Continuous intravenous infusion constant rate

With continuous intravenous infusion, the rate of drug entry into the body is constant. In the majority of cases, the elimination of a drug is first-order, that is, a constant fraction of the agent is cleared per unit time. Therefore, the rate of drug exit from the body increases proportionately as the plasma concentration increases and at every point in time is proportional to the plasma concentration of the drug. 

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 ... 

Trimethaphan camsylate is a ganglionic-blocking drug that inhibits both sympathetic and parasympathetic autonomic activities. It has a rapid onset and brief duration of action and must be admiifistered by continuous intravenous infusion with constant monitoring of blood pressure. Trimethaphan camsylate is particularly useful in aortic dissection because it can be titrated carefully to permit smooth control of blood pressure and because it decreases cardiac output and left ventricular ejection rate. Tachyphylaxis... 

Figure 10.5 Scheme for constant input rate, continuous intravenous infusion. Q, constant input or infusion rate X, mass (amount) of drug in the blood or body at time f X , mass of unchanged drug in urine at time t K, first-order elimination rate constant. 

An intravenous infusion involves a continuous flow of drug into a patient at a rate defined by the infusion rate constant, Rini, with units of mass/time. Discussions of infusion normally present the infusion rate constant as inf, which may be confused with a true reaction rate constant. Therefore, this presentation of infusion uses a less ambiguous variable, Rmi, for the infusion rate constant. 

Following an intravenous injection of 10 mg propranolol, McAllister (1976) found the values of the elimination rate constant (K) and the apparent volume of distribution (V) to be 0.00505 0.0006min and 295 SSL, respectively (mean SD for six patients). These values were then used to calculate the loading dose (I ) and infusion rate (Q) necessary to instantly obtain and then continuously maintain propranolol plasma concentrations of 12, 40 and 75 ngmL . ... 

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