Clearance Flashcards
What is renal clearance?
The volume of body fluid cleared of a substance per unit time. (L.hr / ml.min)
Why is renal clearance predicted?
To predict the method of elimination or metabolism of a drug and to predict its changes in plasma concentration over time.
What substance is used to measure GFR and which to measure RBF?
GFR uses creatinine
RBF uses PAH
What is clearance of any substance equal to?
Excretion rate / plasma concentration
What can be determined if renal clearance of a substance is 0?
Not filtered or excreted by the kidneys e.g. large proteins
What can be determined if renal clearance < GFR?
Filtered and partially reabsorbed
rate of being cleared < rate of filtration
What can be determined if renal clearance = GFR?
Filtered but not reabsorbed
rate of being cleared = rate of filtration
What can be determined if renal clearance GFR?
Filtered and partially secreted
rate of being cleared is < blood flow but >filration
What can be determined if renal clearance = RBF?
Filtered and secreted
What can be determined if renal clearance > RBF?
Produced in the kidney
What is whole body clearance?
The sum of organ clearance measured using the concentration changes of a substance over time.
What is the equation for whole body clearance using the curve and what does it show?
Cl = dose / Area in curve
If a substance is cleared QUICKLY the area under the curve is SMALL showing a LARGE clearance value.
What is the rate of removal proportional to and what equation is this shown by?
Rate of removal is proportional to concentration as long as Tm is not reached.
dC / dt = -kC
How is the concentration calculated using Cmax?
C = CmaxE ^ -kt
Concentration will fall exponentially with time.
How is half life calculated?
T1/2 = ln2 / k = 0.6931 / k
or
C = Cmax(1/2) ^no. of T1/2
What is Vd and clearance used to predict?
Cl = KVd (volume of plasma from which a substrate is completely cleared by kidney per unit time)
Predicts how the concentration of a substance will change in circulation overtime.
What is K?
The elimination rate constant
What piece of information is required to calculate a Pt’s Vd?
Weight as Vd is measured in L.Kg
What is meant by bioavailability?
The portion of a substance reaching systemic ciruclation UNCHANGED
Absorbed dose =
Dose x bioavailability
Vd in ECF (L) =
Vd of drug (L.kg) x mass (kg)
Cl of L.hr =
Cl of L.min x 60
Cpeak = (Peak concentration
Absorbed dose / Vd
K =
Clearance / Vd
Why are regular drug dose intervals used?
To allow the plasma concentration of the drug to fall before the next, to prevent accumulation.
How does a dose affect rate of Cl?
Clearance is independent to dose.
A higher dose will still be cleared within the same amount of time. Dose only affects the starting concentration.
After an overdose, when can the normal drug dose be resumed?
After the normal drugs interval time without causing accumulation, as the overdose concentration will have fallen to the resting concentration within this time. Body can eliminate any dose within the same period of time.
When is an overdose an issue for resuming normal drug intervals?
When pt has renal failure due to the reduced GFR meaning that the concentration of the drug fails to reach the resting concentration between each dose.
As excretion is slower, accumulation will occur.
For how long will accumulation of a drug occur in a pt with renal failure?
Until the drug reaches an equilibrium with the body fluids.
What needs to be altered about a drug for a pt with renal failure?
Need to increase the interval time between each dose.
