals with a known history of sensitivity to halogenated anaesthetics including symptoms of malignant hyperthermia.

Warnings

When using isoflurane to anaesthetise an animal with a head injury, consideration should be given as to whether artificial ventilation is appropriate to maintain normal CO2 levels, so that cerebral blood flow does not increase.

As with all halogenated anaesthetics, repeat anaesthesia within a short period of time should be approached cautiously.

Use of greater than 1.3-1.6 MAC should be avoided in cats.

Isoflurane produces respiratory depression in horses. It is recommended that horses, anaesthetised for longer than 2 hours, should be mechanically ventilated to prevent atelectasis, hypoxaemia and respiratory alkalosis.

Respiratory failure occurs at concentrations exceeding 2 MAC. Respiratory complications, especially in cats are often linked to inadequate consideration of reducing the MAC of isoflurane required.

Interactions

Interactions with other drugs should always be kept in mind before any anaesthesia.

The simultaneous administration of isoflurane and the following products requires strict supervision of the patient's clinical and biological condition.

•Muscle relaxing agents: Intensification of the action of depolarising relaxants and, especially, nondepolarising relaxants such as atracurium, pancuronium or vecuronium. Thus it is recommended that ±1/3 to 1/2 of the usual dose of these substances is administered. The disappearance of the myoneural effect takes longer with isoflurane than with other conventional anaesthetics. Neostigmine reverses the effect of the nondepolarising muscle relaxants, but does not reverse the direct neuromuscular depression of isoflurane.

•The concurrent use of sedative or analgesic drugs may reduce the level of isoflurane required to induce and maintain anaesthesia. For example, opiates, alpha-2-agonists, phenothiazines and benzodiazepines have been reported to reduce the MAC values. Particular caution should be exercised if administering dissociative combinations to a dog or cat already anaesthetised with isoflurane. Respiratory complications are often linked to inadequate consideration of reducing the MAC of isoflurane.

Isoflurane has a weaker sensitising action on the myocardium, to the effects of circulating dysrhythmogenic catecholamines, than halothane.

Isoflurane has been reported to interact with dry carbon dioxide absorbents to form carbon monoxide. In order to minimise the risk of this in rebreathing circuits, and the possibility of elevated carboxyhaemoglobin levels, absorbents should not be allowed to dry out.

Side-effects

Hypotension, respiratory depression with hypercapnia and arrhythmias have been reported. These can often be controlled by decreasing the level of anaesthesia.

In sensitive dogs, isoflurane anaesthesia can induce a hypermetabolic state in the skeletal muscles, which leads to a high oxygen consumption and a clinical syndrome known as malignant hyperthermia.

Operator warning

Do not breathe the vapour. The recommended occupational exposure standard (OES) on an 8 hour time weighted basis is 50 ppm.

Operating rooms must be provided with adequate ventilation and active extraction (scavenging equipment) to prevent the accumulation of isoflurane vapours. Induction and recovery areas should be well ventilated.

Make sure that the ventilation system in the operating room assures a number of air changes that is at least equal to 12 times the percentage of isoflurane in the anaesthetics multiplied by the applied flow rate (in litres per minute) and divided by the volume of the operating room (in m3).

Wash any splashes from skin and eyes immediately. It is recommended that pregnant workers should limit any exposure to isoflurane. In the event of severe accidental exposure remove the operator from the source of the exposure and seek urgent medical advice.

Advice to doctors: maintain a patent airway and give symptomatic and supportive treatment.

Anaesthetic and scavenging equipment should be checked frequently to ensure that it functions properly.

For animal treatment only.

Overdose

Overdosage with isoflurane produces marked hypotension and respiratory depression. In the event of an overdosage, or an adverse effect, stop drug administration, establish a clear airway and initiate assisted or controlled ventilation with pure oxygen. Administration of fluids may be beneficial.

From an environmental point of view, it is considered good practice to use charcoal filters with scavenging equipment. Do not dispose container or unused product into the environment. Care should be taken when dispensing isoflurane into the vaporiser. Spillage should be contained or removed immediately, using sawdust, sand or other inert absorbent, to a well ventilated place.