Types of Agents

Contrast agents are classified according to osmolality (T.a.,ble,,2.9.4-.1...). High-osmolality agents have been in clinical use since the 1950s, and they are often called ionic contrast agents because they are all organic salts ( Fig 294-1). Low-osmolality agents have been in use since 1986, and they are commonly called nonionic agents because all but one are organic molecules without electronic charge ( Fig 2.9.4.-2.). A number of studies have demonstrated that low-osmolality contrast agents are associated with at least a 50 percent reduction in the incidence of both minor and severe reactions ( ,T.a.b.l.e.,2.9,4.-.,1,). Anecdotally, injection of low-osmolality agents is associated with less subjective effects, such as flushing, heat, chemical taste, and osmotic diuresis. Currently in the United States, low-osmolality contrast agents cost on average 8 to 10 times more than an equivalent dose of a high-osmolality agent. The use of low-osmolality contrast agents is surrounded by some controversy due to the significant cost differential for a relatively modest difference in the incidence of severe reactions, which are rare.

TABLE 294-1 Comparison of High- and Low-Osmolality Contrast Agents

Rl 1

R2 I

FIG. 294-1. The general chemical structure of the anion of a high-osmolality contrast agent. The R groups are either diatrizoate or iothalamate, depending on the manufacturer. The corresponding cation is either sodium or meglumine.

RI 1

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