Constant rate infusions (CRIs) are essential tools in veterinary anesthesia, allowing for steady drug delivery and consistent therapeutic effects. This reference guide covers the most commonly used CRI drugs, their available concentrations, and dosing guidelines.
Inotropes and Vasopressors #
Dopamine #
Available Concentration:
- 40 mg/mL (400 mg in 10 mL vials)
Dosing Guidelines:
- Low dose (renal): 1-3 μg/kg/min
- Medium dose (inotropic): 3-7 μg/kg/min
- High dose (vasopressor): 7-10 μg/kg/min
- Calculate mL/hr = dose (μg/kg/min) × weight (kg) × 0.06 / concentration (mg/mL)
- Must dilute
- extravascular administration can cause tissue necrosis
Dobutamine #
Available Concentration:
- 12.5 mg/mL (250 mg in 20 mL vials)
Dosing Guidelines:
- 2-20 μg/kg/min
- Titrate to effect, starting at lower end
- Calculate mL/hr = dose (μg/kg/min) × weight (kg) × 0.06 / concentration (mg/mL)
Clinical Notes:
- Primarily β₁-adrenergic effects (positive inotrope)
- Minimal chronotropic and vasopressor effects at lower therapeutic doses
- Preferred over dopamine for cardiac support with minimal afterload effects
- Must dilute
Norepinephrine #
Available Concentration:
- 1 mg/mL (4 mg in 4 mL ampules)
Dosing Guidelines:
- 0.05-1.0 μg/kg/min
- Start at lower dose and titrate to effect
- Calculate mL/hr = dose (μg/kg/min) × weight (kg) × 3.6 / concentration (μg/mL)
Clinical Notes:
- Potent α-adrenergic and moderate β-adrenergic effects
- Significant vasoconstriction so use lowest effective dose to minimize risk of decreased tissue perfusion
- Drug of choice for vasodilatory shock
- Must be administered via central line or large peripheral vein
- Must dilute
- Extravascular administration can cause tissue necrosis
Anesthetic and Analgesic Agents #
Fentanyl #
Available Concentration:
- 50 μg/mL (100 μg/2 mL ampules)
Dosing Guidelines:
- Loading dose: 2-5 μg/kg IV
- CRI: 3-10 μg/kg/hr (0.05-0.16 μg/kg/min)
- Higher doses (10-20 μg/kg/hr) for major surgical procedures
Clinical Notes:
- Short duration of action necessitates CRI for sustained effect
- Significant respiratory depression at higher doses
- May reduce inhalant requirements by 25-75% (MAC sparing effect)
- Can be vagomimetic so heart rate management may be needed
Lidocaine #
Available Concentration:
- 20 mg/mL (2%) (50 mL vials/ampules)
Dosing Guidelines:
- Loading dose: 1-2 mg/kg IV slowly (over 5-10 minutes)
- CRI for dogs: 25-80 μg/kg/min (1.5-4.8 mg/kg/hr)
- Cats: CAUTION – use reduced doses (10-20 μg/kg/min) or avoid completely
Clinical Notes:
- Antiarrhythmic, analgesic, anti-inflammatory properties
- MAC-sparing effect (reduces inhalant requirements by 15-25%)
- Contraindicated in cats with hepatic dysfunction
- Toxicity signs: muscle tremors, seizures, cardiovascular depression, gi upset
Ketamine #
Available Concentration:
- 100 mg/mL (10 mL vials)
Dosing Guidelines:
- Loading dose: 0.5-1 mg/kg IV
- Subanesthetic CRI:
- Dogs: 2-10 μg/kg/min (0.12-0.6 mg/kg/hr)
- Cats: 1-5 μg/kg/min (0.06-0.3 mg/kg/hr)
- Higher doses for dissociative anesthesia: 10-20 μg/kg/min
Clinical Notes:
- NMDA antagonist providing analgesia at subanesthetic doses
- MAC-sparing effect (reduces inhalant requirements by 15-25%)
- Preserves cardiovascular function at analgesic doses
- May increase sympathetic tone and heart rate
- Can be used alongside opioids for multimodal analgesia
Propofol #
Available Concentration:
- 10 mg/mL (20 mL vials)
Dosing Guidelines:
- Induction: 2-6 mg/kg IV (titrated to effect)
- CRI: 0.25-0.5 mg/kg/min (6-24 mg/kg/hr)
- Sub-anesthetic CRI: 0.05-0.1 mg/kg/min
Clinical Notes:
- Rapid redistribution and clearance
- Minimal accumulation with short-term use (<6 hrs)
- Prolonged use may result in delayed recovery, particularly in cats
- Heinz body formation with prolonged use in cats
- Must be used within 6 hours once vial is opened (no preservatives)
Alfaxalone (Alfaxan®) #
Available Concentration:
- 10 mg/mL (10 mL vials)
Dosing Guidelines:
- Induction: 1-2 mg/kg IV (dogs), 2-5 mg/kg IV (cats)
- CRI: 0.07-0.1 mg/kg/min (4-6 mg/kg/hr)
- Sub-anesthetic CRI: 0.025-0.05 mg/kg/min
Clinical Notes:
- Wider safety margin than propofol
- Less respiratory depression compared to propofol
- Minimal cardiovascular effects at clinical doses
- Preservative-free but contains cyclodextrin (longer shelf-life after opening)
- Recovery is less smooth than from propofol
Practical Tips for CRI Administration #
- Always double-check calculations
- Use dedicated infusion pumps when possible
- Label all infusions clearly with drug name, concentration, and rate
- Monitor patient response closely and adjust rates as needed
- Consider premixed commercially available preparations for drugs frequently used
Remember that individual patient factors including species, breed, age, and concurrent disease states may necessitate adjustments to these guidelines. Always titrate to effect while closely monitoring vital parameters.
Calculation Formulas: #
- Standard calculation: mL/hr = dose (μg/kg/min) × weight (kg) × 0.06 / concentration (mg/mL)
- When concentration is in μg/mL: mL/hr = dose (μg/kg/min) × weight (kg) × 3.6 / concentration (μg/mL)
- For μg/kg/hr dosing: mL/hr = dose (μg/kg/hr) × weight (kg) / concentration (μg/mL) × 1/60
