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

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

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

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

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

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

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

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

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

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

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)

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)

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

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

Another Way to Determine Volume Concentration 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

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

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

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

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

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