The steady state is the condition when the rate of drug administration is equal to the rate of drug elimination.
When the drug is administered as a constant rate IV infusion, there will be one steady state concentration as long as the rate of administration and the rate of elimination do not change.
The time required to achieve steady state is dependent on the drug half life. It takes about 5-6 half lives to achieve steady state. The steady state concentration during constant rate iv infusion is directly proportional to the infusion rate and inversely proportional to the total body clearance of the drug.
The total body clearance is the volume of the plasma or blood which is completely cleared from the drug per unit time. It has units of volume/time.
The CLT for a drug is constant within a patient (dose and concentration independent) when the elimination processes follow first-order kinetics.
The total body clearance is a measure of the efficiency of all eliminating organs in eliminating the drug and it is the sum of all organ clearances (i.e. CLT is the sum of the renal clearance, hepatic clearance and all other organ clearances).
The elimination rate constant and the half life (the dependent pharmacokinetic parameters) are dependent on (is determined from) the total body clearance and the volume of distribution (the independent pharmacokinetic parameters).
CLT
Vd = k andCLT
Vd =0.693
t 1/2The metabolite total body clearance is the volume of the plasma or blood which is completely cleared from the metabolite per unit time. It has units of vloume/time.
The CLT(m) for the metabolite is constant within a patient (dose and concentration independent) when its elimination processes follow first-order kinetics.
The CLT(m) is a measure of the efficiency of all eliminating organs in eliminating the metabolite.
The metabolite elimination rate constant and the metabolite half life (the dependent pharmacokinetic parameters) are dependent on (is determined from) the metabolite total body clearance and the metabolite volume of distribution (the independent pharmacokinetic parameters).
CLT(m)
Vd(m) = k andCLT(m)
Vd(m) =0.693
t 1/2(m)Cpaverage ss is the average drug concentration at steady state.(amount/volume).