Upload
henry-king
View
216
Download
0
Embed Size (px)
Citation preview
Measurement in Veterinary Pharmacology
CHAPTER 6 - 2CHAPTER 6 - 2
Dr. Dipa Brahmbhatt VMD MpHDr. Dipa Brahmbhatt VMD MpH
[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/ 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