Vetrica Policies – Anaesthetic Induction Agents

This picture shows a bottle containing a drug called propofol. We use propofol to induce anaesthetic in our patients¹.

The first part of anaesthesia, when the animal is being put under anaesthetic, is called the induction phase. Anaesthetic induction in dogs and cats is generally achieved by giving an injection of some sort. It’s possible to use a face mask and simply use the anaesthetic gases that we use for anaesthetic maintenance, but it is generally much quicker, and nicer for the animal to give an injection which rapidly causes unconsciousness, and follow this up with gas.

Traditionally anaesthesia has been induced with a barbiturate called thiopentone. While very effective and safe, especially for healthy animals, there are a number of problems associated with the use of thiopentone.

The main problem is that a patient recovers from thiopentone not by it being removed from the body, but by it being re-distributed from the nervous system (i.e. brain), to body fat. Only after this does the drug become gradually deactivated and excreted.

In a normal healthy animal this does not matter. In very young or old, sick or lean animals, (typically greyhounds, whippets and their crosses), there may not be very much body fat for the drug to become re-distributed to. This means the main way that the patient recovers from the drug is by deactivation which takes a long time. Recovery in these patients can be very prolonged.

Also, as the drug leaks back out of the body fat following recovery, the drug can have a further sedating effect. We call this the “hangover” effect, and in some patients this can be very marked. Affected animals are typically restless and stagger about. Injury or damage to surgical wounds are possible.

Thiopentone itself is very irritant. If the injection goes around rather than directly into a vein, as can happen if the patient jumps at the moment of injection, the irritation caused can be very severe, and the tissues surrounding the affected area (usually the right fore-leg for right handed vets) will die leaving a large ulcer. This is called a “slough”, and can take several weeks to heal.

Propofol does not suffer from any of these problems. Following injection, the patient rapidly loses consciousness. This gives us enough time to place a tube into the throat so we can deliver the anaesthetic gases. The drug is rapidly removed from the patient’s system by deactivation in the liver and lungs. Once the patient has recovered from propofol there is no possibility of a hangover effect because it has been completely deactivated. There is no problem with recovering from propofol by thin animals. Propofol is not irritant when injected around a vein, eliminating the possibility of a slough occurring.

Because, the drug is so rapidly eliminated, the risk of overdose is much less than with thiopentone. At the end of all but the shortest anaesthetics, virtually all of the propofol has been cleared from the body, which is certainly not the case with barbiturates. Propofol is an altogether safer, more pleasant way to be anaesthetised, giving a faster, more complete recovery from anaesthetic.

The price of propofol has come down recently, but is still almost double the cost of thiopentone². Traditionally, propofol has been reserved for the very old or young, sick or thin animals. At Vetrica, we believe all animals should benefit from the improved safety of propofol. We do not use barbiturates to induce anaesthesia.

Vetrica only uses isoflurane as a volatile anaesthetic agent…

Note: Not all patients are induced with propofol. Occasionally we use no injectable induction agents, especially for small mammals such as rodents, guinea pigs and rabbits.

Some cats and other small mammals, but not dogs, receive another drug called ketamine which does not have to be injected directly into a vein. This makes it particularly useful for very fractious cats which are difficult to safely restrain for an intravenous injection. Cats undergoing spay or castration procedures are also routinely anaesthetised with ketamine unless you ask us to do otherwise. (There is an additional fee for this).

All other factors being equal, anaesthetic techniques utilising ketamine are probably less safe than as is described above, which is why we restrict its use to these specific conditions.