Zero-order absorption models assumptions

Zero-order absorption occurs when drug enters the systemic circulation at a constant rate. An IV infusion, in which a drug solution is delivered directly into the systemic circulation at a steady flow rate, represents an idealized zero-order absorption case. Because of this, standard zero-order absorption models are typically called IV infusion models and are designed specifically for the IV infusion case. This particular section deals with the one-compartment IV infusion model, so as in the previous one-compartment bolus IV model, the body is modeled as a single compartment with the implication that the distribution process is essentially instantaneous. As with the other standard models, the exact meaning of the assumptions inherent in this model are described next. Model equations then are introduced that allow the prediction of plasma concentrations for drugs with known PK parameters, or the estimation of PK parameters from measured plasma concentrations. Modification of the one-compartment IV infusion (zero-order absorption) model to approximate other types of steady drug delivery are described in Section 10.8.5. 

The standard one-compartment IV infusion (or zero-order absorption) model makes three inherent assumptions about the ADME processes that occur during and after drug delivery ... 

This assumption is the same for aU zero-order absorption models. See Section 10.8.1.1 for the details regarding this assumption. 

The use of a two-compartment model with zero-order absorption results in five inherent model assumptions. The specific nature and implications of each of these assumptions are described in this section. 

Multicompartment model equations can be written for instantaneous absorption, zero-order absorption, or first-order absorption. For any of these particular absorption situations, the assumptions described previously for the corresponding absorption in one- and two-compartment models remains exactly the same for multicompartment models. 

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