Weight-based Calculations

Use the following formula to get your total required dose:

Weight (kg) x Dosage Ordered (per kg) = Required Dose.

Convenient conversions worth memorizing:

• 1 kg = 2.2 lbs.
• In the old empiric system, 15 drops = ml.

HOWEVER...

Different pipettes can create different-sized drops. The two most common types of automated devices to deliver IV medication are

1. macrodrip IV set which delivers larger volumes and rates: 10 drops per milliliter--10 gtt/ml and
2. microdrip IV set which is used for smaller volumes of administration: 60 drops per milliliter--60 gtt/ml. (So it takes 60 drops in a microdrop set to deliver 1ml of the drug.)

 

EXAMPLE

So if 5 mg/kg/hour is the order for your patient who weighs 44 pounds, from a solution of 200 mg in a 5-ml vial, you derive the following by determining the kg, concentration, then applying the equalities for gtts and minutes:

1) We first need to turn 44 lbs into kilograms.

44/2.2 = 20. Therefore, this patient is 20 kg.

You won't ever have to convert kg to lbs, but if you did, you'd multiply the lbs by 0.45. (e.g., 5 lbs x 0.45 = 2.25 kg.) An easy way to remember the difference is to think:

• Bigger ⇒ Smaller, then DIVIDE. (Kg to pounds is going bigger to smaller.)
• Smaller ⇒ Bigger, then MULTIPLY. (Pounds to Kg is going smaller to bigger.)

2) Then we can multiply the weight in kilograms by the dosage ordered/hr to get the required dose/hr In this case:

20kg x 5mg/kg/hr = 100 mg/hr.

Afterward, plug the desired dose into our regular drug calculation formula of the desired dose (here, 100 mg) divided by the amount on hand and then multiply by drop set to get a Weight-based IV drip.

For example, in this example, you need to give 100 mg/hr of this drug. You have a vial that is labeled 200 mg/5 ml, so you need the concentration (mg/ml):

3) 200 mg/5 ml = 40 mg/ml = CONCENTRATION.

4) 100 mg/40 mg/ml = 2.5 ml ANSWER.

5) Multiply by the drop set to get a Weight-based IV drip.

 

Medication Drip Calculations

The formula for a medication drip is

VTBI (volume to be infused) x Drop Factor ÷ by concentration.

ml X gtts/ml ÷ [concentration] = drip rate.

"Dose on hand" refers to the total availability of the drug in each unit of distribution (bag, vial, etc). Morphine comes in 10 mg vials, your dose on hand would be 10 mg even if you're desired dose was only 4 mg.

EXAMPLE

You're ordered to infuse lidocaine at 2 mg/min. You carry lidocaine 2g in 500-ml bags. What is your drip rate?

Since the formulas used call for mg, ml, and minutes,

1. First, convert grams to milligrams: 2 g → 2000 mg.
2. Next, determine the concentration: 2000 mg per 500 ml = (2000/500) X (mg/ml) = 4 mg/ml.
3. Since you're using a 60-drop set, you multiply your VTBI by the drip factor. Then divide by the concentration of 4 mg/ml.
4. If it's 4 mg per ml, then the 2 mg/min you've been ordered to drip means 1/2 ml/minute.
5. 1/2 ml/minute X 60 ggts/ml = 30 ggts/minute.
6. The mL in the numerator and denominator cancel, and it becomes 1/2 X 60 gtts/min.
7. ANSWER: The correct drip rate in this situation would be 30 gtts/min