Medical Pharmacology – Chapter 7 – Time-response relationships
and dosage schemes
Dosis; determines the intensity (=strength) of effects
The concentration of drug is not constant over time and
concentration curve depends on rate of resorption, distribution
and rate of elimination.
Elimination rate; determines the length of time a known amount of
drug (= dose) remains in the body and thereby its duration of
action is known
Elimination according to:
First-order kinetics (is most cases!); rate of elemination at
any time is proportional to the drug concentration (in the
plasma) at that time
Concentration decrease over a defrined time-interval will be a constant
percentage (fraction) of the concentration at start of the interval
There is a half order. In normal scale curved line. In log scale straight
line
Zero-order kinetics: rate of elimination is independent of drug
concentration.
Concentration decrease will be a fixed amount per unit of time
There is no half order. In normal scale straight line
Few drugs (aspirin, ethanol, phenytoin) with large doses --> plasma drug
concentration is much greater than the Km --> drug metabolism is zero order
Lot of drugs with smaller doses --> plasma drug concentration is less than
Km --> drug metabolism is first order
Graph build up as the following
1. Absorption phase (only by oral administration) (going up)
2. Distrubtion phase (first part going down)
3. Elimination phase (last part going down)
Determining the half time (time needed to halve the
concentration of the substance being eliminated in the plasma)
Only look to the part of the elimination phase. Then determine
the half order
Apparent volume of distrubtion (Vd);
Liquid volume over which the drugs seems to spread.
Vd= amount of drug in the body (dose) / concentration in the blood in L or
L/kg
Vd= intravasculair administered dose A / concentration in the plasma (A)
at t=0 (determined by extrapolation of plasma concentration values to t=0)
(trek de lijn van elimination fase door wat er dan op t=0 staat moet je
pakken zie evt. lecture 5) in L or L/kg
and dosage schemes
Dosis; determines the intensity (=strength) of effects
The concentration of drug is not constant over time and
concentration curve depends on rate of resorption, distribution
and rate of elimination.
Elimination rate; determines the length of time a known amount of
drug (= dose) remains in the body and thereby its duration of
action is known
Elimination according to:
First-order kinetics (is most cases!); rate of elemination at
any time is proportional to the drug concentration (in the
plasma) at that time
Concentration decrease over a defrined time-interval will be a constant
percentage (fraction) of the concentration at start of the interval
There is a half order. In normal scale curved line. In log scale straight
line
Zero-order kinetics: rate of elimination is independent of drug
concentration.
Concentration decrease will be a fixed amount per unit of time
There is no half order. In normal scale straight line
Few drugs (aspirin, ethanol, phenytoin) with large doses --> plasma drug
concentration is much greater than the Km --> drug metabolism is zero order
Lot of drugs with smaller doses --> plasma drug concentration is less than
Km --> drug metabolism is first order
Graph build up as the following
1. Absorption phase (only by oral administration) (going up)
2. Distrubtion phase (first part going down)
3. Elimination phase (last part going down)
Determining the half time (time needed to halve the
concentration of the substance being eliminated in the plasma)
Only look to the part of the elimination phase. Then determine
the half order
Apparent volume of distrubtion (Vd);
Liquid volume over which the drugs seems to spread.
Vd= amount of drug in the body (dose) / concentration in the blood in L or
L/kg
Vd= intravasculair administered dose A / concentration in the plasma (A)
at t=0 (determined by extrapolation of plasma concentration values to t=0)
(trek de lijn van elimination fase door wat er dan op t=0 staat moet je
pakken zie evt. lecture 5) in L or L/kg
