Levels of anaesthesia can be altered easily and rapidly with Isoflurane. Hence it is recommended that only vaporizers which are reliable and produce a predictable concentration of Isoflurane should be employed. MAC (Minimum Alveolar Concentration) is 1.31% in adult horses, 1.28% in dogs, 1.68% in cats and 1.34% in birds.

The premedication regimen should be selected which is appropriate to the animal, its clinical condition and the procedure to be performed.

Induction of anaesthesia in the adult horse is usually achieved by the use of a barbiturate, ketamine and/or a muscle relaxant and all of these agents have been shown to be compatible with the administration of Isoflurane. In the dog and cat, anaesthesia is normally induced with a barbiturate or propofol and these agents have also been shown to be compatible with Isoflurane.

In the cat and in birds, ketamine may also be used for the induction of anaesthesia.

In view of the fact that Isoflurane is a relatively potent and rapidly acting anaesthetic agent, it is essential that the delivered concentration is known. Hence an accurately calibrated, temperature compensated and agent specific vaporiser should be used. However, it has on some occasions been satisfactorily administered from a simple in-circuit vaporiser.

Horses: Under normal conditions, anaesthesia would only be induced with Isoflurane in foals. This can be achieved using concentrations of 3 to 5 per cent Isoflurane with either oxygen or in nitrous oxide/oxygen (in a ratio of 2:1) as a carrier gas. This may be administered by nasal endotracheal tube, nasal insufflation or by mask. This technique will normally produce general anaesthesia within a period of 5 to 10 minutes when oral endotracheal intubation may be performed. Once anaesthesia has been induced by an intravenous agent in the mature horse, concentrations of 3 to 5 per cent of Isoflurane should be administered initially to ensure an adequate transition to Isoflurane anaesthesia.

Dogs and Cats: Anaesthesia can readily be achieved in the premedicated dog and cat with Isoflurane at concentrations of between 2 and 4 per cent. Either oxygen alone or nitrous oxide/oxygen in a ratio of 2:1 may be used as a carrier gas and administered by mask. When anaesthesia is induced by the intravenous route, concentrations of Isoflurane of up to 3 per cent should be administered to achieve a smooth transition to Isoflurane anaesthesia.

Birds : Anaesthesia can be induced in birds by the administration of Isoflurane in oxygen at delivered concentrations of between 2 and 4 per cent. This should be administered by mask which can be readily adapted from those used in the small domestic animal.

The concentrations of Isoflurane necessary to maintain anaesthesia in all species are lower than those required to induce anaesthesia. This is, obviously, related to the MAC value of the agent in a particular species. As a general guideline, an end-tidal concentration of 1.5 MAC will be adequate for the maintenance of anaesthesia under most conditions.

Horses : Delivered concentrations from the vaporiser of up to 3 per cent Isoflurane in oxygen are required to maintain surgical anaesthesia in the horse.

Dogs and Cats : Delivered concentrations from the vaporiser of up to 2.5 per cent Isoflurane in oxygen are required to maintain surgical anaesthesia in the dog and cat. This may be reduced to a figure of up to 2 per cent when nitrous oxide and oxygen are used as a carrier gas.

Birds : Delivered concentrations from the vaporiser of up to 2.5 per cent Isoflurane in oxygen are normally required to maintain anaesthesia in birds.

The level of arterial blood pressure during maintenance of anaesthesia is an inverse function of the administered Isoflurane concentration in the absence of other complicating factors. Excessive degrees of hypotension, unless they are specifically related to a reduced circulating blood volume, may be due to depth of anaesthesia and, in such a situation, may be corrected by reducing the delivered concentration of Isoflurane. However, it may sometimes be necessary in the horse, as with other inhalational anaesthetic agents, to administer an inotropic drug.

Recovery from Isoflurane is normally uneventful though it may be more reactive in the horse.