October 2023

In this VETgirl blog, Tiffany Gendron, CVT, VTS (ECC) discusses why a constant rate infusion (CRI) is so important in veterinary medicine! If you hate calculating CRIs – or don’t know how to, you’ll want to keep reading to learn how. Make sure to tune back in a few weeks to check out Part 2 of Why & How to Calculate a CRI: Part 2 HERE also!

Don’t Let a CRI Make You Cry: Why & How to Calculate a CRI Part 1

by Tiffany Gendron, CVT, VTS (ECC)
VETgirl Veterinary CE Coordinator

Does the term “CRI” (constant-rate infusion or continuous rate infusion) immediately make you tachycardic or want to hide in the breakroom? Believe me, I’ve been there and done that. It shouldn’t!  Remember… you safely calculate and administer injectable agents every day, so, with practice, you can safely and confidently calculate and deliver a CRI too! Let’s get into the nitty gritty: a CRI or manually controlled infusion provides the veterinary team the ability to deliver a continuous rate of medication over a set period of time. 

If you’re thinking, “I don’t need to use CRI’s”, think again! Their use reaches well beyond analgesia and has become a mainstay of therapy in veterinary medicine. 

Benefits of a CRI Include:

  • Allows for precise delivery of medications at therapeutic levels
  • Decreases the prevalence dose-related side effects seen with higher intermittent dosing
  • Helps to eliminate the peaks and valleys of drug plasma associate with intermittent bolus dosing 
  • Provides optimal clinical effects

You Can Use a CRI in the Veterinary Clinic for the following reasons:

  • Electrolyte supplementation
  • Vasopressor support
  • Multimodal analgesia
  • Total intravenous anesthesia
  • Insulin therapy 
  • Nutritional support (via an enterally placed feeding tube)
  • And SO MUCH MORE!   

Before you jump on the CRI bandwagon, let’s chat about CRI considerations!

Administration considerations when delivering a CRI: 

  • Venous access
  • Compatibility of required solutions for infusion
  • Dilution requirements
  • Stability of solution
  • Patients’ fluid status
  • Underlying disease 
  • Organ function

multi-lumen catheter on surgical drape image for blog

Venous access requirements depend on several factors including maintenance fluids requirements, injectable medications the patient is receiving, duration of administration of these medications, drug compatibility, solution osmolality, and patient blood sample collection requirements. These factors often lead one to consider the use of multiple peripheral intravenous catheters, multi-lumen catheters, or peripherally inserted catheters (PIC).

A clear understanding of pharmacokinetics is essential. Pharmacokinetic properties include: absorption, distribution, metabolism, and excretion. This will vary from drug to drug, and is affected by organ dysfunction, body water, protein levels, diuresis (e.g., due to intravenous fluid therapy), etc. 

The half-life of a drug is defined as the time necessary for plasma or blood concentrations to decline by 50% (1). When a drug is administered as a CRI, therapeutic plasma concentrations are not achieved until three to five elimination half-lives have occurred (Boothe). This is why we will often administer a bolus before starting a CRI to increase plasma concentrations in a desirable time frame (e.g., lidocaine). It is important to remember that a patient may exhibit a clinical response before the steady-state drug concentrations are achieved. 

The nursing staff should be familiar with acceptable dose ranges and infusion rates of the drugs they are using. You also must consider equipment availability, stability of drug(s) post-preparation, and the half-life of the drug, all of which will influence dosing, formulation, and equipment selection.

In a few weeks in Part 2 of this blog, we will review how to specifically calculate a CRI with drug examples! Check back in a few weeks for Part 2 and bring a calculator!


  • Boothe DM,  Chapter 1 . In: Small Animal Clinical Pharmacology Therapeutics. 2nd ed. St.Louis, MO: Saunders/Elsevier: 2012:5-26.
  • Reves JG. “Smart Pump” technology reduces errors. Anesthesia Patient Safety Foundation 2003;18(1):825. Accessed October 2023 at https://www.apsf.org/article/smart-pump-technology-reduces-errors/
  • Baeckert M, Batliner M, Grass B, et al. Performance of modern syringe infusion pump assemblies at low infusion rates in the perioperative setting. Br J Anaesth 2020;124(2):173-182. doi: 10.1016/j.bja.2019.10.007. Epub 2019 Dec 19. PMID: 31864721.
  • Schmidt N, Saez C, Seri I, Maturana A. Impact of syringe size on the performance of infusion pumps at low flow rates. Pediatr Crit Care Med. 2010;11(2):282-6. doi: 10.1097/PCC.0b013e3181c31848. PMID: 19935442.
  • Neff, S., Neff, T., Gerber, S., & Weiss, M. Flow rate, syringe size and architecture are critical to start-up performance of syringe pumps. EJANEG 2007;400;(7),602-608. doi:10.1017/S0265021506002328.
  • Silverstein DC, Hopper K, Rosenstein PG, Hughes D. Chapter 143. In: Small Animal Critical Care Medicine. 2nd ed. St.Louis, MO: Saunders/Elsevier: 2009:763-765.
  • Plunkett SJ. Appendices. In: Emergency Procedures for the Small Animal Veterinarian. 3rd ed. St. Toronto: Saunders/Elsevier: 2013:828-830.
  • Martin EP, Mukherjee J, Sharp CR, Sinnott-Stutzman VB. Evaluation of the sterility of single-dose medications used in a multiple-dose fashion. Can Vet J. 2017;58(11):1187-1190.

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