Dosage calculation pharmacology 2nd year.pptx

Drug dosage calculation

1. 2.4 DOSAGE CALCULATIONS

2. Demonstrate the correct method of calculation of drug usage in patients including those used in special situations

3. At the end of the session, you should be able to. Calculate appropriate doses for individual patients based on age, body weight, and surface area. Calculate the dose of drug using appropriate formulae in a given clinical case in children Calculate the dose of drug using appropriate formulae in a given clinical case in elderly Calculate the dose of drug using appropriate formulae in a given clinical case in patients with renal dysfunction and other pathological conditions like CCF, Liver disease .  

4. Expression of drug concentration The drug concentration can be expressed as weight (mg or gm ) for solid medicaments. For liquid formulations like syrups and injections, the concentration may be expressed as: Weight in volume (W/V)- mg or gm or U in ml e.g., insulin 40 U/ml, dopamine 200mg/5ml, 5% dextrose solution (5gm in 100ml) b) Volume in volume (V/V) - e.g. 70% ethyl alcohol (70ml absolute alcohol in 100 ml of aqueous solution).

5. Contd.., In case of semisolid medications like ointments, gel and creams , the drug concentration is expressed as: Weight/ volume- 1 gm of solute in 100 ml of solvent (1% W/V) Volume/ volume- 1 ml of solute in 100 ml of solvent (1% V/V) Weight/ weight- 1 gm of solute in 100 g of solvent (1% W/W)

6. Dose calculation based on body weight and body surface area a) Based on weight (expressed as mg/kg body weight): b) Based on body surface area:

7. Pediatric dose calculation based on age New born and infants have low GFR and immature tubular transport. Hepatic enzymes inadequately developed. Hence need for dose reduction.

8. Dose Calculation – in Renal Failure Creatinine clearance Calculated by Cockroft Gault’s formula.

9. Normal clearance is 120ml/min and 75% reductions shows renal dysfunction. Dose modification

10. Liver disease ( Cirrhosis) Hepatic metabolism of drugs is decreased leading to increased blood levels. No specific formula available for dose calculation

11. DRIP (DROP) Factor of IV Tubings The size of a drop in an IV setup depends upon the width of the IV tubing: thinner tubing produces smaller drops and wider tubing produces larger ones. There are two major categories of IV tubing: Macrodrip tubing is wider and so produces larger drops. It is the tubing most commonly used for routine IV administration, such as infusion of IV fluids that do not contain sensitive medication. Macrodrip tubing comes in 3 sizes: 10 drops/mL, 15 drops /mL and 20 drops / mL.

12. Contd.., Microdrip tubing is narrower and so produces smaller drops. It is used for children and infants , or to infuse sensitive medications where precision in the flow rate is essential. Microdrip tubing (sometimes called minidrip ) comes in only one size : 60 drops /mL For an IV setup on a volumetric pump or other electronic infusion device, the flow rate measured in mL/h. This rate is fed into the device without having to count drops in a drip chamber.

13. PROBLEMS

14. 1. What is the quantity of Mannitol present in 60ml of 20% Mannitol ?

15. 1% solution is 1g in 100mL Therefore 20% solution will be 20g in 100mL 100mL contains 20g Therefore 60mL contains 60 X 20/100 = 12g of Mannitol

16. 2. How many µg of Adrenaline is present in a 30mL vial of 2% Xylocaine with adrenaline? Adrenaline available as 1:200000

17. 1:200000 means 1g in 2,00,000 mL Conversion of 1gm to µg [1g =1000 mg ; 1mg =1000 µg] 1g =1000 X 1000 µg 2, 00,000 ml contains 1g of adrenaline Alternatively 2,00,000 ml contains 1000 x 1000 of adrenaline . Therefore 30 ml contains 1000 x 1000 x 30 = 150 µg of 2,00,000 Adrenaline is present in 30mL of 2% Xylocaine

18. 3. A 30 year old man weighing 50Kg develops hypotension after myocardial infarction. He was prescribed Inj. Dopamine 8µg/min/Kg. Calculate the rate of infusion in drops per minute if strength of Dopamine ampoule(1mL) = 200mg

19. Dose required = 8 x 50 = 400 µg/min Dissolve the Dopamine ampoule in 500 mL NS 200mg (200000µg) is present in 500 mL 400µg is present in = 500 x 400 200000 = 1mL per minute = 15 or 20 drops per minute (depending on drop factor of IV Set)

20. 4. Calculate the Paediatric dose of INH and Rifampicin for a 10 year old child using Dilling’s formula. [ Adult dose of INH - 300mg OD and Rifampicin - 600mg OD]

21. Calculation Dilling’s Formula to calculate child dose = Age of the Child /20 x Adult dose Dose of INH for the child of 10 years = 10/20 x 300 = 150mg OD Dose of Rifampicin for the child of 10 years = 10/20 x 600 = 300mg OD

22. 5. Using Young’s Formula Calculate the dose of Paracetamol and Metronidazole for a 3 year old child. [ Adult dose of Paracetamol is 500mg TID and Metronidazole is 400mg TID.]

23. Calculation Young’s Formula to calculate child dose: = Age/(Age +12) X Adult Dose Child dose of Paracetamol = 3/3+12 X 500 = 100mg TID Child dose of Metronidazole = 3/3+12 X 400 = 80mg TID

24. 06. The adult dose of Gentamicin is 80mg/BD. How much Gentamicin should a 1 year old child weighing 12kg receive?

25. Weight of the child = 12 kg Clark’s formula =(Adult dose X Body weight ) / 70 = (80X 12) / 70 = 13.7 mg

26. 07. Calculate the Creatinine Clearance with the following data; Age:60 years, Weight: 54 Kg, Normal serum creatinine= 0.6-1.2 mg/ dL Creatinine Clearance Estimate by Cockcroft- Gault Equation

27. Creatinine clearance using Cockcroft Gault equation = (140-age) X Weight in Kg 72 x Serum Creatinine = (140-60) x 54 72 x 1.2   = 50 mL / min   Note: In females the equation has to be multiplied by 0.85

28. 08. Calculate the creatinine clearance of a drug whose IV dose is 240mg and plasma conc. Is 6mg/ lt , t1/2 of the drug is 4hrs

29. t½ = 0.693x Vd / Cl Cl = 0.693x Vd / t1/2 Vd = Total dose administered/ Plasma conc. = 240/ 6 = 40 lts So, Cl = 0.693x 40/4 = 6.93ml/min

30. 09. A 40 yr old male patient ( body wt 70kgs) has a serum creatine of 4mg/dl. He has proteus mirabilis induced UTI, sensitive to Gentamicin. Calculate the Gentamicin dose which should be administered to this patient. Gentamicin dose in a patient with normal renal function is5mg/ kg once a day. Renal clearance of Gentamicin is 78ml/min in a normal renal function patient according to literature

31. a) Calculate the creatinine clearance in this patient using Cockcroft Gault equation - (140 – Age) x body wt / Serum creatine x 72 - (140 – 40)x 70 / 4 x 72 = 24.3ml/min b) Dose of Gentamicin In normal renal clearance i.e 78ml/min, it is5mg/ kg For 24.3 ml /min clearance – 5x 24.3/78 = 1.56mg/kg

32. 10. XY is a 60yr old , 85kg male with ventricular fibrillation who requires therapy with IV Lidocaine . He has liver cirrhosis ( Child pugh score = 11).The Vd of Lidocaine range is 51L to 221L . Suggest an initial IV Lidocaine dosage regimen designed to achive steady state Lidocaine conc. equal to 4µg/ml

33. Vd = 51L to 221L. Since patient has cirrhosis, Lower limit is taken i.e. 51L Css = 4µg/ml LD (Loading dose) = Cpss x Vd LD = 4mg/L x 51L ( note : 4µg/ml = 4mg/ L) LD = 204mg

34. 11. calculate the dose of anticancer drug A in a child having body surface area of 0.78 square meter, while adult dose is 20mg/day ?

35. Dose based on Body Surface area: = BSA(in m 2 ) x Adult dose/ 1.73 = 0.78 x 20/1.73 = 9mg/day

36. THANK YOU