Angiographic contrast agents

Angiographic contrast agents

Contrast agents are complex organic compounds which are water soluble and have iodine atoms bound to a benzene ring. High-osmolality ionic agents are salts of tri-iodinated benzene derivatives with sodium or methylglucamine. The number of particles double while dissolving in water due to ionization. Hence they become hypertonic, with osmolality over six times that of blood. They generally have more adverse effects due to their high osmolality, which causes pain and warmth as well as other adverse responses like nausea and vomiting. High osmolality ionic contrast agents are not being used for angiography now.

Nonionic contrast is formed by replacing the carboxyl group of the benzene ring with a nonionizing side chain. Osmolality of non ionic agents are half that of ionic agents. Low osmolality contrasts have less nephrotoxicity. Both high osmolality and low osmolality contrasts produce similar radiographic opacification for equivalent number of iodine atoms. Low osmolality contrasts have six iodine atoms compared to three in conventional contrasts so that fewer particles are needed for a given iodine concentration and hence the osmolality is reduced by half.

Still later iso-osmolar contrasts were developed, with osmolality equal to that of plasma. They are used in cases of borderline renal function as they have less chance of leading to contrast induced acute kidney injury. Iodixanol, an iso-osmolar contrast is a non-ionic dimer composed of two covalently bound tri-iodinated benzene rings.

Osmolality of the contrasts cause shift of fluid from the extravascular compartment into the intravascular compartment causing initial volume expansion and slight drop in hematocrit. Hence hemodynamic measurements are better made before the administration of contrast agents. The enhanced volume is removed by renal excretion within a few minutes. Any delay in excretion will compromise the cardiac function in those with left ventricular dysfunction and may precipitate pulmonary edema. Osmotic diuresis which ensues later may reduce the intravascular volume later. In cyanotic patients in whom dehydration may cause thrombotic episodes, adequate pre-procedure hydration is required. Adequate pre-procedure hydration is also useful in reducing the risk of contrast induced acute kidney injury.