Basic definitions and formulae Flashcards
(12 cards)
What is an amphoteric compound?
One that can behave as an acid or a base
What is the Henderson hasselbalch equation?
An equation which describes how the pH of a system affects the degree of ionisation of acids and bases.
Can be used to calculate pH and when the solution is in equilibrium I.e. the pKA. When 50% of the drug is ionised and 50% unionised.
pH = pKa + log [A-]/[HA]
Where pKA = -log Ka
What is HER?
What affects it?
How do we calculate it?
What are the different categories of HER we see?
The fraction of drug removed form the blood after one pass through the liver
Affected by: hepatic blood flow, absorption into hepatocytes and the hepatic enzyme activity
Calc: Ci - Co / Ci
Her > 0.7 is a high ratio. This means drugs are rapidly removed as the enzyme systems aren’t saturated, so metabolism is dependent on flow (hypotension etc.). Examples are propofol, fentanyl and GLAM.
Under 0.3 is considered low. Enzymes are easily saturated so metabolism is capacity limited and not affected by flow.
- Low and highly protein bound = big change in BA with decrease in protein.
- greatly effected by enzyme inducers and inhibitors
- drugs = phenytoin and warfarin, theophylline.
What is drug metabolism?
A process that modifies substances, to inactivate, deactivate, toxic y or detoxify these.
Generally takes lipophillic (non polar drugs) and converts them into polar compounds that are water soluble and therefore more easily excreted in the urine.
Some drugs are prodrugs that need to be metabolised in order to become active e.g. enalapril
How can we calculate VD?
VD = dose / C0
Loading dose / plasma concentration
How can we calculate clearance?
Rate of elimination / plasma concentration
How can we calculate half life?
0.693tau
How are VD, CL and T1/2 interrelated?
VD is proportional to t1/2 x cl
T1/2 is proportional to VD/Cl
Clearance is proportional to Vd/t1/2
Triangle with Vd on top.
What is a compartment model?
Mathematical model that can predict the drug handling characteristics of the body
What is an exponential function?
Rate of change of the quantity of a substance is directly proportional to the quantity of a substance at that time.
C = dC/dt
What is e?
Eulers constant, which is the base of the natural logarithm
2.71828
What is the michaelis menten equation?
Predicts the rate of a biological reaction according to the concentration of the substrates and specific enzyme characteristics.
V = Vmax [S] / Km + [S]
V = velocity
Km = michaelis constant which is the substrate concentration where the velocity is half of maximum
