Build an MLK bag to last 24 hours
You're starting an MLK CRI on a 22 lb dog recovering from a splenectomy. You'd like to run the infusion at 1 mL/kg/hr and want enough drug in the bag to last 24 hours. Target doses: morphine 0.2 mg/kg/hr, lidocaine 1.6 mg/kg/hr, ketamine 0.4 mg/kg/hr (all within published ranges). Stock concentrations: morphine 5 mg/mL, lidocaine 20 mg/mL (2 %), ketamine 100 mg/mL. What pump rate (mL/hr) do you set, what bag size should you choose, and how much of each drug do you add?
Hint
Convert weight to kg first. Then the planned duration (24 hr) is what you multiply each per-hour quantity by: pump rate × duration tells you how much fluid you need (which sets the bag size), and dose × weight × duration tells you the mg of each drug.
Another hint
(1) lb → kg via 2.2 lb/kg. (2) Pump rate (mL/hr) = weight × per-kg rate. (3) Bag volume needed = pump rate × 24 hr; round up to the next standard bag size. (4) Each drug: total mg = dose × weight × 24 hr; stock volume = total mg ÷ stock concentration. (5) Remove the combined drug volume from the bag.
Show worked answer
-
Convert the patient's weight from lb to kg.
$$\frac{22 \,\cancel{lb}}{2.2 \,\cancel{lb}/kg} = 10 \,kg$$ -
Pump rate (mL/hr) = weight × per-kg rate. kg cancels.
$$10\,\cancel{kg} \times 1 \,\tfrac{mL}{\cancel{kg}\cdot hr} = 10 \,\tfrac{mL}{hr}$$ -
Bag volume needed for a 24-hour infusion = pump rate × duration. hr cancels, leaves mL.
$$10 \,\tfrac{mL}{\cancel{hr}} \times 24 \,\cancel{hr} = 240 \,mL$$ -
240 mL isn't a standard bag size. Round up to the next standard size (a 250 mL bag) so you have enough fluid for the full 24 hours. The drug amounts below are still calculated for 24 hr of infusion; the extra ~10 mL of carrier doesn't change the per-hour dose because that's set by the pump rate, not the bag size.
-
Total morphine over 24 hr = dose × weight × duration. Two cancellations: kg cancels kg, hr cancels hr, leaving mg.
$$0.2 \,\tfrac{mg}{\cancel{kg}\cdot\cancel{hr}} \times 10\,\cancel{kg} \times 24\,\cancel{hr} = 48 \,mg$$ -
Volume of morphine stock to draw = mg ÷ concentration.
$$\frac{50 \,\cancel{mg}}{5 \,\cancel{mg}/mL} = 10 \,mL$$ -
Lidocaine and ketamine follow the same pattern.
$$\text{Lidocaine: } 1.6 \times 10 \times 25 = 400 \,mg \;\to\; \tfrac{400}{20} = 20 \,mL$$ -
$$\text{Ketamine: } 0.4 \times 10 \times 25 = 100 \,mg \;\to\; \tfrac{100}{100} = 1 \,mL$$
-
Total drug volume = 10 + 20 + 1 = 31 mL. Remove 31 mL of saline from the 250 mL bag, then add the three drugs. The bag now holds 50 mg morphine, 400 mg lidocaine, and 100 mg ketamine in 250 mL (enough to last 25 hr at 10 mL/hr, comfortably more than the 24 hr of planned infusion). Pre-loading the bag for its full duration keeps the drug concentration exactly on target throughout the infusion.
-
Note: this protocol is dog-only. Cats are uniquely sensitive to lidocaine cardiotoxicity (myocardial depression, arrhythmias, and CNS effects can occur at the MLK lidocaine dose). For feline multimodal analgesia, consider a single-agent fentanyl, buprenorphine, or hydromorphone CRI instead.
Pump rate 10 mL/hr in a 250 mL bag (240 mL needed for 24 hr, rounded up; the bag lasts 25 hr at this rate). Add 50 mg morphine (10 mL), 400 mg lidocaine (20 mL), and 100 mg ketamine (1 mL) after removing 31 mL of saline. Loading the bag for its full duration keeps the drug concentration exactly on target.