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Constant rate infusions — a veterinary nurse's guide

02 April 2014
10 mins read
Volume 5 · Issue 3
Figure 1. Graphical representation of plasma levels associated with intermittent bolus injections (inj) compared with a contant rate infusion (CRI).
Figure 1. Graphical representation of plasma levels associated with intermittent bolus injections (inj) compared with a contant rate infusion (CRI).


Constant rate infusions (CRIs) are ever more commonly used in veterinary anaesthesia and critical care. Veterinary nurses are often required to set up, administer and monitor infusions. Understanding the mechanics of CRIs, as well as the pharmacology of individual drugs is vital for a veterinary nurse working on such cases. Uses for CRIs during anaesthesia can broadly be divided in to three areas: partial intravenous anaesthesia (PIVA); total intravenous anaesthesia (TIVA); and second line treatment for hypotension. Drugs used in CRIs include opioids, sedatives, and catecholamines; commonly used drugs are discussed including benefits and contraindications.

Constant rate infusions (CRIs) have become more commonly used in veterinary practice over the past decade. A CRI refers to a drug being injected at a specified dose over a continuous period, usually by the intravenous route. Technically, where a drug is administered continuously but with changes being made to the dose, it is termed as a variable rate infusion (VRI), however the term CRI is usually applied to both situations. When CRIs are used to reduce inhalational anaesthetic agent requirement, it is known as partial intravenous anaesthesia (PIVA). Where a CRI is used alone to provide anaesthesia, it is termed total intravenous anaesthesia (TIVA) (Duke, 2013).

CRIs of many drugs provide balanced anaesthesia protocols as they provide varying amounts of analgesia, narcosis and muscle relaxation, allowing reduced reliance on inhalational agents. This reduced reliance on inhalation agents is beneficial as CRI agents generally cause less depression of cardio-respiratory systems than traditional inhalation agents, and using different agents in a multi-modal combination again reduces negative side effects. CRIs may also be continued into the recovery phase benefitting the patients with a smoother and more comfortable recovery. They also allow for careful titration of dosage and weaning (Duke, 2013).

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