RN Nursing · Medication Administration and Dosage Calculations · Practice question
The order is to infuse nitroglycerin at 3 mcg/min for a patient with chest pain; 30 mg of nitroglycerin has been added to 250 mL of 0.9% NS. The nurse needs to calculate the rate, in milliliters per hour, at which to set the IV pump. Round to nearest tenth.
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✓
1.5 ml/hr
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2.5 ml/hr
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2 ml/hr
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3 ml/hr
Answer & explanation
Correct: 1.5 ml/hr
To find the infusion rate in mL/hr, use dimensional analysis. The concentration is 30 mg nitroglycerin in 250 mL, and the ordered dose is 3 mcg/min. First convert the concentration to mcg/mL: 30 mg = 30,000 mcg; so 30,000 mcg ÷ 250 mL = 120 mcg/mL. Next, find mL/min: 3 mcg/min ÷ 120 mcg/mL = 0.025 mL/min. Convert to mL/hr by multiplying by 60 minutes: 0.025 × 60 = 1.5 mL/hr. This matches option A, confirming the keyed answer is correct. A common error is forgetting to convert mg to mcg before dividing, which would yield a rate 1,000 times too small. Another mistake is dividing the ordered dose by the total volume without accounting for the total drug, or failing to multiply by 60 to convert from per minute to per hour. Students should always confirm that units match at each step of the calculation — dose units in the numerator and concentration units in the denominator must cancel cleanly to yield a volume-per-time answer.
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