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What are three important pharmacokinetic parameters?

1. volume of distribution

2. clearance

3. bioavailability


What is volume of distribution and how is it measured?

Relates the amount of drug in the body to the concentration of drug in the plasma

Vd = amount of drug in body over plasma drug concentration


What does Vd (volume of distribution) represent?

Vd is the volume that would be required to contain all of the drug in the body at the same concentration as it is in the blood.


What does a low Vd mean?

high in the vascular and low in the extravascular


what does a high Vd mean?

low in the vascular and high in the extravascular


What is the main use of the volume of distribution?

The main use of the volume of distribution is to determine the loading dose to quickly achieve a target plasma concentration.


How to determine Vd?

Vd = dose over C0

C0 = concentration at time zero


What are the two phases of a Vd graph?

The rapid fall phase = distribution

The slower phase = elimination


What is clearance defined as?

defined as the volume of blood cleared of drug per unit time

clearance predicts the rate of elimination of a drug in relation to the drug concentration


What are the two major sites of drug elimination

kidneys and the liver


What is first-order elimination?

For a drug eliminated with first-order kinetics, CL is a constant, ie, the ratio of the rate of elimination to the plasma concentration is the same regardless of plasma concentration


why do most drugs follow first- order elimination?

This occurs because the physiological mechanisms of drug elimination (enzymes and transporters) are not

the plasma concentration is below the Km


What is half-life?

Half-life determines the rate at which blood concentration rises during a constant infusion and falls after infusion is stopped

at steady state


During a constant IV infusion of a drug (half-life)

50% of Css is reached after 1 half-life
• 75% after 2 half-lives
• 87.5% after 3 half-lives
• 93.75% after 4 half-lives

vise versa for elimination


how to change the Css?

2x the infusion rate


What factors impact half-life

obesity = increases

pathologic fluid = increases

aging = increased

cardiac, liver, renal failure = increase

CYP induction = decreases

CYP inhibition = increases


Why do some drugs show saturation kinetics?

Drug metabolism and tubular secretion are
saturable processes.

When drug concentration exceeds Km, nonlinear
kinetics is observed


Which drugs show saturation kinetics?

aspirin at high doses




properties of saturation kinetics

elimination is zero order

a constant amount of drug is eliminated per unit time

The rate of elimination is maximal and independent of drug concentration


What does a saturation kinetics graph look like?



What is not applicable with saturation kinetics?

the concept of “4 half-lives to steady state” is NOT applicable for drugs with nonlinear elimination kinetics