The apparent volume of distribution V

Concentrations (mass per unit volume or amount per unit volume), not masses (mg or pg), are usually measured in plasma or serum (more often than blood). Therefore, a term is needed to relate the measured concentration (Cp) at a time to the mass of drug (X) at that time. This term is defined as the apparent volume of distribution (V). Please note that the apparent volume of distribution (V) is simply a proportionality constant whose sole purpose is to relate the plasma concentration (Cp) and the mass of drug (X) in the body at a time. It is not a physiological volume. 

Beakers A and B contain equal but unknown volumes of water. 

Only beaker B contains a small quantity of charcoal (an adsorbing agent). 

Let us assume that we add 1 g of potassium iodide (KI), which is soluble in water, to each beaker. 

Using a suitable analytical procedure, the concentration ( j,gmL ) of potassium iodide in each beaker is determined. 

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The plasma drug concentration reached after distribution is complete is a result of the dose and the extent of uptake by tissues. The extent of distribution can be described by relating the amount of drug in the body to the concentration. This parameter is known as the volume of distribution. This volume does not indicate a defined physiological dimension but the volume of fluid required to contain all the drug in the body at the same concentration as in the plasma or blood. Therefore, it is often called the apparent volume of distribution (V) and is determined at steady state when distribution equilibrium has been reached between drug in plasma and tissues. 

Following drug absorption, individual drugs distribute throughout the body in a characteristic manner. The apparent volume of distribution (V ) is the quantity of drug in the body (L/kg body weight) divided by the plasma concentration at steady state. Volume distribution also represents the amount of water that is needed for a drug to dissolve to reach an... 

Changes in body composition seen throughout infancy and childhood have an impact on the apparent volume of distribution (V). Newborn infants have higher total body water on a L/kg basis than older populations therefore, drugs distributed to water have higher V values in newborns when compared to older children... 

Apparent volume of distribution The apparent volume of distribution (V ) is an important pharmacokinetic parameter that reflects the above determinants of drug distribution in the body. relates the amount of drug in the body to the concentration in the plasma. (See Chapter 3 and Table 1-2.)... 

After intravenous dosing, atropine distributes rapidly, with only 5% remaining in the blood compartment after 5 min (Bcrghem et at., 1980). The apparent volume of distribution (V i) is 1-1.7 liters/kg. Atropine is partly metabolized in the liver by microsomal monooxygenases to noratropine, tropinc, atropine-A/-oxide, and tropic acid (Van der Meer et at., 1983) and partly excreted unchanged in the urine. Elimination kinetics can be fitted to a two-compartment model with a clearance of 5,9-6,8 ml/kg/min and a half-life of 2.6—4.3 hr in the elimination phase (Aaltonen etal., 1984 Kanto etal., 1981 Virtanen etal., 1982). Since the renal plasma clearance (656 18 ml/min) was found to approach the renal plasma flow (712 38 ml/min), tubular... 

Please note that one cannot obtain the apparent volume of distribution (V) from urinary excretion data. 

Determination of the apparent volume of distribution V or Vd) and fraction of drug absorbed F)... 

Please note that the elimination half (ti/a), the elimination rate constant (K), the apparent volume of distribution (V) and the systemic clearance (Cls) of a drug are independent of the route... 

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 . ... 

Pharmacokinetic parameters, such as elimination half life (ti/2), the elimination rate constant (K), the apparent volume of distribution (V) and the systemic clearance (Cl) of most drugs are not expected to change when different doses are administered and/or when the drug is administered via different routes as a single or multiple doses. The kinetics of these drugs is described as linear, or dose-independent, pharmacokinetics and is characterized by the first-order process. The term linear simply means that plasma concentration at a given time at steady state and the area under the plasma concentration versus time curve (AUC) will both be directly proportional to the dose administered, as illustrated in Fig. 15.1. 

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