Rate of absorption Flashcards
What are the three processes that describe rate of absorption
Bolus, zero-order, first-order
Describe bolus rate of absorption
Bolus input means absorption is instantaneous. Approximated by rapid IV injection
Describe zero-order rate of absorption
Means the absorption rate is constant for some defined period of time. e.g. constant rate IV infusion/ oral administration (due to no absorption in stomach and constant ‘infusion’ from stomach to small intestine). Many drugs are absorbed at the same rate because it is the gastric emptying not the drug that determines the absorption into the blood
Describe first-order rate of absorption
Means the absorption rate is proportional to the amount of drug at the absorption site. Typically higher immediately after the dose is given and the rate then decreases as the drug is absorbed.
What is the rate limiting step in oral administration
Stomach emptying time or slow release formulation
What is the formula for first order absorption
t1/2= 0.7/KA
KA is the specific drugs rate constant.
0.7 is the natural log of 2 whatever the fuck that means.
How can the absorption half life be calculated?
Using the rate constant (KA) and the natural log of 2 (0.7 duh)
When is peak concentration in a first order absorption
When rate of absorption is equal to rate of elimination
How many half life does it take for more than 90% complete absorption for all drugs
4
What is the key clinical implication for the extent of absorption?
IV/ oral dose conversion
What is the key implication for understanding rate of absorption
Knowing when peak concentration will be, which may be a good predictor of peak effect.
What is the relevance of generic medicines and rate/extent of absorption?
The new medicine can substitute the original if its shown to have the same rate and extent of absorption.
