Pharmacokinetics Language English Format: PDF Price - Tutorsindia

PharmacokineticsElimination: Elimination rate constant,...V50 LLiverCl = 5 L/hVolume = 50 litre.5 litres cleared perhour.10% of system iscleared per hour...K = 0.1 h -1K = Cl/VFigure 4.5 Clearance considered along with a drug’s volume of distributionWe can either say ‘One tenth of the drug mass is eliminated per hour’ or ‘One tenth of the volume that the drug occupiesis cleared per hour’; either way, the elimination rate constant K is 0.1h -1 .The relationship was expressed above as:K = Cl / VHowever, for clinical calculations it is more likely to be used in its re-arranged form:Cl = K x V4.2.5 Expressing clearance relative to body weightSection 3.6.2 showed how volume of distribution could be expressed relative to body weight and clearance can be treatedin a similar manner. If the clearance of a drug in a patient was 10L/h and he/she weighed 50Kg, then clearance could beexpressed as 10L/h / 50Kg = 0.2 L/h/Kg.This approach can then be used to predict likely clearance in a particular patient. It is known that in healthy young adults,the average clearance of theophylline is approximately 0.04L/h/Kg. So, if a patient weighs 75Kg, we would predict thattheir clearance to be 0.04L/h/Kg x 75Kg = 3L/h.Unfortunately, clearance is subject to much greater modification by disease, smoking and the use of other drugs thanis volume of distribution. Consequently, predicted clearance values tend to be less precise than predicted volumes ofdistribution. These effects can be so great, that predicted clearances may need to be adjusted to take account of some ofthe more powerful influences such as smoking.45Download free ebooks at bookboon.com