Presenter – Dan Holden B.Vet.Med. DVA DipECVAA CertSAM MRCVS, The County VeterinaryClinic and independent veterinary anaesthesia consultancy service. Dan was involved in the establishment of the ICU at the RVC and was responsible for the University of Bristol’s internal medicine emergency service and small animal intensive care unit for 8 years.
According to my old boss ‘the safest option for inducing general anaesthesia is by using the induction agent you are most familiar with. I am certainly at my most comfortable clutching a syringe of propofol, secure in my knowledge and experience of this particular agent. So was my old boss right? Should I just stick to what I know or should I be reaching for something different, something new?
To find out I logged into last week’s veterinary webinar organised by ‘The Webinar Vet’ led by Dan Holden and, judging by his long list of qualifications and extensive experience in referral and first opinion practice, he is definitely the man to answer my question.
According to Dan there are four main players in the intravenous agent category: Propofol, alfaxalone, ketamine and etomidate. Of course I won’t discuss each of these drugs in great depth, you will have to log into the webinar to get this information, but I can fill you in on two of the most commonly used drugs, propofol and alfaxalone. Both produce a rapid smooth induction of anaesthesia. Propofol is rapidly metabolised by the liver in dogs, but in cats clearance from the liver is no different to thiopentone. For this reason propofol is cumulative in cats and prolonged propofol anaesthesia can cause oxidation and heinz body anaemia in this species.
Alfaxalone is rapidly metabolised by the liver in both the dog and cat and could be considered more appropriate for a cat if a short acting induction agent is required. On the other hand, alfaxalone can cause a sudden, rapid and excitable recovery with adequate premedication being necessary to minimise these side effects. So where a calm recovery is necessary, propofol could be considered the drug of choice.
Etomidate is an induction agent that vets are less likely to be familiar with but Dan suggests this, or perhaps a derivative of etomidate, may be a drug we encounter more of in the future. Etomidate is a true hypnotic and in the human field was used continuously in intensive care units. Its use was later found to increase death rates in patients and was consequently found to block adrenocorticol function. However it is unknown how significant this is in small animals especially after just one injection and as etomidate has almost no cardiovascular effects and is cerebroprotective, it may be a drug that proves very useful in high risk patients or those with cardiovascular instability.
There are many more ways to induce anaesthesia including the use of intramuscular and volatile agents. This is all covered by the veterinary webinar, but for the purpose of answering my question, I believe the differences between the properties of propofol and alfaxalone say it all. They are both very effective and useful induction agents and being familiar with either one of these agents has obviously got its advantages.
The key, according to Dan, with both these agents is to ensure the doses are kept down to a bare minimum. However there may be certain circumstances where one agent is more appropriate than another and a sound knowledge base behind each drug is crucial to ensure the safest method is used for those sometimes challenging cases.
The Stethoscope MRCVS