Updated: May 17
A common question for us is about using morphine lidocaine and ketamine as a combination infusion for analgesia.
Note - you can substitute morphine for methadone at exactly the same doses. Morphine: 60mg/500ml bag of fluid (i.e. 6ml of 10mg/ml morphine) lidocaine: 500mg/500ml bag of fluid (i.e. 25ml of 20mg/ml lidocaine) ketamine: 60mg/500ml bag of fluid (i.e. 0.6ml of 100mg/ml ketamine)
Use a 500ml bag of saline and remove 31ml before adding any of the analgesics.
Dose is 1ml/kg/hour.
Lidocaine should not be used in cats due to toxicity concerns so in cats only morphine/ketamine combinations should be used. Our preferred option for cats is methadone by injection every 4 hours (0.2-0.3mg/kg IV) plus a ketamine infusion.
Titrate the rate according to pain scoring - if the pet is comfortable, decrease by half and reassess one hour later. If the pet is painful, increase by half and reassess an hour later.
In one study from the RSPCA the most common adverse effect seen from a methadone infusion at 0.1mg/kg/hr was dysphoria - and mostly in cats. In these cases either reduce the rate or stop for an hour and then reassess.
The infusion can be used pre, intra and post operatively. It is perfect for a trauma case requiring stabilisation prior to surgery. We would use lower rates in conscious patients and increase the rate intra-operatively to 2ml/kg/hr. Ideally this will be used alongside local anaesthetic techniques. Don't forget that the CRI will have a marked volatile sparing effect and you should decrease your vaporiser setting accordingly. Reduce the rate towards the end of surgery. Sometimes you may see dysphoria in recovery with higher rates or cumulation. Low dose acepromazine or alpha 2 agonist can be used according to the patient to manage this.
For an in-depth understanding of using CRIs in your practice, you will find all of the information you need in our CRI masterclass.
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This post was written by Matt Gurney.
Matt & Carl established Zero Pain Philosophy to provide educational resources & telemedicine 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.