Lecture 2. Pharmacokinetics 2 Flashcards
What does IV bolus mean?
100% drug given all at once
What is the apparent volume of distribution (Vd)?
Amount in body at equilibrium / Plasma drug concentration
What causes a low apparent volume of distribution?
Low lipid solubility + low tissue binding
What causes a high apparent volume of distribution?
High lipid solubility + High tissue binding
What is clearance?
Rate of elimination / concentration of input
What are the units of clearance?
Volume/time (ml/min)
What is the total body clearance?
The sum of all the clearances occuring simultaneously by different organs in the body
Where is clearance frequently measured?
Hospitals
Why is creatine used to measure clearance rate instead of inulin?
Only one blood sample is needed to measure clearance with creatine
How to calculate creatine clearance?
Rate of production / serum creatine
What are the two ways to measure clearance?
Single blood sample and/or renal clearance: uring collection over 24 hours
What is the normal creatine clearance for healthy women?
88-128ml/min
What is the normal creatine clearance for healthy men?
97-137ml/min
Does clearance increase or decrease with age?
Decrease
What is first order elimination kinetics?
Rate of elimination is proportional to drug concentration
What is elimination rate constant (Kel)?
Proportion of drug eliminated per unit time (min⁻ ¹)
What is the time constant?
1/Kel
If you plot a natural log of concentration graph against time, what is the gradient?
-Kel
What is the half life?
The time taken for the drug concentration to fall by half
How do you calculate half life?
ln2/Kel
What can half-life help determine?
The dosing interval
What is the rate of absorption proportional to?
The amount of drug unabsorbed
The more drug you have, the more drug is absorbed
What does the area under the curve measure?
The amount of drug absorbed
How can bioavailability be calculated from areas under the curve?
AUC dosage form / AUC IV injection
What are the properties of a loading dose?
Acute conditions
Status asthamticus
Status epilepticus
Long elimination half life
What happens during a continuous infusion?
Slowly build up drug concentration over time
What is zero order kinetics also known as?
Saturation kinetics
What does zero order kinetics mean?
Rate of elimination is not proportional to drug concentration but is constant
What are examples of substances that follow zero order kinetics?
Ethanol, phenytoin
Is there any change in absorption with age?
Little change
Why can therapeutic drug monitoring be used for?
Measuring the concentration of drug in the body
To determine the most effective dose or to avoid toxicity
Can measure drug in blood, urine (or saliva if unbound)
What is the use of urine sampling?
Useful as do not need blood sample
Useful for drugs which are fully/partially eliminated in the urine
Volume of urine is very variable so do not use drug concentration
Express quantity of drug as absolute amount
What is a disadvantage of urine sampling?
Drug must be excreted via kidneys
How can a drug be measure if it has been metabolised?
Measure the metabolite
