RN Nursing · Medication Administration and Dosage Calculations · Practice question
The physician orders magnesium sulfate 500 mg/hr IV. The medication is supplied 4 g/250 mL. Calculate the flow rate in milliliters per hour. (Round to the nearest whole number. Use a leading zero if it applies. Do not use a trailing zero.) mL/hr
Answer & explanation
Correct:
To calculate the flow rate, first convert the ordered dose and the available concentration to compatible units. The order is magnesium sulfate 500 mg per hour, and the available concentration is 4 g per 250 mL. Convert 4 g to milligrams: 4 g × 1,000 mg/g = 4,000 mg. The concentration is therefore 4,000 mg per 250 mL. Determine how many milligrams are in each milliliter: 4,000 mg ÷ 250 mL = 16 mg/mL. Now calculate how many milliliters per hour are needed to deliver 500 mg/hr: 500 mg/hr ÷ 16 mg/mL = 31.25 mL/hr. Rounding to the nearest whole number gives 31 mL/hr. A common error would be failing to convert grams to milligrams before dividing, which would produce an answer 1,000 times too small, or inverting the concentration ratio. Another potential mistake is rounding 31.25 up to 32 rather than down to 31 as instructed by standard rounding rules for values less than 0.5. Magnesium sulfate is a high-alert medication used in obstetrics for seizure prophylaxis in preeclampsia and tocolysis in preterm labor; accurate dosage calculation is critically important because toxicity can cause respiratory depression, loss of deep tendon reflexes, and cardiac arrest. Serum magnesium levels and clinical monitoring are required during infusion.
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