Pharmacokinetics of contrast media: experimental results in dog and man with CT implications

The pharmacokinetics of contrast media such as sodium iocarmate, sodium ioxithalamate, metrizamide, sodium/meglumine diatrizoate, and sodium ioxaglate (a recently introduced medium: a hexaiodinated monoacid dimer) was compared in 25 dogs. A biphasic phenomenon was observed due to the rapid diffusion of contrast media from plasma into tissue (interstitium) followed by slow urinary excretion. A bicompartmental analysis was performed in dogs for each agent, showing that the tissue distribution of ioxithalamate is higher than that of other media. Sodium/meglumine ioxithalamate, sodium iocarmate, and sodium/meglumine ioxaglate were also compared in 20 human subjects. The diffusion and excretion phases observed in man appear to be slower than in the dog. Significant variations of iodine plasma concentration from one patient to another were recorded for the same medium and at the same interval after injection. Significant differences were observed between ioxithalamate and iocarmate or ioxaglate plasma concentrations due to the greater tissue diffusibility of ioxithalamate. The mechanisms affecting contrast media diffusibility are discussed: osmolarity, liposolubility, protein binding, and molecular size. Variations in contrast medium concentration in plasma noted in different patients with the same medium are explained by variations in tissue distribution, contrast medium volume, patient age, and patient hydration. Some computed tomographic (CT) implications of these pharmacokinetic studies are discussed. The need for highly diffusible media in routine CT and for less diffusible media in CT angiography is emphasized.