Measurement in Veterinary Pharmacology
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Transcript of Measurement in Veterinary Pharmacology
Measurement in Veterinary Pharmacology
CHAPTER 6 - 2CHAPTER 6 - 2
Dr. Dipa Brahmbhatt VMD MpHDr. Dipa Brahmbhatt VMD [email protected]@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/ kgWeight 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 / # doses100mg / 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