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Drug Calculations

Drug Calculations. A comprehensive review from basic math review to critical care intravenous calculations. By Jenna Underwood, BSN, RN. Math Review – Calculator Skills. Always check your answer against common sense. Have an idea if your answer will be big or small.

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Drug Calculations

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  1. Drug Calculations A comprehensive review from basic math review to critical care intravenous calculations By Jenna Underwood, BSN, RN

  2. Math Review – Calculator Skills • Always check your answer against common sense. • Have an idea if your answer will be big or small. • Calculators calculate EXACTLY as you put it in. YOU need to remember order of operations. • When in doubt, put it into the calculator again to confirm your answer.

  3. Math Review – Rounding Place value Example: 531.067 What place value is the 6 in?

  4. Math Review – Rounding To round a number: • Look at the number after the requested place value • If that number is 5 or above (5, 6, 7, 8, 9) round the requested place value number up to the next numeric value • If that number is below 5 (1, 2, 3, 4) leave the requested place value alone. • The numbers after the requested place value either become zeroes (if before the decimal) or are left off entirely (if after the decimal) Example: 531.067 Round 531.067 to the nearest hundredth: Number after: 7 It is 5+ so round up: 6 becomes 7 New number is 531.07 (leave off thousandths) Round 531.067 to the nearest tens place: Number after: 1 It is below 5 so leave alone: 3 remains 3 New number is 530 (zero for the ones place and everything else after is left off)

  5. Math Review - Decimals 53.45 + 21.3 + 7.0006 = • Addition and Subtraction • Line up decimals • Add trailing zeroes as necessary • Perform the necessary function 53.45 21.3 +7.0006 81.7506 53.4500 21.3000 +7.0006 81.7506 • Value • Line up decimals • Compare numbers from left to right Put 4.1, 4.4, 3.1 and 3.06 in order from least to greatest 4.1 4.4 3.1 3.06 3.06, 3.1, 4.1, 4.4 Compare 3.1 & 3.2 3.1 3.2 3.1 < 3.2 Compare 3.1 & 3.01 3.1 3.01 3.1 > 3.01

  6. Math Review - Fractions • Parts of a fraction Numerator 1 4 Fraction bar Denominator

  7. Math Review - Fractions • Add and subtract with like denominators • Simply add (or subtract) the numerators • Keep the denominator the same

  8. Math Review - Fractions • Equivalent fractions 3 5 ? 10 = 3 5 6 10 x2 Start here! x?...2

  9. Math Review - Fractions • Add and subtract with unlike denominators Find equivalent fractions with LCD then add x5 1 2 5 10 LCD is 10 (2x5) x5 4 5 x2 8 10 x2

  10. Math Review - Fractions • Value • Find equivalent fractions with the same denominator • Compare the numerators Common denominator is 30 (3*2*5)

  11. Math Review – Roman Numerals

  12. Math Review – Roman Numerals • I = 1 • V = 5 • X = 10 • Add if numbers to right are smaller than those immediately preceding them (ex: VI = 6) • Subtract if numbers to right are larger than those immediately preceding them (ex: IV = 4)

  13. Drug Calculations - Abbreviations • BID • TID • SubQ • IM • Twice a day • Bis in die • Three times a day • Ter in die • Subcutaneous • Intramuscular • PRN • NPO • PR • q__h (q2h) • As needed • Pro re nata • Nothing by mouth • Nil per os • By rectum • Per rectum • Every __ hours • Quaque __ hora • IV • PO • ac • pc • intravenous • By mouth • Per os • Before meals • Ante cibum • After meals • Post cibum

  14. Drug Calculations - Abbreviations

  15. Measurements – Basic units

  16. Measurements - Systems

  17. Measurements – Time • Military time • Enables quick look at AM/PM distinction • (eliminates confusion) • No colon (:) • AM is always the normal time • Single digit numbers have a leading 0 (ex – 0800) • PM is the normal time plus 12 • 3:3O PM would be (3 +12 = 15) 1530

  18. Measurements – Time • Adding and subtracting time • 42 minutes, 1 hour 12 minutes and 38 minutes (what is the total time this IV bag has infused?) • Convert all to minutes by multiplying the number of hours by 60 minutes and add the minutes. • 42 min + 72 min + 38 min = 152 minutes • Now convert back to hours by dividing by 60 minutes (152/60 = 2 hours 32 minutes) • The remainder is the minutes

  19. Measurements – Using Devices • Syringe • Needle syringe – withdraw and measure in upright position with syringe below medication vial. • Oral syringe – withdraw and measure vertically with syringe above medication vial. Be sure to note the line increments on the syringe.

  20. Measurements – Using Devices • Dose cup • ALWAYS measure with the cup flat on a counter or other flat surface. • Eyes should be level with the dose cup • Be sure to note the dosage increments

  21. Word Problems • Distracters – extra information that is NOT needed to solve the problem. • What information is unnecessary for the solving this problem? • After getting rid of the distracters, re-read the problem. An adult patient comes into the hospital with an acute infection. The doctor orders Ampicillin 500 mg IM q4h. The hospital stores Ampicillin 500mg in powdered form. The directions for reconstitution state to add 1.8 ml of diluent to yield a concentration of 250 mg/ml. How much Ampicillin will the nurse give the patient? The doctor orders Ampicillin 500 mg IM q4h. After reconstituting, Ampicillin yields a concentration of 250 mg/ml. How much Ampicillin will the nurse give the patient?

  22. Problem-Solving Skills • Skim the problem. • What answer is the problem looking for? • Note the requested information • Underline IMPORTANT information • If distracters are distracting, cross them out. • When you get an answer ALWAYS go back and look to see if it makes sense. You wouldn’t give 20 pills, would you? • Rework problem if necessary

  23. Drug Calculations – Conversion Factors • 1 cc = 1 ml • 1000mg = 1 g • 1000 mcg (µg) = 1 mg • 1 tbsp = 3 tsp • 1 tsp = 5 ml • 1 oz = 30 ml • 1 gr = 60 – 65 mg • 1 cup = 8 fl. oz • 1 kg = 2.2 lb

  24. Drug Calculations – NO Formulas

  25. Drug Calculations – Dimensional Analysis • Use units to help determine how to set up the problem… • if the units are right, you KNOW you’re right. • Process of cancelling units • No memorized formulas • Works with ANY kind of problem!

  26. Drug Calculations – Tablets The doctor orders spironolactone 25mg for a patient experiencing mild heart failure. The nurse has 50 mg tablets of spironolactone on hand. How many tablets will the nurse administer?

  27. Drug Calculations – Tablets The doctor orders Diabinese 0.1 g po daily for a patient with type 2 diabetes. The nurse has Diabinese 100 mg tablets in the Pyxis. How many tablets will the nurse administer?

  28. Drug Calculations – Syrups/Liquids A client is ordered 50 milligrams of Amoxicillin trihydrate orally. 125 milligrams in 5 milliliters of Syrup is available. How many milliliters will you administer?

  29. Drug Calculations – Syrups/Liquids The doctor has ordered promethazine with codeine gr 1/6 po q6h prn for cough. On hand is promethazine with codeine solution 10mg per 5 ml. How much syrup should the nurse administer?

  30. Drug Calculations – IV/Parenteral Drugs The medication order states that 12,000 units of heparin are to be added to 250 milliliters of D5W. The patient is to receive 1200 units per hour IV. How many milliliters per hour will your patient receive?

  31. Drug Calculations – IV/Parenteral Drugs The doctor orders D5LR 1500 ml IV to run for 12 hours on a postoperative patient. The tubing has a drop factor of 20 gtt/ml. How many gtt/min will the nurse count out in order to comply with this order?

  32. Drug Calculations – IV/Parenteral Drugs Your shift is 0700 to 1500. You make rounds at 0730 and find an IV of D5 ½ NS is regulated on an electronic infusion pump at the ordered rate of 75 ml/hr with 400 ml remaining. The order specifies a continuous infusion. At what time should you anticipate hanging the next IV bag? How long until the bag is empty Check and change IV bag on or before 1250 or 12:50 PM Convert it to hours

  33. Drug Calculations – Weight-based/Pediatric Order: dicloxacillin sodium 125 mg po q6h for a child who weighs 55 lb. The recommended dosage of dicloxacillin sodium for children weighing less than 40 kg is 12.5 to 25 mg/kg/day po in equally divided doses q6h for moderate to severe infections. Is this dosage safe? Minimum Maximum Safe Dose!

  34. Drug Calculations – Weight-based/Pediatric The recommended dosage of tobramycin for adults with serious infections that are not life-threatening is 3 mg/kg/day in 3 equally divided doses q8h. What should you expect the total daily dosage of tobramycin to be for an adult with a serious infection who weighs 80kg?

  35. Drug Calculations – Critical Care The doctor orders Procainamide 1 g IV in 500 ml D5W to infuse at 2 mg/min for a patient you are caring for in the ICU. You should program the IV pump at what rate?

  36. Drug Calculations – Critical Care The doctor orders dobutamine IV to increase the patient’s cardiac output. The dobutamine is supplied at a concentration of 250 mg in 250 ml D5W. The doctor orders the dobutamine to infuse at 5 mcg/kg/min. The patient weighs 176 lbs. At what rate should the nurse program the IV pump?

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