Updated: Oct 14
After last week’s CRI masterclass I received some questions I thought worth sharing. Rule – if someone if asking it, someone else is thinking it and it’s worth sharing!
Why is fentanyl efficacy questionable in cats? I know this to be said and feel also myself it is often not a good choice and we don't use it often. Just to understand the area further.
Q1: In the Carrozzo study they looked at 5mcg/kg/hr fentanyl versus 3mcg/kg/hr. They did show that in the 5mcg/kg/hr group the thermal thresholds were increased - so were able to document an effect. To me this means if we use fentanyl in cats we should use 5mcg/kg/hr and should start with a loading dose of 5mcg/kg. They said the effect wore off rapidly after ending the infusion - but that's what we want from an infusion! No hangover. For me, ketamine is a more effective option.
How long may a methadone CRI be expected to last? We don't personally use methadone CRI but often we receive a referral that has been on methadone CRI.
Methadone CRIs were examined in an RSPCA clinic and reported in an abstract at BSAVA Congress in 2017. They showed that in dogs and cats the infusion at 0.1mg/kg/hr was effective, but that cats experienced more dysphoria than dogs. If the pain scores are low then you could observe these dogs and administer the next methadone when pain score increases - I would be thinking about 2hrs after stopping the infusion but that's just my experience as there are no studies looking at this specifically.
How long does a ketamine CRI last approximately? If we don’t want to continue ketamine CRI post op should we turn it off ~45 minutes before the patient recovers to reduce dysphoria?
You don't need to stop the ketamine infusion 45mins prior to recovery. If we see emergence delirium I don't always attribute that to the ketamine - rather the anaesthetic as a whole. Our practice is to decrease ketamine to a recovery rate (2-5mcg/kg/min) when closing and then move to recovery. If we see emergence delirium we treat that with 1mcg/kg dexmed IV.
Do you struggle with eye ulcers in ketamine use in cats/brachycephalics? A few surgeons wish not to use this in the brachy patients for this reason.
Opioids and ketamine both appear to reduce blink rate in cats so we lubricate their eyes - provided it does not stress them. Same for brachys. We usually run ketamine overnight in the spinal patients and stop next morning. If they are still painful we may restart.
How long approximately does ketamine take to produce effect as a CRI when not using a loading dose in conscious patients?
In those conscious cats that I start on ketamine I generally see a clinical improvement in 30-60 mins, without the need for a loading dose. In one study, the loading dose did cause sedation in those cats.
Do you administer individual loading doses for an MLK CRI?
The most common infusion combo for me is fentanyl and ketamine and I will use loading doses for both of those. With MLK you've probably given a methadone premed so you've already loaded there. You could then give 0.5mg/kg ketamine IV before starting surgery.
We love questions, as you can see. I hope these answers prove useful to some of you! If you are keen for more, then please catch up on the CRI masterclass webinar here.
These questions were answered by Matt Gurney.
Matt & Carl established Zero Pain Philosophy to provide educational resources to veterinary professionals enabling optimal management of pain.
Matt Gurney is an RCVS & European Specialist in Veterinary Anaesthesia & Analgesia and works at Anderson Moores Veterinary Specialists. Matt is President of the European College of Veterinary Anaesthesia & Analgesia.
Carl Bradbrook is an RCVS & European Specialist in Veterinary Anaesthesia & Analgesia and is President of the Association of Veterinary Anaesthetists. Carl works at Anderson Moores Veterinary Specialists.