CRI Calculator

When to use a CRI versus a bolus

A constant rate infusion delivers a drug at a steady mass-per-time rate so plasma concentration plateaus near a target — useful when the therapeutic window is narrow and peak/trough swings from repeat bolusing would push the patient into adverse territory or back below efficacy. The clinical situations where this matters most: intra-operative analgesia (lidocaine, fentanyl, ketamine), MAC-sparing during inhalant anesthesia, refractory ventricular arrhythmias (lidocaine in dogs), and ongoing nausea (metoclopramide CRI in dogs unresponsive to bolus dosing).

Reach for an intermittent bolus instead when the desired effect is short-lived and re-dosing is acceptable (most pre-op opioids, single-shot ketamine for induction), when the drug has a long half-life that already produces steady plasma levels (most antibiotics — gentamicin SID is not a CRI candidate), or when you do not have pump capacity for a continuous line.

The math the calculator runs for you

The fundamental equation is the same one any anesthesia text spells out: mL/hr = (dose × weight × time-conversion) ÷ concentration. The trap is unit-handling: a published lidocaine dose of 50 mcg/kg/min needs to land at the pump as a mL/hr rate, and the conversion has to cross both micro→milligram and minute→hour boundaries. Done by hand at the table with a hypotensive patient on the table is exactly when decimal mistakes happen. The presets in this calculator carry the unit conventions for each drug; the math itself is unit-aware so a typo in the bag concentration produces an obviously-wrong mL/hr rather than a quietly-wrong one.

Worked example: lidocaine CRI in a 25-kg dog

You want analgesic-dose lidocaine at 50 mcg/kg/min intra-op. The pharmacy delivers a 250-mL bag of 0.9% saline containing 500 mg of lidocaine (concentration 2 mg/mL, equivalent to 2,000 mcg/mL).

• Target rate: 50 mcg/kg/min × 25 kg = 1,250 mcg/min.
• Per hour: 1,250 mcg/min × 60 = 75,000 mcg/hr = 75 mg/hr.
• At bag concentration 2 mg/mL: 75 mg/hr ÷ 2 mg/mL = 37.5 mL/hr.

Enter weight 25 kg, dose 50 mcg/kg/min, concentration 2 mg/mL and the calculator returns 37.5 mL/hr — the same answer, with the unit conversions auto-handled.

Common CRI drugs in small-animal and exotic practice

• Lidocaine (dogs, 25–80 mcg/kg/min): MAC-sparing and analgesia. Cats are far more sensitive — most references cap CRI at ~25 mcg/kg/min and many anesthesiologists avoid it altogether in cats.
• Ketamine (2–10 mcg/kg/min): sub-anesthetic CRI for NMDA-mediated wind-up pain. Often paired with lidocaine and an opioid (the "FLK" or "MLK" combo).
• Fentanyl (2–10 mcg/kg/hr): pure-mu intra-op analgesia. Watch for bradycardia and hypoventilation; pair with an anticholinergic when appropriate.
• Butorphanol (0.1–0.4 mg/kg/hr): mild-to-moderate visceral analgesia, often used in exotics where mu-agonists are sometimes avoided. Less respiratory depression than fentanyl.
• Dexmedetomidine (1–5 mcg/kg/hr): sedation and sympatholysis for ICU agitation or post-op restlessness. Watch cardiovascular effects closely.
• Metoclopramide (1–2 mg/kg/day, divided as CRI): antiemetic and prokinetic. CRI dose distinct from the intermittent SC/IV bolus rule on the drug page — both protocols appear in the formulary.

Each of these has its own ExoticRx drug page with the full dose ranges, source citations, and species-specific notes — the calculator handles the math, the drug page is where you verify the protocol.

CRI in exotic species — what changes

The math does not change. The pharmacology does. Most published CRI protocols come from dog and cat data; extrapolating to rabbits, ferrets, birds, and reptiles needs caution because metabolism, plasma protein binding, and adverse-effect profiles shift between species. Where the ExoticRx formulary has species-specific CRI rules (e.g. rabbit ketamine CRI for analgesia), each carries the primary-source citation. Where it does not, dose extrapolation is the clinician's call — and a written informed-consent note in the chart is good practice.

Frequently asked questions

Does this calculator handle bolus loading doses before the CRI?

No — this tool computes the steady-state infusion only. Loading doses are usually a single-bolus calculation off the drug page, then the CRI starts. A common protocol pairs a 1–2 mg/kg lidocaine bolus over 1–2 minutes with the maintenance CRI shown here.

What if I'm using a custom dilution not in the presets?

Use Custom drug and enter the final bag concentration (the mg/mL after you've added drug to the carrier fluid). The calculator does not care what the original vial concentration was — it only needs the concentration of what's actually going into the patient.

Why is the answer for cats so different from dogs?

The math is the same; the appropriate dose range is different. Cats are more sensitive to lidocaine, ketamine, and several other CRI drugs. The presets show the dose range, but the clinician chooses where in that range to dose. Reference the species-specific drug page before locking in a number.

Can I print the rate for the chart?

Yes — hit Cmd/Ctrl-P after entering the inputs. The current calculator inputs and result print cleanly without the navigation and clinical notes. Several practices keep a paper CRI worksheet in the anesthesia binder; print this page and it works the same.

Does the calculator work offline?

The calculator runs entirely in the browser — once the page has loaded, the math works without an internet connection. Useful for ambulatory practices and exotic field work where connectivity is unreliable.

The presets reflect common protocol ranges; clinical doses vary by patient, indication, and species. Always verify against current literature and the source citation on the relevant drug page. Reference only — not veterinary advice. See the full disclaimer.