Time problems explanation

You will sometimes be asked to determine how long a fluid will take to infuse. If a question asks you, “How long,” your answer can only be given in the form of time. After all, if you said you were going to the grocery, and someone said, “How long will you be gone?” you wouldn’t answer, “Oh, about 125 ml/hr!”

Whether you initially determine your answer in hours or minutes will depend on what you have given in the problem.

Hours

How long will it take a liter of NS to infuse at 75 ml/hour? You have hours in the problem, so let’s find hours.

hr = 1 hr/75 ml x 1000 ml = 13.33 hours

What is this in hours and minutes? It is 13 hours plus 0.33 hour. One hour = 60 minutes, so it is 13 hours plus 0.33 hr x 60 min/hr = 20 minutes.

Be sure in calculating the minutes part of your hours that you take the decimal portion of the hour to two places to calculate the minutes, not just to one place.

Minutes

The doctor has ordered a 750 ml bolus dose of NS. Tubing size is 20 gtt/ml, and the drip rate is 30 gtt/minute. How long will it take to infuse? You have minutes in the problem, so let’s start there.

      min = 1 min/30 gtt x 20 gtt/ml x 750 ml = 500 min

Now turn this into hours and minutes. Eight hours = 480 minutes. 500 minutes - 480 minutes = 20 minutes, so the answer is 8 hours and 20 minutes. Alternatively, divide 60 into the 500 minutes = 8.33 hours, and calculate the minutes portion as described previously under hours.

You must always give your answers to time problems in the form of ___ hours and ___ minutes. Answers such as 453 minutes or 2.79 hours will be considered incorrect.

  Note regarding rounding here:

For minutes, the portion after the decimal is seconds, and we don’t care about seconds. Therefore, the decimal portion of minutes is used only to round to the nearest minute.

For hours, the portion after the decimal is minutes, and we do care about minutes. Therefore, take the decimal to a (rounded) two places and calculate how many minutes that portion of an hour is.

Some sample problems

1. A liter of IV fluid is to infuse at 24 gtt/min using 20 gtt/ml tubing. How long will it take to          empty the bag?

     min = 1 min/24 gtt x 20 gtt/ml x 1000 ml = 13.88 hr = 13 hr 53 min

2. You hang a liter bag of NS at 2330. It is infusing at 80 ml/hr. At what time will the bag be          empty?

hr = 1 hr/80 ml x 1000 ml = 12.5 hours, so the bag will be empty at 1200 (noon the next day)

3. The patient has 500 ml of D5NS running at 32 gtt/min using 10 gtt/ml tubing. The infusion      was started at 0830. At what time will it be complete?

min = 1 min/32 gtt x 10 gtt/ml x 500 ml = 156.3 = 156 min or 2 hr 36 min

The infusion will be complete at 1106.

4. 500 ml of NS is to infuse. The drip rate is 44 gtt/min and the drop factor is 15 gtt/ml. How        long will it take for this infusion to finish?

    min = 1 min/44 gtt x 15 gtt/ml x 500 ml = 170 min = 2 hr 50 min

5. The patient has 750 ml of D5W to infuse at 85 ml/hr. How long will it take?

     hr = 1 hr/85 ml x 750 ml = 8.82 hr = 8 hr 49 min