Flow rate/drip rate problems

DEFINITIONS

Flow rate = infusion rate = hourly rate. These are interchangeable terms, and all of them indicate a pump is in use. The pump can “read” only ml/hr, so if any of these terms are used to describe what you are looking for, your answer must be in ml/hr. Example: “At what rate will this medication infuse?” You are looking for an infusion rate, so your answer will be in ml/hr. (Note: this is ml per hour. ml/0.5 hour, ml/0.75 hr, ml/1.5 hr, ml/15 minutes are not flow rates.)

Drip rate – this is another way of determining how fast a medication is going in (infusing). If asked for a drip rate, your answer should always be in gtt/min. (Gtt is drops.) The answer must always be a whole number, as there is no way to accurately measure a partial drop.

Drop factor is simply the size of the tubing. It is literally the number of drops it takes to make up one ml, using that particular tubing. It is therefore always given as gtt/ml. Watch your step – that’s drops per ml, not per minute. Don’t confuse this with drip rate. In a problem, this may be described as a calibrated set.

A KEY FORMULA

DRIP RATE = DROP FACTOR X FLOW RATE

This would be written so: gtt/min = gtt/ml x ml/hr x 1 hr/60 min or, combining the 2nd and 3rd factor, as gtt/min = gtt/ml x ml/60 min A FURTHER LOOK AT TUBING SIZE (drop factor)

There are two types of tubing: macro and micro.

MICRO tubing is used primarily with pediatric and geriatric patients. It is used when the flow rate (ml/hr) is less than 100. (There are two exceptions to this: if you are giving blood, or if you are hanging a viscous (thick and sticky) fluid, you must use macro tubing, regardless of the flow rate.) Micro tubing comes in only one size: 60 gtt per ml.

Please note: Many facilities will choose to buy only one size tubing to save money, and this will be macro tubing, regardless of flow rate or patient size.

Therefore, if you are doing drip rate problems in the clinical setting, you can and must use only the drop factor of the tubing available at your facility.

Back to our tubing:

MACRO tubing is used only when the flow rate (ml/hr) is 100 or more. (Many hospitals have only macro tubing, so you may have to use it even though the medication and rate really calls for micro tubing.) Macro tubing comes in three sizes: 10, 15, and 20 gtt per ml. In solving problems on the test or in clinical, sometimes the drop factor will not be given/known, so you must decide if macro or micro tubing should be used (based on flow rate). If it is macro, you can use any of the three sizes of tubing, and (if your math is correct) your answer will be correct. Remember: If you are in the clinical setting, you must use the tubing size available at that facility in doing your calculations.

Look back at macro and micro tubing again for a moment. Note that it is the flow rate that determines the tubing size, NOT the volume to be infused. When you are trying to determine what tubing size to use in a given problem, it is important to remember that.

ANOTHER CONSIDERATION

If a problem involves heparin or insulin, something that comes in units, be very careful to check what response the question is asking for in your answer, because it may be units or ml. If a question is asking about dosage or dosage per hour, the answer must be in the form (ml, mg, units, etc.) the medicine comes in. Thus, if a question about insulin asks for hourly dosage, your answer would be in units/hr.

Some examples of flow rate/drip rate problems

1. Your patient has D5.45NS infusing at 150 ml/hr. What is the drip rate?

     Drip rate = drop factor x flow rate. 150 ml/hr is macro, so you can use 10,15, or 20 gtt/ml for the drop factor. We’ll use 10.

     gtt/min = 10 gtt/ml x 150 ml/60 min = 25 gtt/min

Note that all answers for our purposes for macro problems are based on 10 gtt/ml, unless tubing size is given in the problem. When a problem is macro and does not specify a tubing size, please use 10 gtt/ml.)

       2. The order is for 1 liter of NS to infuse in 6 hours. What flow rate would you set on the             pump?

        ml/hr = 1000 ml/6 hours = 166.7 ml/hr

In the next problem, another formula pertains: FR = concentration x doctor’s order. formula is true only when the doctor’s order is given in something other than ml/hr, such as mg/hr, mEq/hr, or,This  as in this problem, units/hr.

 3. A heparin drip is ordered to infuse at 900 units/hr. The IV solution is 25,000 unit's                     heparins in 250 ml D5W. What is the infusion rate?

         Infusion rate = flow rate = concentration x doctor’s order

         ml/hr = 250 ml/25,000 units x 900 units/hr = 9 ml/hr

4. The doctor has ordered 1500 ml of NS to be infused over the next 12 hours.                              Drip rate? gtt/min = 10 gtt/ml x 1500 ml/12 hours x 1 hr/60 min = 20.8 = 21 gtt/min

5. What is the infusion rate for NS infusing at 43 gtt/min using 15 gtt/ml tubing?

      ml/hr = 1 ml/15 gtt x 43 gtt/min x 60 min/1 hr = 2580/15 = 172 ml/hr

6. What is the drip rate for heparin 10,000 units/100 ml infusing at 8.4 ml/hr?

    gtt/min = 60 gtt/ml x 8.4 ml/60 min = 8.4 = 8 gtt/min

7. The medication is available in a 5 ml vial. You add it to a 50 ml vial and infuse it over 30          minutes. What is the hourly rate?

    ml/hr = 55 ml/30 min x 60 min/hr = 110 ml/hr

8. You are to infuse 150 ml of a medication prior to a patient’s test at 1300. It is now 1030. At      what rate will you set the pump?

     ml/hr = 150 ml/2.5 hours = 60 ml/hr

9. Calculate the drip rate for 150 ml infusing in 45 minutes using 15 gtt/ml tubing.

     gtt/min = 15 gtt/ml x 150 ml/45 min = 2250/45 = 50 gtt/min

10. You hang a liter bag of IV fluids at 0200 at 40 gtt/min using 10 gtt/ml

      tubing. How much fluid will your patient receive by the end of your shift at 0700?

      ml = 1 ml/10 gtt x 40 gtt/min x 60 min/hr x 5 hours = 1200 ml

This introduces you to the basic types of problems you will see as flow rate/drip

rate problems.