Thursday, August 1, 2013

Three things I learned: veterinary anaesthesia

surprised cat
Did you realise that anaesethetic morbidity and mortality is slightly higher in cats?
Anaesthetist Dr Sanaa Zaki presented a webinar on “The Changing Face of Small Animal Anaesthesia” as part of the ASAVA’s 2013 webinar series.

Dr Zaki referred to recent studies of anaesthetic mortality in humans which ranged from 85 to 467 deaths per million in human patients (in developed and developing countries respectively, from 1990s-2000s) vs an average of less than 1 per cent in veterinary studies.

The vast majority of complications and mortalities (>50 per cent) occurred in the post-op period, within the first three hours of recovery from anaesthesia.
Not surprisingly, monitoring and pulse oximetry is associated with reduced risk of anaesthetic mortality.

Higher mortality is associated with pocket pets and cats (interestingly, those which were intubated and on IV fluids had a higher risk – but I wonder if this is because these were likely sicker cats in the first place?).

According to Dr Zaki, “There are no safe drugs, there are only safe anaesthetist. Any drug in unskilled hands can be quite dangerous – even those with a good safety margin”.

Given the higher risk of cats, much of the talk was focused on feline patients. So here's what I learned.
  1. Cats may have mydriasis for up to nine hours following administration of opioid analgesia. During this period they may have impaired vision and don’t appreciate bright lights. It also lasts longer than the analgesia provided. (On a slightly related note, methone CRIs definitely have a cumulative effect – a trend I’ve noted – and should be weaned down within 12-24 hours).
  2. In cats, light anaesthesia may be safer than heavy sedation. For very feisty cats Dr Zaki uses tiletamine/zolazepam at 2-3mg/kg + methone at 0.1-0.4mg/kg IM.
  3. I’ve not used sevofluorane before, but apparently it works MUCH quicker than isofluorane and its easy to overdose cats this way if one is not very careful.

At the end of the talk Dr Zaki provided an excellent review of analgesia. I like the way she talked about different types of pain, distinguishing physiological pain (in proportion to the degree of the stimulus and a warning sign to the body) from inflammatory pain (initiated by tissue damage, doesn’t serve a useful purpose) and neuropathic pain (due to specific injury to neuronal tissue and modulation of the pain pathway). There is a ye olde school belief which comes up at veterinary conferences intermittently which is that post-op pain is good because it forces the animal to rest. 

Most veterinarians know this is not the case, but Dr Zaki made a point of reminding everyone anyway - and I am sure her patients are very grateful.