Measurement in Veterinary Pharmacology

1. Measurement in Veterinary Pharmacology CHAPTER 6 - 2 Dr. Dipa Brahmbhatt VMD MpH dbrahmbhatt@vettechinstitute.edu

2. Dose Calculations • Must know correct amount of drug to administer to a patient • Must be in same system of measurement • Weight conversion factor: 2.2 lb = 1 kg • Remember that drugs can be measured in mcg, mg, g, gr, ml, l, units • Remember that drugs can be dispensed or administered in tablets, ml, l, capsules

3. Dose Calculations Dosage of a drug: 2 mg/ kg Weight of animal: 22 lbs

4. Dose Calculations • STEP 1: Convert weight into kg • 1 kg = 2.2 lbs • X = 22 lbs • X = (22 lbs/ 2.2lbs) * 1 kg = 10 kg • STEP 2: From dosage to dose • Dosage: 2mg/kg • Dose: 10kg * 2mg/kg = 20 mg

5. Dose Calculations • STEP 3: From dose to amount administered • 20 mg • Tablets: 40 mg, 80 mg and 100 mg • ½ of the 40 mg tablet (assuming that tablet is scored!!) • ½ tablet – 40 mg • Solution: 10 mg/ ml • (20 mg/ 10 mg) * 1 ml = 2 ml • 2 ml of 10mg/ml drug solution • Total dose: BID for 7 days • Tablet: 7 tablets • Solution: 28 ml of the 10mg/ml solution

6. Dose Calculations • Units (U) or International Units (IU) • Insulin, heparin, penicillin • 1000 lb cow; 50,000 U/kg; 300,000 U/mL • (1000lb/2.2kg) * (50,000 U/kg) = 22,727,272.73 • (22,727,272.73) * (1mL/300,000 U) = 75.8 mls

7. Dose calculations - Verification • Calculating # doses • # doses = total amount of medicine/ strength of each dose • Vet prescribes 200 mg and each dose is 20 mg, # doses? = 200 mg/ 20 mg = 10 doses • Single dose 1 gram, # doses in 10 g = 10 doses • 1 dose is 200 mg, # doses in 10 g = 10,000 mg/200mg = 50 doses

8. Dose calculations - Verification • Determining amount in each dose = Quantity in each dose / # doses 100mg / 20 doses = 5 mg in each dose

10. Solutions • Solutions are mixtures of substances not chemically combined with each other • The dissolving substance of a solution is referred to as the solvent (liquid) • The dissolved substance of a solution is referred to as the solute (solid or particles) • Substances that form solutions are called miscible • Substances that do not form solutions are called immiscible

11. Working with Solutions • The amount of solute dissolved in solvent is known as the concentration • Concentrations may be expressed as ratio strengths: parts per drug per parts per solution: parts (per some amount), weight per volume, volume per volume, and weight per weight • Usually reported out as percents or percent solution • Remember that a percent is the parts per the total times 100

12. Rules of Thumb When Working with Solutions • Parts: parts per million (ppm) means 1 mg of solute in a kg (or L) of solvent • % conc. of 1:1000 epinephrine? • % conc. = (1/1000)*100 = 0.1% • Liquid in liquid: the percent concentration is the volume per 100 volumes of the total mixture (1 ml/100 ml, 5 oz/ 100 oz) • Solids in solids: the percent concentration is the weight per 100 weights of total mixture(60 mg/100 mg, 4.5 g/ 100 g)

13. Rules of Thumb When Working with Solutions • Solids in liquid: the percent concentration is the weight in grams per 100 volume parts in milliliters (dextrose 5% = 5 g/100ml = 5000mg/100ml = 50mg/ml)

14. Percent Concentration Calculations • Pure drugs are substances that are 100% pure • Stock solution (pure drugs) is a relatively concentrated solution from which more dilute solutions are made • Ratio-proportion method: one method of determining the amount of pure drug needed to make a solution • (Amount of drug/amount of finished solution) = (% of finished solution/100% (based on a pure drug)) • NB: that the amount of drug used to prepare a solution is added to the total volume of the solvent

15. Percent Concentration Calculations • How much NaCl needed to make 500mls of 0.9% solution • X = (0.9g NaCl/ 100 ml) * 500 ml = 4.5 g (4.5 ml density is 1 mg/ml) • Prepare 1L of 4% formaldehyde fixative solution from 37% stock solution • X = (4%/ 37%) * 1000 = 108 ml of stock solution • 1000 ml – 108 mls of stock = 892 ml of solvent

16. Another Way to Determine VolumeConcentration Volume concentration method: • Vs = volume of the beginning or stock solution • Cs = concentration of the beginning or stock solution • Vd = volume of the final solution • C d = concentration of the final solution Vs x Cs = Vd x Cd

17. Volume concentration method Vs x Cs = Vd x Cd • How much water must be added to 1L of 90% alcohol to change it to 40% • Vs x Cs = Vd x Cd = 1,000mls x 90% = Vd x 40% • Vd = 2250 mL • 2250 mL – 1000 mL = 1250 mL

18. Volume concentration method • How much 1:25 solution of NaCl is needed to make 3 L of 1:50 solution • Convert to % • (1/25)*100 = 4%; (1/50) * 100 = 2% • Vs x Cs = Vd x Cd = Vs x 4% = 3 L x 2% • Vs = 1.5 L

20. Drug Concentrations in Percents • Drug concentrations are sometimes listed in percents • Parts per total = parts (in g) per 100 • The front of the vial specifies the concentration (for example, 2% lidocaine) • Use X g/100 ml to determine dose • Dosage: 3mg/ kg; weight: 15 lb dog; 2% lidocaine • (15 lb/ 2.2 lb/kg) * 3mg/kg = 20.45 mg • 20.45 mg / 20 mg/ml = 1.02 mL

21. Reconstitution Problems • Drug is in powder form because it is not stable when suspended in solution • Cefazolin sodium, azithromycin, ampicillin • Such a drug must be reconstituted (liquid must be added to it: sterile water, saline, dextrose) • The label should state how much liquid to add • Powder may add to the total final volume of liquid being reconstituted • Label a reconstituted drug with the date prepared, the concentration, and your initials

22. Reconstitution Problems • Cefazolin sodium: 500 mg + 2mL = 225mg/ml (reconstituted vol. increases to 2.2 mL) • Ampicillin – 25 g (adds 20.5 mL of volume) • 104.5 mL (200 mg/mL) • 79 mL (250 mg/mL) • 41 mL (400 mg/mL) • Patient needs 800mg hence 2 mL