One-compartment open model for continuous intravenous infusion

4 One-compartment open model for continuous intravenous infusion 

In the previous discussion of the one- and two-compartment models we have loaded the system with a single-dose D at time zero, and subsequently we observed its transient response until a steady state was reached. It has been shown that an analysis of the response in the central plasma compartment allows to estimate the transfer constants of the system. Once the transfer constants have been established, it is possible to study the behaviour of the model with different types of input functions. The case when the input is delivered at a constant rate during a certain time interval is of special importance. It applies when a drug is delivered by continuous intravenous infusion. We assume that an amount Z) of a drug is delivered during the time of infusion x at a constant rate (Fig. 39.10). The first part of the mass balance differential equation for this one-compartment open system, for times t between 0 and x, is given by  

The solution of the first part of the model is obtained by straightforward integration  

If the infusion is maintained for a sufficiently long time x, one obtains the condition for a steady-state plasma concentration which no longer changes with time. The condition follows immediately from eq. (39.34), by setting rto infinity  

Note that the steady-state plasma concentration varies proportionally with the rate of infusion and inversely with the plasma clearance C/p, the definition of 

---

Fig. 39.10. (a) One-compartment open model for continuous intravenous infusion of a dose D from a reservoir. The rate of infusion is k. (b) Time courses of the content in the reservoir Xn of the plasma concentration Cp and of the content of the elimination pool... 

---