Regular insulin CRI for a 33 lb DKA dog
A 33 lb dog presents in DKA with BG 480 mg/dL. You prepare a regular insulin CRI bag using the standard 2.2 U/kg method: add 2.2 U/kg to 250 mL of 0.9 % NaCl, then run the first 50 mL through the line to saturate the tubing (insulin binds to plastic). At what mL/hr should you start the infusion to deliver 0.05 U/kg/hr?
Hint
Convert weight first. Both the loading dose (2.2 U/kg) and the hourly dose (0.05 U/kg/hr) need kg. Then the 50 mL line prime is the trick: you've thrown away 50 mL but kept all the insulin.
Another hint
(1) lb → kg via 2.2 lb/kg. (2) Total U in bag = 2.2 U/kg × kg. (3) Effective concentration = total U ÷ remaining mL (200, not 250). (4) Target hourly dose ÷ effective concentration → mL/hr.
Show worked answer
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Convert the patient's weight from lb to kg.
$$\frac{33 \,\cancel{lb}}{2.2 \,\cancel{lb}/kg} = 15 \,kg$$ -
Total units added to the bag = 2.2 U/kg × weight.
$$2.2 \,\tfrac{U}{\cancel{kg}} \times 15 \,\cancel{kg} = 33 \,U$$ -
After priming 50 mL through the line, 200 mL of bag remains. The 33 U are now in 200 mL of effective volume.
$$\frac{33 \,U}{200 \,mL} = 0.165 \,\tfrac{U}{mL}$$ -
Target hourly dose: 0.05 U/kg/hr × 15 kg.
$$0.05 \,\tfrac{U}{\cancel{kg}\cdot hr} \times 15\,\cancel{kg} = 0.75 \,\tfrac{U}{hr}$$ -
Pump rate = hourly dose ÷ concentration. U cancels, leaves mL/hr.
$$\frac{0.75 \,\cancel{U}/hr}{0.165 \,\cancel{U}/mL} \approx 4.5 \,\tfrac{mL}{hr}$$ -
Why regular insulin? Only short-acting (regular) insulin is suitable for IV CRI in DKA. Its rapid onset and short half-life let you titrate against bedside BG checks. The longer-acting preparations (NPH, glargine, detemir) and the meal-time analogs (lispro, aspart) are designed for subcutaneous use; their kinetics make precise IV titration impossible.
Start the insulin CRI at ≈ 4.5 mL/hr.