18 Intravenous (IV) Flow Rate Calculations

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An IV administration set is used to administer (or infuse) an IV solution into a patient. When working with an IV administration set it is important to know how to obtain two types of “flow rates”. The first type of flow rate is commonly known as the rate of administration, or IV rate. It is generally expressed in terms of mL/hour or mL/minute and is the speed at which the IV solution is administered to the patient.

$$\frac{\text{mL}}{\text{hr}} = \frac{\text{vol to be infused (mL)}}{\text{infusion time (hr)}}$$

$$\frac{\text{mL}}{\text{min}} = \frac{\text{vol to be infused (mL)}}{\text{infusion time (min)}}$$

It is usually rounded up or down to the nearest whole number since IV administration pumps typically don’t have decimal places.

The drop factor, or pump calibration is the number of drops of solution in 1 mL of IV fluid. Each IV administration set is labeled with one. However, not all IV sets have the same drop factor so be absolutely sure of the correct drop factor before beginning. Drops, drips and gtt all mean the same thing so the following drop factors are all equal:

$$\frac{\text{gtt}}{\text{mL}} = \frac{\text{drops}}{\text{mL}} = \frac{\text{drips}}{\text{mL}}$$

The second type of flow rate is the drip rate, which is the number of drops per minute delivered in that IV administration set:

$$\frac{\text{gtt}}{\text{min}} = \frac{\text{drops}}{\text{min}} = \frac{\text{drips}}{\text{min}}$$

Take note that the drip rate has a denominator in minutes, but the drop factor has a denominator in mL.

The drip rate is obtained by multiplying the drop factor by the rate of administration:

$$\text{drip rate} = \text{drop factor} \times \text{rate of administration}$$

$$\frac{\text{gtt}}{\text{min}} = \frac{\text{gtt}}{\text{mL}} \times \frac{\text{vol to be infused (mL)}}{\text{infusion time (min)}}$$

$$\frac{\text{gtt}}{\text{min}} = \frac{\text{gtt}}{\text{mL}} \times \frac{\text{mL}}{\text{min}}$$

The IV frequency, or IV schedule, is the time it takes to infuse a specific volume of solution, expressed in hours. It’s calculated by dividing the volume to be infused over the rate of administration:

$$\text{q (x) hr} =$$

$$\frac{\text{vol to be infused (mL)}}{\text{rate of administration in mL / hr}}$$

$$\text{q (x) hr} = \frac{\text{mL}}{\text{mL / hr}} = \text{mL} \times \frac{\text{hr}}{\text{mL}}$$

The IV frequency is commonly rounded down to the nearest whole number to ensure that the patient receives enough IV solution.

Now that we have a basic understanding, let’s try some examples.

Let’s say that a nurse will be administering a 300 mL SVP (small volume parenteral) over 3 hours (the rate of administration) using an 11-drop set (the drop factor). What is the drip rate?

$$\text{3 hours} \times \frac{\text{60 min}}{\text{1 hour}} = \text{180 min} \;\text{;}$$

$$\frac{\text{(x) gtt}}{\text{min}} = \frac{\text{11 gtt}}{\text{mL}} \times \frac{\text{300 mL}}{\text{180 min}}$$

$$\frac{\text{11 gtt} \times \text{300 mL}}{\text{1 mL} \times \text{180 min}} = \frac{\text{18.33 gtt}}{\text{min}}$$

Sometimes we’ll come across problems that give us more information than we need to know to solve the problem. In these situations let’s focus on the most relevant information. For example, if we’re asked to find the rate of administration in mL/min for the following order then let’s focus on the rate of administration already provided in mL/hour: 1,500 mL KCl 20 mEq and K Acetate 30 mEq in D5W administered at 120 mL/hour. To solve, we first change the hours to minutes and then set up a proportion:

$$\frac{\text{120 mL}}{\text{1 hour}} = \frac{\text{120 mL}}{\text{60 min}} \;\text{;}$$

$$\frac{\text{120 mL}}{\text{60 min}} = \frac{\text{(x) mL}}{\text{1 min}}$$

$$\frac{\text{120 mL} \times \text{1 min}}{\text{60 min}} = \frac{\text{(x) mL}}{\text{1 min}}$$

$$\frac{\text{120 mL} \times \text{1 min}}{\text{60 min}} = \frac{\text{2 mL}}{\text{1 min}}$$

A prescription reads as following — Rx: NS (Normal Saline) 1-L IV 250 mL/hour. How do we find the IV frequency of a 1-L IV bag? Well, let’s use the formula for IV frequency of course 🙂

$$\text{q (x) hr} = \frac{\text{1,000 mL}}{\text{250 mL / hr}} =$$

$$\text{1,000 mL} \times \frac{\text{hr}}{\text{250 mL}} = \text{q 4 hr}$$

Now it’s your turn. Find the following:

A. Find the IV frequency of a 2 L solution infused into a patient with a rate of administration of 500 mL / hr

B. Find the drip rate of 700 mL solution administered to a patient over 1 hr using a 15 gtt /mL set.

C. Convert a 120 mL / hr rate of administration to mL / min

D. Find the IV frequency of a 1,800 mL solution with a rate of administration of 300 mL / hr

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