An order is received for heparin 18 units per kg per hour on a patient whose weight is 125lb. The IV bag comes as a concentration of 50 units /mL. Calculate the infusion rate in terms of mL/hr.

The calculation requires a three-step process. First, convert the patient's weight from pounds (lb) to kilograms (kg) by dividing by the conversion factor of 2.2 lb/kg. Second, calculate the total hourly dose in units by multiplying the patient's weight in kg by the prescribed dose rate. Finally, determine the infusion rate in mL/hr by dividing the total hourly dose (units/hr) by the available concentration (units/mL).
1. Weight in kg: 125 lb / 2.2 lb/kg = 56.82 kg
2. Total dose: 56.82 kg 18 units/kg/hr = 1022.76 units/hr
3. Infusion rate: 1022.76 units/hr / 50 units/mL = 20.455 mL/hr, which rounds to 20.45 mL/hr.

A. 5.15 mls/hr: This result likely stems from a significant miscalculation, possibly involving incorrect conversion factors or misplaced decimals during the calculation steps.
B. 10.23 mls/hr: This is approximately half the correct rate, suggesting a mathematical error such as an unnecessary division by two.
C. 40.9 mls/hr: This is double the correct rate, indicating a potential multiplication error where division was required or an error in the initial weight conversion.
E. 18 mls/hr: This answer incorrectly uses the dose rate (18) as the final infusion rate, failing to account for the patient's weight and the drug concentration.

1. Stoklosa, M. J., & Ansel, H. C. (2013). Pharmaceutical Calculations (14th ed.). Lippincott Williams & Wilkins. Chapter 11, "Isotonic and Buffer Solutions," and Chapter 13, "Intravenous Admixtures and Flow Rates," cover the principles of dilution, concentration, and rate calculations. The methodology for weight-based dosing is a fundamental concept discussed.

2. University of Kentucky College of Pharmacy. (n.d.). Dosage Calculations Review. This resource provides practice problems and foundational principles for pharmaceutical calculations, including weight-based dosing and IV infusion rates. The core steps of converting weight, calculating total dose, and determining the final rate are standard procedures taught. (Available via university pharmacy program websites).

3. University of Washington School of Pharmacy. (2018). Pharmaceutical Calculations Workbook. Section on "Calculations for Intravenous Admixtures," pp. 45-52. This workbook details the step-by-step process for calculating infusion rates for weight-based medications like heparin, emphasizing the dimensional analysis used in the explanation.