# Liv 52

By A. Eusebio. University of Pittsburgh at Bradford. 2018.

The negative slope of the residual line is referred to as alpha (α) discount liv 52 100 ml without prescription, and α is the distribution rate constant for the two-compartment system order 200 ml liv 52 mastercard. A dose of drug is administered by rapid intravenous injection buy liv 52 60 ml free shipping, and the concentrations shown in Table 6-1 result. The last four points form a straight line, (similar to Figure 6-5) so back-extrapolate a line that connects them to the y-axis. Then, for the first five points, extrapolated values can be estimated at each time (0. Subtracting the extrapolated values from the actual plasma concentrations yields a new set of residual concentration points, similar to those values shown in Table 6-2. Plot the residual concentrations (on the same semilog paper) versus time and draw a straight line connecting all of your new points (similar to Figure 6-7). Note that α must be greater than β, indicating that drug removal from plasma by distribution into tissues proceeds at a greater rate than does drug removal from plasma by eliminating organs (e. Plasma drug concentrations with a two-compartment model after an intravenous bolus dose. For a one-compartment model (Figure 6-8), we know that the plasma concentration (C) at any time (t) can be described by: -Kt Ct = C0e (See Equation 3-2. The equation is called a monoexponential equation because the line is described by one exponent. The two-compartment model (Figure 6-9) is the sum of two linear components, representing distribution and elimination (Figure 6-10), so we can determine drug concentration (C) at any time (t) by adding those two components. Therefore: -αt -βt Ct = Ae + Be This equation is called a biexponential equation because two exponents are incorporated. For the two-compartment model, different volume of distribution parameters exist: the central compartment volume (Vc), the volume by area (Varea, also known as Vβ), and the steady-state volume of distribution (Vss). As in the one-compartment model, a volume can be calculated by: For the two-compartment model, this volume would be equivalent to the volume of the central compartment (Vc). The Vc relates the amount of drug in the central compartment to the concentration in the central compartment. If another volume (Varea or Vβ) is determined from the area under the plasma concentration versus time curve and the terminal elimination rate constant (β), this volume is related as follows: This calculation is affected by changes in clearance (Cl). The Varea relates the amount of drug in the body to the concentration of drug in plasma in the post-absorption and post-distribution phase. Although it is not affected by changes in drug elimination or clearance, it is more difficult to calculate. One way to estimate Vss is to use the two-compartment microconstants: or it may be estimated by: using A, B, α, and β. Because different methods can be used to calculate the various volumes of distribution of a two- compartment model, you should always specify the method used. When reading a pharmacokinetic study, pay particular attention to the method for calculating the volume of distribution. Clinical Correlate Here is an example of one potential problem when dealing with drugs exhibiting biexponential elimination. Recall that A steeper slope equals a faster rate of elimination resulting in a shorter half-life. If a terminal half-life is being calculated for drugs such as vancomycin, you must be sure that the distribution phase is completed (approximately 3-4 hours after the dose) before drawing plasma levels. Plasma drug concentrations with a one-compartment model after an intravenous bolus dose (first-order elimination). Plasma drug concentrations with a two-compartment model after an intravenous bolus dose (first-order elimination). The plasma drug concentration versus time curve for a two- compartment model is represented by what type of curve?