Anesthesia & Sedation

Ketamine CRI

Adjunctive analgesic CRI for moderate-to-severe pain in dogs and cats. NMDA antagonism reduces pain wind-up, making it useful perioperatively, for chronic pain states, and for post-operative wind-down. Also a component of the MLK multimodal CRI. Two CRI regimens are available: surgical maintenance and lower-dose postsurgical/general analgesia.

Stock: 100 mg/mL (vet) · Schedule III

Unit-confusion alert. Plumb's flags ketamine CRI dosing as a high-alert risk: "Care should be taken to not confuse CRI dosages (ie, mg/kg/HOUR vs µg/kg/MINUTE)." The two units differ by ≈60×. The calculator below shows both, verify both match your protocol before infusing.
Cardiovascular cautions. Ketamine increases HR, BP, and myocardial O₂ consumption via sympathetic stimulation. AVOID in cats with HCM, patients with significant hypertension, heart failure, arterial aneurysm, hyperthyroidism, or pheochromocytoma. Use caution in renal/hepatic insufficiency, seizure disorders, increased IOP, and pharyngeal/laryngeal/ tracheal procedures.
Recovery period. Eyes remain open, apply ophthalmic lubricant. Minimize handling and noise during recovery. Schedule III controlled substance, follow appropriate dispensing/recording protocols.

Two indication modes

Surgical maintenance, intraoperative analgesia. Range 10–20 µg/kg/min (= 0.6–1.2 mg/kg/hr). Default 10 µg/kg/min. Per Plumb's: precede with 0.5 mg/kg IV loading dose if anesthesia was induced with a non- ketamine agent.

Postsurgical / general analgesia, postop wind-down or stand-alone analgesia. Range 2–10 µg/kg/min (= 0.12–0.6 mg/kg/hr). Default 2 µg/kg/min for 24 hr. Plumb's "general analgesia" (0.1–0.6 mg/kg/hr loading 0.5 mg/kg) overlaps fully with this range.

Indication

Surgical (10–20 µg/kg/min) Postsurg / general (2–10)

Species

Dog Cat

Both species supported. Cat selection surfaces additional cautions (HCM avoidance, seizure rate, renal-excretion considerations).

Patient weight

lb kg

Maintenance dose

µg/kg/min mg/kg/hr

Plumb's gives doses in both conventions; switch the unit to match your protocol. Result panel always shows both for cross-check.

Awaiting input

Enter a patient weight to see the result.

Reference

How the calculation works

Ketamine stock is 100 mg/mL. The dose is entered in µg/kg/min or mg/kg/hr, both are equivalent and the calculator always shows both for cross-check. Converting mg/kg/hr to a pump rate:

$$\text{mL/hr} = \frac{\text{weight}_{\text{kg}} \times \text{dose}_{\text{mg/kg/hr}}}{\text{stock}_{\text{mg/mL}}}$$

To convert µg/kg/min to mg/kg/hr (divide by 1000, multiply by 60):

$$\text{mg/kg/hr} = \frac{\text{dose}_{\mu g/kg/min} \times 60}{1000}$$

Worked example with current inputs

Enter a patient weight to see the worked example.

Special populations & warnings

Cautions and populations

Cats

Plumb's documents several cat-specific concerns. AVOID in cats with HCM (ketamine raises HR/BP/MVO₂). Up to 20% of cats given ketamine alone may experience seizures at therapeutic doses, diazepam is suggested if treatment is needed. Self-limiting hyperthermia has been documented at 5–10 mg/kg; low-dose acepromazine (0.01–0.02 mg/kg IV) may help. Cats excrete ketamine almost exclusively unchanged in urine, consider dose reduction or alternative agents in renal dysfunction. Anecdotal reports of acute CHF in cats with mild-to-moderate heart disease.

Small patients (low pump rates)

The 100 mg/mL stock at typical analgesic CRI rates produces very low volumes, a 4 kg cat at 2 µg/kg/min postsurgical works out to about 0.005 mL/min, well below most pump accuracy thresholds. For small patients, dilute ketamine into a small fluid bag for accurate delivery. Plumb's: ketamine may be diluted with sterile water for injection, 5% dextrose, or 0.9% sodium chloride. The MLK protocol provides a useful ready-to-use dilution recipe (see /mlk).

Drug interactions

α₂ agonists (dexmedetomidine, medetomidine, xylazine): may reduce ketamine metabolism via CYP2B11 inhibition. Doses commonly reduced when co-administered.
Barbiturates, benzodiazepines, opioids, halothane: may prolong ketamine recovery. Lower doses of each agent often used in combination.
Thyroid hormones: may induce hypertension and tachycardia when combined with ketamine in humans.
Theophylline / aminophylline: may lower seizure threshold (in humans).
Compatibility: physically compatible in same syringe with xylazine, fentanyl, hydromorphone, lidocaine, midazolam, morphine. Do NOT mix with barbiturates or diazepam in same syringe, precipitation.

Overdose

Ketamine has a wide therapeutic index (≈5× pentobarbital). Overdose presents as significant respiratory depression, treat with mechanical ventilation rather than analeptics. There is no specific reversal agent. In cats, yohimbine with 4-aminopyridine has been suggested as a partial antagonist.

Sources

Primary: Plumb's Veterinary Drugs, ketamine monograph (current edition). Sections used: Prescriber Highlights, Uses/Indications, Pharmacology/ Actions, Pharmacokinetics, Contraindications/Precautions/ Warnings, Adverse Effects, Overdose/Acute Toxicity, Drug Interactions, Dosages (dogs and cats, adjunctive analgesia section), Compatibility/Compounding Considerations.
Secondary: Silverstein DC, Hopper K, eds. Small Animal Critical Care Medicine. 3rd ed. St. Louis, MO: Elsevier; 2023. Chapter 134 (Analgesia and Constant Rate Infusions), Table 134.1 (Commonly Used Analgesic Agents), p. 789.