Contrast media as extracellular fluid space markers: adaptation of the central volume theorem

We present some theoretical observations on the pharmacokinetics of the conventional intravascular radiographic and gadolinium chelate MR contrast agents. These agents may be viewed from a physiological perspective as extracellular fluid (ECF) space markers. It can be shown that the passage of such agents through the body may be described by a variant of the "central volume theorem", if the central volume is taken to be that of the entire extracellular fluid. A consequence is that, in a manner akin to nuclear medicine techniques utilizing technetium-99m diethylene triamine pentacetic acid (DTPA), measures of glomerular filtration rate (GFR) related to total ECF volume rather than to the normal body surface area may be obtained from direct or indirect measurements of iodine or gadolinium plasma levels. Typical measurements are presented. A simple means is also described of generating curves of the fraction of contrast medium excreted from the body, applicable to both patients with normal and with abnormal renal function.