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Can anybody help to understand difference between Vd(area) and Vd(ss)
When they are used in different situation?
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Steady-state volume of distribution of a drug (VdSS):
This is an estimate of drug distribution independent of elimination processes. It is most useful for predicting the plasma concentrations following multiple dosing to a steady-state or pseudo-equilibrium. VdSS is proportional to the amount of drug in the body versus the plasma concentration of the drug at steady state (pseudo-equilibrium).
Vdss= (D x F x AUMC)/ AUC2
Volume of Distribution Vd:
The volume of fluid that would be required to contain the amount of drug in the body if it were uniformly distributed at a concentration equal to that in the plasma. This value does not distinguish between a drug that is widely distributed and one with high affinity binding with restricted distribution. The volume of distribution is more properly termed the apparent volume of distribution as it does not correlate to any actual body volume.
A proportionality constant relating plasma concentration of a drug to the total amount of the drug in the body at any time after pseudo distribution equilibrium has been attained.
Vd is also equal to the total amount of drug in the body divided by the concentration of drug in the plasma.
The apparent volume of distribution Vd (mL/kg) can be calculated as:
Vdarea = (D x F) / AUC x (B270.3.w3)
(AUC = area under conc./time curve from t=0 to t= infinity; F = the bioavailability for a drug administered by a given route; D = the dose administered; = the slope of the terminal disappearance portion of the plasma concentration-time profile when plotted as a natural logarithm of concentration on the Y axis versus time on the X axis).
A less accurate but more straightforward calculation than Vdarea is VdB :
VdB = (D x F)/B (B270.3.w3)
(D is the administered dose, F is the bioavailability of the dose administered by that route, B (ml/L) is a value obtained by extrapolating the linear terminal phase of the drug disposition curve to its intercept on the ordinate (plasma drug concentration axis).
Note: VdB generally overestimates the true volume of distribution and Vd area is generally an overestimate relative to Vdss.
An approximation of the apparent volume of distribution can be calculated as:
Vd = D/Cmax
(D is the dose of drug administered, Cmax is the measured peak serum concentration of drug.
The volume of an individual's body into which a drug is apparently distributed. Expressed in litres per kilogram of bodyweight (L/kg).
Note that this volume is rarely a true physiological fluid or tissue space, but rather a mathematical constant relating the amount of drug in the body to its concentration in the plasma.
Amount of drug (mg) = distribution (L/kg) x plasma concentration (ug/ml)
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