Basic Pharmacokinetic Principles Flashcards
pharmacokinetics
the actions the body takes on a drug
can be defined in terms of volumes and clearances
pharmacodynamics (PD)
the action a drug has on the body
volume of distribution
that volume in which a substance/marker/drug appears to be homogeneously distributed (dissolved)
an apparent volume of distribution
volume of distribution = amount of substance/concentration
more accurately described as plasma equivalent apparent volume of distribution
the hydraulic model
can be used to compare to drug distribution int he body
- water soluble substances travers aqueous channels between compartments
aqueous channels vary in size
Describe the body’s water distribution.
first law of pharmacokinetics
amount of drug in body = concentration * volume of distribution
clearance
a term used to describe the time-dependent movement of drug
includes intercompartmental clearance and elimination clearance
rate of elimination = clearance * concentration
intercompartmental clearance
describes the movement of drug between compartments
elimination clearance
describes the metabolic alteration or removal of drug from the blood/body
rate constant (k) = clearance (liters/min)/volume(liters)
time constant (minutes) = volume (liters)/clearance(liters/min)
elimination clearance rate constant
the fraction or percentage of the volume that is cleared per unit time (minutes)
T(1/2) = 0.693/k
elimination clearance time constant
the time it takes for about 63% of the volume to be cleared
95% of a substance is cleared in three time constants
extraction ration
extraction ratio = clearance/flow
renal clearance
that part of the volume of distribution from which a substance is irreversibly removed
creatinine clearance = (U x V)/P
U = concentration of creatinine in the urine
V = time measured collection of urine (urine production rate)
P = concentration of creatinine in the plasma
second law of pharmacokinetics
target (steady state) concentration = infusion rate/clearance
clearance vs. elimination rate
clearance is a physiologic process assumed not to change over the course of observation
elimination rate is constantly changing due to the concentration of the parent drug constantly decreasing as a result of drug being continuously eliminiated from the system