The minimum dose is given rapidly iv as a bolus for induction, and then the animal usually wakes up as drug is redistributed away from the brain. However, subanaesthetic plasma levels will still be sedative and potentiate other anaesthetic drugs. Most anaesthetics are very lipid soluble (so that they enter the brain quickly) but this means that they redistribute to fat which then forms a depot and releases the drug back into the circulation.
Selecting the correct dose is most important: the dose required for induction depends on
The dose required is reduced by: premedication - especially α2 agonists, hypovolaemia, old age, debilitation, low plasma proteins, protein bound drugs - eg NSAIDs, anaemia, individual variation.
With rapidly metabolised drugs like propofol and alphaxalone, it is possible to maintain anaesthesia by continuously infusing the drug (often with a short acting opioid like alfentanil to supplement analgesia). This has become popular in human anaesthesia but requires an infusion pump which is too expensive for most veterinary practices. A much less accurate way is to mix all the ingredients in a drip bag and drip it in. This is sometimes used in horses with a mixture of xylazine, ketamine and guaifenesin (“triple drip”). Human anaesthetists who like playing with lots of kit have tried controlling infusion pumps by computer feed back systems for hands off anaesthesia with reasonable success. The way of the (distant) future?
