It’s a relatively quiet night on your first shift as an emergency veterinarian. You’re seeing the normal rotation of skin, ears, and diarrhea cases, with just a few hospitalized patients to monitor through the night. One of those patients is Rex, a 20 kg MN mixed breed dog who presented to the day shift after being hit by a car.

The daytime veterinarian determined that Rex was in pretty good shape, overall, except for a tibial fracture, which she stabilized with a Robert-Jones bandage until a surgeon can repair the fracture (which will likely happen in one to two days). She started Rex on some pain medication, but mentioned that he may need a change in his pain management protocol as his sedation wears off. 

During your shift, Rex appears to become increasingly uncomfortable. You decide to administer a morphine-lidocaine-ketamine (MLK) constant rate infusion (CRI), because you’ve heard that this is a very effective pain control cocktail in dogs. You remember learning a shortcut for how much of each drug to add to a fluid bag for a MLK drip, but you don’t trust your memory. A quick Google search turns up multiple recipes, each of which is slightly different, so you decide to do your own calculations.

After some research, you find the following dosing information: 

Morphine: 0.1 to 0.4 mg/kg/hr

Lidocaine: 1 to 2 mg/kg/hr

Ketamine: 0.12 to 0.6 mg/kg/hr¹

Rex is eating and drinking well, so he doesn’t really need additional fluids. You decide that you will administer this MLK CRI at a rate of 1 ml/kg/hr, or 20 ml/hr. You want to start with a 500 ml bag of 0.9% NaCl for drug delivery, because you don’t know how long Rex will require this medication. 

What volume of each drug do you need to add to this 500 ml bag of 0.9% NaCl

Do the Math 

Step 1: Determine how long the fluid bag will last

Rex is going to receive fluids at a rate of 20 ml/hr and there are 500 ml of fluids in the bag. Divide 500 ml by 20 ml/hr to determine how many hours the fluid bag will last. 

(500 ml) / (20 ml/hr) = 25 hours

Step 2: Determine the quantity of morphine (in mls) that Rex will need to receive in 25 hours 

You elect to deliver a mid-range dose of each drug in this cocktail. That way, you will have room to increase or decrease your dose by adjusting Rex’s fluid rate, if necessary. 

First, ask yourself: how many mgs of morphine will you need to add to the fluid bag, in order to deliver a dose of 0.2 mg/kg/hr for 25 hours?

(0.2 mg/kg/hr) x (20 kg) = 4 mg/hr 

(4 mg/hr) x (25 hrs) = 100 mgs of morphine in the fluid bag 

Next, convert mgs of morphine to mls of morphine. Your practice has 5 mg/ml morphine. How many mls do you need to add to deliver 100 mgs of morphine?

(100 mg) / (5 mg/ml) = 20 ml morphine in the fluid bag 

Step 3: Determine the quantity of lidocaine (in mls) that Rex will need to receive in 25 hours 

How many mgs of lidocaine will you need to add to the fluid bag, in order to deliver a dose of 1.5 mg/kg/hr for 25 hours? 

(1.5 mg/kg/hr) x (20 kg) = 30 mg/hr 

(30 mg/hr) x (25 hrs) = 750 mgs of lidocaine in the fluid bag 

Your practice has 20 mg/ml lidocaine. How many mls do you need to add to deliver 750 mgs of lidocaine?

(750 mg) / (20 mg/ml) = 37.5 ml lidocaine in the fluid bag 

Step 4: Determine the quantity of ketamine (in mls) that Rex will need to receive in 25 hours  

How many mgs of ketamine will you need to add to the fluid bag, in order to deliver a dose of 0.3 mg/kg/hr for 25 hours? 

(0.3 mg/kg/hr) x (20 kg) = 6 mg/hr 

(6 mg/hr) x (25 hrs) = 150 mgs of ketamine in the fluid bag 

Your practice has 100 mg/ml ketamine. How many mls do you need to add to deliver 150 mgs of lidocaine?

(150 mg) / (100 mg/ml) = 1.5 ml ketamine in the fluid bag

Prepare the Fluid Bag

Based on your calculations, you have decided to add the following to Rex’s 500 ml bag of 0.9% NaCl: 20 ml of morphine (5 mg/ml), 37.5 ml of lidocaine (20 mg/ml), and 1.5 ml of ketamine (100 mg/ml). 

But wait! Before you do that, you need to remove an equivalent amount of 0.9% NaCl from the bag. This ensures that you will end up with a final volume of 500 ml of fluids/drugs in the bag. 

Add the drug volumes together to determine how much fluid to remove: 

(20 ml) + (37.5 ml) + (1.5 ml) = 59 ml 

You remove 59 ml of fluid from the 500 ml bag of 0.9% NaCl, then add the following to the bag: 

20 ml of morphine (5 mg/ml) 

37.5 ml of lidocaine (20 mg/ml) 

1.5 ml of ketamine (100 mg/ml) 

Administer the MLK CRI

Once you have added your drugs to the 500 ml bag of 0.9% NaCl, you can set up the fluid bag like any other fluid bag. Hook up an extension set and connect the fluid line to Rex’s IV catheter. Finally, set up a fluid pump to deliver this MLK CRI at a rate of 20 ml/hr and monitor his response to treatment. 

References

1. Lukasik, V. 2015. Constant Rate Infusions in Small Animal Anesthesia: A Guide to the Practitioner. World Small Animal Veterinary Association World Congress Proceedings.