Clearance Flashcards
What is the science that links dose and concentration and what are the two components of this?
Pharmokinetics which is made up of drug clearance and drug volume of distribution
What is the science that links concentration and effect and what are the two components of this?
Pharmacodynamics which is composed of maximum effect (Emax) and the concentration producing 50% of the maximum effect (C50)
What is clearance?
The relationship between concentration and the rate of elimination of the drug from the body
What is rate out at a steady state?
Rate out= clearance x concentration
Which are the two key organs which clear drugs?
Liver (90L/h)
Kidneys (70L/h) : glomerular filtration (all drugs 6 L/h) and tubular secretion (variable)
Which drug has the highest clearance rate?
Glyceryl trinitrate (used in angina) 150L/h
What is the clearance of morphine?
60 L/h (clearance limited by blood flow)
What is the significance of the clearance rate of gentamicin and digoxin?
Gentamicin 6 L/h as only cleared by glomerular filtration
Digoxin 9 L/h as metabolised by kidney and liver
Which two drugs have a very slow clearance rate?
Theophylline 3 L/h liver
Warfarin 3 L/day liver
What are the three classifications of clearance?
Constant (99% of drugs)
Concentration dependent
Flow dependent
Describe constant clearance
Also called first order/linear
Anything to do with glomerular filtration is always first order
This refers to where clearance is apparently independent of concentration or organ blood flow
Describe concentration dependent clearance
Also called mixed order
Rout = (Vmax/ Km +C) x C
Km is the concentration producing 50% of Vmax
Describe flow dependent clearance
Morphine
Elimination is dependent on blood flow to the liver
