Measurement in Veterinary Pharmacology

CHAPTER 6 - 2CHAPTER 6 - 2

Dr. Dipa Brahmbhatt VMD MpHDr. Dipa Brahmbhatt VMD MpH

dbrahmbhatt@vettechinstitute.edudbrahmbhatt@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

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 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

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