Easy way to calculate drugs MONA ALHARBI

Easy way to calculate drugs MONA ALHARBI

Objective At the end of this lecture you should be able to : Calculate dosage drugs. Calculate drip rate (drops/min). Conversion of dosages to ml/hour Calculating the length of time for IV infusions Minimize mistake during dosage calculation.

Out line : Dosage calculation : 1- PO dosage calculation ( solid ) 2- PO dosage calculation ( liquid ) 3- IV dosage calculation Calculate infusion rates : 1- IV rate ( gtt/ min) 2- IV rate ( ml/ hr ) 3- mcg/ min as ml / hr 4- mcg/kg/min as ml /hr Calculate infusion time : 1- IV time ( hr )

Dose calculation : 1- PO dosage calculation ( solid ) i.e. tablets, caplets, capsules, lozenges. The Formula : Dose ordered Example : Dose available Give 70 milligrams of Lasix (furosemide) PO now. Pharmacy has sent a package of 20- milligram Lasix tablets. How many tablets will you give? Using the formula: dose ordered = 70 mg dose available = 20 mg tablets 70 mg 20 mg = 3.5 tablets

Dose calculation : 2- PO dosage calculation ( liquids ) calculates liquid medications such as acetaminophen elixir The Formula : Example : Dose ordered Dose available x volume available give 650 mg of liquid acetaminophen (Tylenol ). The liquid acetaminophen comes in a concentration of 120 mg per 5 ml. How much medication would you give? Answer: 650 mg = dose ordered 120 mg = dose available 5 ml = volume available 650 x 5 = 27 ml. 120

Dose calculation : 3- IV dosage calculation calculates IV medications such as furosemide and digoxin The Formla : Dose ordered Dose available x volume available (Actually this is the same formula used in the "PO dosage calculation ) Example : Dr. Smith has ordered a heparin infusion of 1000 units/hour for John Doe in bed 7. The infusion is to be mixed as 25,000 units in 500 ml.9% NS. What rate will the IV pump be set to? 1000 units/hr = dose ordered 25,000 units = dose available 500 ml = volume available 1.000 units x 500 = 20 ml 25.000 units

Calculate infusion rates : 1- IV rate ( gtt/ min) To calculates IV rates without using an IV pump The Formla : 1 - To calculate rate using minutes : volume of infusion x calibration = gtt / min minutes 2 - To calculate rate usin hours : volume of infusion x calibration = gtt / min hours x 60 There are two giving sets of drip rate (calibration): The standard giving set (SGS) has a drip rate of 20 drops per ml for clear fluids (i.e. sodium chloride, glucose) and 15 drops per ml for blood. The micro-drop giving set or burette has a drip rate of 60 drops per ml. (The drip rate of the giving set is always written on the wrapper if you are not sure. )

Calculate infusion rates : 1- IV rate ( gtt/ min) Example : 1) Using IV tubing alone (calibrated at 60), prepare an IV to infuse 1 liter over 8 hours. How many drops per minute should the IV be set for? See the example below. Hours = 8 volume = 1000 calibration = 60 Actual formula: 100 mlx 10 = 16.7 or 17 gtt/min 8 x 60

Calculate infusion rates : IV rate ( ml/ hr ) calculates IV rates when given a time limit when IV must be infused The Formla : 1 - To calculate rate using minutes : volume of infusion = ml/hr ( minutes /60) 2 - To calculate rate using hours : volume of infusion = ml/hr hours Example : If an order was written to infuse a liter of IV fluid every 8 hours, at what rate would the IV pump be set for? Answer is 125 ml/hour.

Calculate infusion rates : 3- mcg/ min as ml / hr calculates IV medication rates such as norepinephrine and nitroglycerine The Formla : First, convert mcg/kg/min into mg/hr. mcg x 60 1000 Next, use the IV dosage formula to solve the rest of the problem. Dose ordered x volume available Dose availabl Example : 1. You have a 500 ml infusion containing 50 mg nitroglycerin. A dose of 10 mcg/min is required. What is the rate in ml/hour? 10 x 60 = 0.6 1000 1. In the example, 0.6 mg is the dose ordered, 50mg is the dose available, 500 ml is the volume. 0.6 x 500 = 6 ml /hr 50 note : 60 converts minutes to hours. 1,000 converts mcg to mg. If doses are expressed in milligrams, then there is no need to divide by 1000

Calculate infusion rates : 4-mcg/kg/min as ml /hr calculates IV medication rates such as nitroprusside and dopamine The Formla : First, convert mcg/kg/min into mg/hr. (mcg x kg ) x 60 1000 Next, use the IV dosage formula to solve the rest of the problem. Dose ordered x volume available Dose available Example : Dr. Smith wants a dopamine drip started at 20mcg/kg/min on John Doe in Bed 7. John Doe's weight is 70 kilograms. He wants an 800mg vial of dopamine mixed in 500 ml of D 5 W. What is the rate in ml/hour? 1- ( 20 mcg x 70 kg ) x 60 = 84 1000 2- In the example, 84 mg is the dose ordered, 800 mg is the dose available, 500 ml is the volume. 84 mg x 500 ml = 52.5 ml / hr 800 mg

Calculate infusion time : 1- IV time ( hr ) Sometimes it may be necessary to calculate the number of hours an infusion should run at a specified rate. Also, it is a good way of checking your calculated drip rate for an infusion. The Formla : 1- Manually controlled infusions number of hours the infusion is to run = volume of the infusion / 2- Infusion or syringe pumps number of hours the infusion is to run = volume of the infusion rate (ml/hour) rate (drops/min) drip rate 60

Calculate infusion time : 1- IV time ( hr ) Example : 1- Manually controlled infusions The doctor prescribes 1 litre of 5% glucose,the drip rate for the infusion is calculated to be 42 drops/min. You wish to check the drip rate; how many hours is the infusion going to run? (SGS = 20 drops/ml) volume of the infusion ( in ml ) = 1000 ml drip rate of giving set = 20 drops/ml Drop rate = 42 drops/min Actual formula : 1000 42 drops / min x 60 = 8 hours 20

Calculate infusion time : 1- IV time ( hr ) Example : 2- Infusion or syringe pumps The doctor prescribes 1 litre of 5% glucose to be given over 8 hours. The rate for the infusion is calculated to be 125 ml/hour. You wish to check the rate; how many hours is the infusion going to run? volume of the infusion ( in ml ) = 1000 ml Drop rate = 125 ml/ hr Actual formla : 1000 ml = 8 hours 125

MINIMIZING ERRORS 1. Write out the calculation clearly. It is all too easy to end up reading from the wrong line. 2. If you are copying formulae from a reference source, double-check what you have written down. 3. Write down every step. 4. Remember to include the units at every step; this will avoid any confusion over what your answer actually means. 5. Do not take short cuts; you are more likely to make a mistake. 6. Try not to be totally dependent on your calculator. Have an approximate idea of what the answer should be. Then, if you happen to hit the wrong button on the calculator you are more likely to be aware that an error has been made. 7. Finally, always double-check your calculation. There is frequently more than one way of doing a calculation, so if you get the same answer by two different methods the chances are that your answer will be correct. Alternatively, try working it in reverse and see if you get the numbers you started with.

Refrences http://www.manuelsweb.com/nrs_calculators.htm POTTER, Patricia A., et al. Fundamentals of nursing. Elsevier Health Sciences, 2013.