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Getting started with fentanyl as a CRI

Updated: May 17, 2023

Fentanyl is a short acting full mu opioid agonist which is used as a continuous rate infusion as well as a bolus for rescue analgesia. This post introduces the use of fentanyl in dogs and cats.

For more detail, you can watch our CRI masterclass webinar here.

Fentanyl is suitable for both dogs and cats and is a useful option for reducing your volatile agent. (For more information on reducing volatile agent use click here). The rate of fentanyl to use depends on which stage of the peri-operative period we are talking about.

If I am planning to use fentanyl for a painful procedure I would use a full mu agonist (morphine or methadone) as premedication. My preference would be to use a locoregional technique, but sometimes this is not possible.

Once the patient is in theatre I would start a fentanyl infusion at 5-10mcg/kg/hr using a syringe driver. To achieve appropriate plasma levels we should give an initial bolus of 2mcg/kg. This can cause apnoea, which you should be aware of. This is not really a concern as you simply need to give an occasional breath to support ventilation. In many cases this apnoea won’t occur, but forewarned is forearmed.

Let’s say we start our CRI at 5mcg/kg/hr plus our initial bolus. This rate is suitable for dogs and cats. We would then monitor the patient for any signs of nociception (reacting to the surgical stimulus). If heart rate, respiratory rate or blood pressure increase above 20% of baseline then we need to provide further analgesia (this assumes that the patient was at a suitable plane of anaesthesia prior to skin incision). At this stage we can give another 1-2mcg/kg IV of fentanyl as a bolus.

The adverse effects we see with opioids are bradycardia and hypoventilation/apnoea.


With a bolus of fentanyl I would expect to see a slight bradycardia, but rarely anything to worry about. Ideally we monitor blood pressure to enable us to understand the effect of the bradycardia on the cardiac output (blood pressure being the closest we have to any cardiac output assessment). If bradycardia does affect blood pressure then an anti-cholinergic such as glycopyrrolate is used to increase heart rate.


Hypoventilation can range from a reduction in either respiratory rate or tidal volume right through to apnoea. After a bolus this apnoea is transient. If you have capnography you can monitor the efficacy of ventilation. At higher rates (10-20mcg/kg/hr) then the patient will probably need continuous mandatory ventilation (IPPV). If rates this high are required I would actually use another CRI alongside, such as ketamine.

Towards the end of surgery I would reduce the fentanyl rate to something that we can continue in recovery. If the pet is receiving a CRI of fentanyl we do not need to give any further methadone or morphine. In recovery I would use a range of 2-5mcg/kg/hr alongside pain scoring. If the pain score on the Glasgow pain scale is >6/24 or 5/20 then the fentanyl rate can be increased. With close pain scoring the fentanyl rate is adjusted over the 24 hr post-operative period to wean off the infusion.

Fentanyl is a useful analgesic for hospitalised patients. This post gives examples of drugs that are suitable for the management of pancreatitis, of which fentanyl is one.

Incorporating fentanyl into your pain management repertoire offers you a degree of flexibility. Fentanyl is suitable for intra-operative, post-operative use as well as in hospitalised patients.

Fentanyl is licensed in the UK for dogs. Although it is not licensed in cats, it is widely used in this species. A dose calculator is available on the Dechra Anaesthesia app which is free to download on the app store.

This post was written by Carl Bradbrook.

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 Vice-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.

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3 comentarios

Omaima Mohammed
Omaima Mohammed
02 feb

where is the references of information?

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Hanne Fiskå Valle
Hanne Fiskå Valle
19 feb 2020

Hi! If I use a premed of 0,3 mg/kg methadone + 0,005 mg/kg dexmedetomedin, would you lower the loading dose and cri dose? By how much? Thanks for a great post!

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Alexis Whiting
Alexis Whiting
17 oct 2022
Contestando a

Same question here!

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