Constant rate infusions provide steady-state plasma drug concentrations without the peaks and troughs associated with intermittent bolus dosing. This results in more consistent analgesia, fewer side effects, and lower total drug requirements. CRI math intimidates many veterinary professionals, but a systematic approach makes it straightforward. Use the Drip Rate Calculator for precise delivery rates and the Drug Formulary for drug concentration references.
All CRI calculations ultimately derive from the same fundamental relationship. Understanding this formula eliminates the need to memorize separate equations for each drug.
The core formula:
Drug to add (mg) = [Desired dose (mg/kg/hr) × Body weight (kg) × Volume of fluid bag (mL)] ÷ Infusion rate (mL/hr)
This formula tells you how much drug to add to a fluid bag so that running the bag at a specified rate delivers the desired dose. Alternatively, when using a syringe pump, you can work backward from the drug concentration to determine the mL/hr rate needed.
For mcg/kg/min dosing (common for fentanyl, lidocaine, ketamine), convert units first:
mcg/kg/min × 60 = mcg/kg/hr ÷ 1000 = mg/kg/hr
Or use the combined formula: Drug to add (mg) = [Dose (mcg/kg/min) × BW (kg) × Volume (mL) × 60] ÷ [Rate (mL/hr) × 1000]
Fentanyl is a potent mu-opioid agonist commonly used as a CRI for perioperative and ICU analgesia. It is approximately 100 times more potent than morphine with a rapid onset (2-5 minutes IV) and short duration (20-30 minutes as a single bolus), making it ideal for CRI delivery.
Dose range: 2-5 mcg/kg/hr (dogs and cats)
Loading dose: 2-5 mcg/kg IV bolus over 1-2 minutes
Stock concentration: 50 mcg/mL (0.05 mg/mL)
Example: 20 kg dog, target 3 mcg/kg/hr fentanyl CRI in a 250 mL bag of LRS running at 10 mL/hr.
Drug to add = (3 mcg/kg/hr × 20 kg × 250 mL) ÷ (10 mL/hr × 1000) = 15,000 ÷ 10,000 = 1.5 mg = 30 mL of 50 mcg/mL fentanyl.
This is a large volume to add, which would change the total bag volume significantly. A better approach: use a syringe pump. Draw up 2 mL fentanyl (100 mcg) in a 10 mL syringe, dilute to 10 mL with saline. Concentration = 10 mcg/mL. Rate = (3 mcg/kg/hr × 20 kg) ÷ 10 mcg/mL = 6 mL/hr on syringe pump.
Fentanyl CRI should always be preceded by a loading dose to rapidly achieve therapeutic plasma concentrations. Without a loading dose, steady state takes approximately 4-5 half-lives (roughly 2-3 hours for fentanyl). Administer fentanyl loading dose slowly and monitor for respiratory depression and bradycardia.
Lidocaine provides multimodal analgesia through sodium channel blockade, reducing central sensitization and providing visceral analgesia. It also has anti-inflammatory, prokinetic, and antiarrhythmic properties, making it particularly useful in GDV patients and those with ventricular tachycardia.
Dose range (DOGS ONLY): 25-50 mcg/kg/min (1.5-3.0 mg/kg/hr)
Loading dose: 1-2 mg/kg IV bolus over 2-5 minutes
Stock concentration: 20 mg/mL (2% lidocaine)
Example: 25 kg dog, target 30 mcg/kg/min lidocaine via syringe pump.
Convert: 30 mcg/kg/min × 60 = 1,800 mcg/kg/hr = 1.8 mg/kg/hr
Total hourly dose: 1.8 mg/kg/hr × 25 kg = 45 mg/hr
Using 2% lidocaine (20 mg/mL): 45 mg/hr ÷ 20 mg/mL = 2.25 mL/hr on syringe pump
Warning: Lidocaine CRI is contraindicated in cats. Cats are extremely sensitive to lidocaine toxicity, with a very narrow therapeutic index. Signs of toxicity include muscle tremors, seizures, nausea, vomiting, and cardiovascular collapse. If systemic lidocaine is used in cats (which is controversial), doses must not exceed 10-15 mcg/kg/min with very close monitoring. Most feline analgesia protocols exclude lidocaine entirely.
Ketamine at sub-anesthetic doses acts as an NMDA receptor antagonist, blocking central sensitization ("wind-up") and providing analgesic synergy with opioids. At CRI analgesic doses, it does not produce dissociation or significant cardiovascular effects.
Dose range: 2-10 mcg/kg/min (0.12-0.6 mg/kg/hr)
Loading dose: 0.5 mg/kg IV (optional, over 1-2 minutes)
Stock concentration: 100 mg/mL
Example: 15 kg dog, target 5 mcg/kg/min ketamine via syringe pump.
Convert: 5 mcg/kg/min × 60 = 300 mcg/kg/hr = 0.3 mg/kg/hr
Total hourly dose: 0.3 mg/kg/hr × 15 kg = 4.5 mg/hr
Dilute 0.5 mL of 100 mg/mL ketamine into 9.5 mL saline = 5 mg/mL solution
Rate: 4.5 mg/hr ÷ 5 mg/mL = 0.9 mL/hr on syringe pump
The MLK combination is a widely used multimodal analgesic CRI that combines three drugs with different mechanisms of action for synergistic pain control. The combination allows lower doses of each individual drug, reducing side effects while providing superior analgesia compared to any single agent alone.
Standard MLK recipe for a 1-liter bag of LRS:
| Drug | Amount to Add | Stock Concentration | Delivery at 10 mL/kg/day |
|---|---|---|---|
| Morphine | 30 mg (2 mL of 15 mg/mL) | 15 mg/mL | 0.3 mg/kg/hr |
| Lidocaine (2%) | 1,500 mg (75 mL of 20 mg/mL) | 20 mg/mL | 50 mcg/kg/min |
| Ketamine | 300 mg (3 mL of 100 mg/mL) | 100 mg/mL | 10 mcg/kg/min |
This recipe is designed for a standard maintenance fluid rate of 10 mL/kg/day (approximately 0.42 mL/kg/hr). If the fluid rate needs to change for clinical reasons, the drug delivery rate changes proportionally. This is a key limitation of in-bag CRI preparations versus separate syringe pump delivery.
When independent rate adjustment is needed, use separate syringe pumps for each drug. This allows you to titrate each component independently based on pain score response without affecting fluid delivery rate. The Drip Rate Calculator can help convert between mL/hr and drops/min for gravity-fed setups.
Syringe pumps are the gold standard for CRI delivery because they provide precise, consistent flow rates independent of the maintenance fluid rate. They allow individual drug titration and are essential for concentrated drug solutions. However, they require specialized equipment and dedicated IV lines.
Gravity infusion (adding drugs to a fluid bag) is more practical in general practice settings. The advantage is simplicity: one bag, one line, one drip rate. The disadvantage is that drug delivery is linked to the fluid rate. If the patient needs more fluids (dehydration correction), drug delivery increases proportionally, and vice versa. Calculate the drug concentration in the bag and verify the delivered dose whenever the fluid rate changes.
Common CRI errors to avoid:
- Unit conversion mistakes (mcg vs mg, mcg/kg/min vs mg/kg/hr)
- Forgetting to account for drug volume added to the bag changing total bag volume
- Running the bag at a different rate than calculated without recalculating drug delivery
- Failing to give a loading dose, leading to prolonged sub-therapeutic levels
- Using lidocaine in cats
- CRI formula: Drug (mg) = [Dose (mg/kg/hr) × BW (kg) × Volume (mL)] ÷ Rate (mL/hr).
- Fentanyl CRI: 2-5 mcg/kg/hr with loading dose; use syringe pump for precision.
- Lidocaine CRI: 25-50 mcg/kg/min for dogs ONLY — it is toxic to cats even at low CRI doses.
- Ketamine CRI: 2-10 mcg/kg/min provides NMDA antagonism without dissociative effects.
- MLK combines three analgesic mechanisms; use separate syringe pumps when independent titration is needed.