The commonest of all multicompartment models is a two-compartment model.
TWO-COMPARTMENT OPEN MODEL
The commonest of all multicompartment models is a
two-compartment model. In such a model, the body tissues are broadly classified
into 2 categories –
1. Central Compartment or Compartment 1 comprising of blood and
highly perfused tissues like liver,
lungs, kidneys, etc. that equilibrate with the drug rapidly. Elimination
usually occurs from this compartment.
2. Peripheral or Tissue Compartment or
Compartment 2 comprising of poorly perfused
and slow equilibrating tissues such as muscles, skin, adipose, etc. and
considered as a hybrid of several functional physiologic units.
Classification of a particular tissue, for example
brain, into central or peripheral compartment depends upon the physicochemical
properties of the drug. A highly lipophilic drug can cross the BBB and brain
would then be included in the central compartment. In contrast, a polar drug
cannot penetrate the BBB and brain in this case will be a part of peripheral
compartment despite the fact that it is a highly perfused organ.
The plasma concentration for a drug that follows a
two-compartment model declines biexponentially as the sum of two first-order
processes – distribution and elimination.
Depending upon the compartment from which the drug
is eliminated, the two-compartment model can be categorized into 3 types:
1. Two-compartment model with
elimination from central compartment.
2. Two-compartment model with
elimination from peripheral compartment.
3. Two-compartment model with
elimination from both the compartments.
In the absence of information, elimination is
assumed to occur exclusively from central compartment.
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