# volume of distribution

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**Concept of Volume of Distribution**

**Distribution volumes:**

**Body water divided into : vascular, interstitial and**

intracellular fluid.

intracellular fluid.

**Concept of Volume of Distribution**

**The real distribution volume of a drug is related to**

body water and can never exceed body water.

body water and can never exceed body water.

**..**

**!!**

**Concept of Volume of Distribution**

**Fluid**

**Volume (L)**

**Volumes**

Plasma

3 - 4

**measured by**

Interstitial

10 - 13

**various test**

Intracellular

**materials**

25 - 28

Total body water

40 - 46

**It is possible for Vd to be close to a recognizable**

volume. This would happen only if the drug is

uniformly

volume. This would happen only if the drug is

uniformly

**distributed**

**in**

**one**

**of**

**these**

**"compartments", but this is rare.**

**Concept of Volume of Distribution**

**In beaker 1, the conc.**

**Dose = 10 mg**

**throughout the beaker is the**

same and the Volume is the

same as the size of the

beaker.

same and the Volume is the

same as the size of the

beaker.

**Conc. = 10 mg/L**

**Conc. = 2 mg/L**

**In beaker 2, after a rapid**

equilibrium, distribution b/n

the solution and the charcoal

may be complete. However,

drug conc. within the beaker

is not uniform. Much of the

drug

equilibrium, distribution b/n

the solution and the charcoal

may be complete. However,

drug conc. within the beaker

is not uniform. Much of the

drug

**is**

**held**

**with**

**the**

**charcoal**

**leaving**

**smaller**

**Solution**

**Charcoal**

**conc. in the solution. After**

**(Plasma)**

**(tissues of the body) measuring the drug conc. in**

**The volume of solution is the same in both**

**the solution, the Volume is**

**beakers = 1 L**

**much larger.**

**Concept of Volume of Distribution**

**Volume of distribution :**

**The apparent volume of distribution is the**

theoretical volume of fluid (not only plasma) into

which the total drug administered would have to

be diluted to produce the concentration in

plasma.

theoretical volume of fluid (not only plasma) into

which the total drug administered would have to

be diluted to produce the concentration in

plasma.

**Volume of distribution has nothing to do with**

the actual volume of the body or its fluid

compartments

the actual volume of the body or its fluid

compartments

**but**

**rather**

**involves**

**the**

**distribution of the drug within the body.**

**Concept of Volume of Distribution**

**For drugs that are highly tissue-bound,**

comparatively little of a dose remains in the

circulation to be measured; thus, plasma

concentration is low and volume of distribution

is high. Drugs that remain in the circulation tend

to have a low volume of distribution.

comparatively little of a dose remains in the

circulation to be measured; thus, plasma

concentration is low and volume of distribution

is high. Drugs that remain in the circulation tend

to have a low volume of distribution.

**Volume of distribution provides a reference for**

the plasma concentration expected for a given

dose. Each drug is uniquely distributed in the

body. Some drugs distribute mostly into fat,

others remain in ECF, and others are bound

extensively to specific tissues.

the plasma concentration expected for a given

dose. Each drug is uniquely distributed in the

body. Some drugs distribute mostly into fat,

others remain in ECF, and others are bound

extensively to specific tissues.

**Concept of Volume of Distribution**

**Acidic drugs (eg, warfarin, aspirin) are highly**

protein-bound and thus have a small volume of

distribution.

protein-bound and thus have a small volume of

distribution.

**Basic drugs (eg, amphetamine, meperidine) are**

extensively taken up by tissues and thus have a

volume of distribution larger than the volume of

the entire body.

extensively taken up by tissues and thus have a

volume of distribution larger than the volume of

the entire body.

**Volume of distribution is a volume into which a drug**

**appears**

**to distribute with a concentration equal to its**

plasma concentration. A parameter used to define the

process of distribution and called "the apparent volume

of distribution (V )".

plasma concentration. A parameter used to define the

process of distribution and called "the apparent volume

of distribution (V )".

**d**

**Concept of Volume of Distribution**

**How Vd is measured ?**

**100**

**dose**

**1000 mg**

**g/L)**

**Extrapolate to**

**Vd find**

**= C = 50 mg/L**

**=**

**0 C**

**50 mg/L**

**0**

**(m**

**50**

**c.**

**Dose**

**= = 1 g**

**20 L**

**40**

**30**

**Plasma Con**

**20**

**Log**

**10**

**0**

**1**

**2**

**3**

**4**

**5**

**6**

**Time**

**20 L**

**=**

**x 100 = 28.6%**

**70 kg**

**Concept of Volume of Distribution**

**How Vd is measured ?**

**100**

**dose**

**1000 mg**

**g/L)**

**Extrapolate to**

**Vd find**

**= C = 50 mg/L**

**=**

**0 C**

**50 mg/L**

**0**

**(m**

**50**

**c.**

**Dose**

**= = 1 g**

**20 L**

**40**

**30**

**Plasma Con**

**20**

**Log**

**10**

**for a 70 kg subject, the Vd**

**0**

**1**

**2**

**3**

**4**

**5**

**6**

**Time**

**in percentage of body wt is:**

**C = 50 mg/L**

**0**

**20 L**

**=**

**x 100 = 28.6%**

**70 kg**

**Concept of Volume of Distribution**

**Schematic representation of the**

**one-compartment model**

**Volume = Vd**

**i.v.**

**Elimination**

**dose**

**Plasma Concentration = C**