Instantaneous absorption models special cases

Special Cases of the One-Compartment Bolus IV (Instantaneous Absorption) Model... 

There are several common special case situations in which the two-compartment bolus IV model can be applied as a reasonable approximation for administration routes other than bolus IV injection. The means by which the two-compartment bolus IV model can be employed in these situations is described in this section. The criteria under which a drug following two-compartment instantaneous absorption can be approximated by a one-compartment instantaneous absorption model are also provided. 

Three special cases are considered for the one-compartment first-order absorption model. Eirst is a relatively rare situation known as a flip-flop situation. Second is the use of the one-compartment first-order absorption model to approximate the plasma concentrations of drugs that follow two-compartment kinetics. The last case considered is the identification of conditions when first-order drug delivery with rapid absorption can be modeled as an instantaneous absorption process. 

Linear (or first-order) kinetics refers to the situation where the rate of some process is proportional to the amount or concentration of drug raised to the power of one (the first power, hence the name first-order kinetics). This is equivalent to stating that the rate is equal to the amount or concentration of drug multiplied by a constant (a linear function, hence linear kinetics). All the PK models described in this chapter have assumed linear elimination (metabolism and excretion) kinetics. All distribution processes have been taken to follow linear kinetics or to be instantaneous (completed quickly). Absorption processes have been taken to be instantaneous (completed quickly), follow linear first-order kinetics, or follow zero-order kinetics. Thus out of these processes, only zero-order absorption represents a nonlinear process that is not completed in too short of a time period to matter. This lone example of nonlinear kinetics in the standard PK models represents a special case since nonlinear absorption is relatively easy to handle mathematically. Inclusion of any other type of nonlinear kinetic process in a PK model makes it impossible to write the... 

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