The chemistry of low-osmolality
contrast agents is reviewed, the effects of these agents on vascular and organ physiology are compared with the effects of conventional ionic
contrast media, and guidelines for intravascular use of the low-osmolality agents in selected high-risk patients are presented. Three low-osmolality
contrast agents, the nonionic media
iohexol (
Omnipaque, Winthrop-Breon) and
iopamidol (
Isovue, Squibb) and the dimeric medium
ioxaglate meglumine-
sodium (
Hexabrix, Mallinckrodt) have recently been introduced into the
contrast-media market. Compared with conventional ionic
contrast media, these new agents demonstrate approximately one third of the osmolality per given
iodine concentration (degree of roentgenographic opacification). Therefore, the risks of hyperosmolarity-induced reactions to
contrast media are lower with the new agents. The low-osmolality agents may be associated with a reduced incidence of
contrast-media-induced
hypersensitivity reactions. Because of their lower osmolality, these agents produce less vessel dilation, vascular endothelial damage, and associated
pain and discomfort than equi-
iodine concentrations of the conventional ionic media. They also demonstrate a reduction in the incidence and severity of
contrast-media-induced renal vasoconstriction and
proteinuria, hemodynamic alterations, negative chronotropic effects, depression of myocardial contractility, and neurotoxicity in the presence of an altered blood-brain barrier. These low-osmolality agents produce fewer undesirable physiological effects than conventional
contrast agents, but the cost of the new products can be more than 10 times as great. Therefore, the new products should be used selectively in patients known to be at increased risk for reactions to intravascular
contrast media. A scoring system was developed to permit rapid recognition of documented single or multiple risk factors and subsequent determination of whether to administer a low-osmolality agent.