In most nonemergent fluoroscopic studies of the gastrointestinal tract, oral or rectal barium sulfate contrast material is preferred over iodinated contrast material (e.g., Gastrografin, Gastroview, Hypaque) because barium demonstrates the mucosa in greater detail and is less susceptible to dilution. However, iodinated contrast agents are indicated in many emergent situations in which barium contrast is contraindicated: (1) suspected or potential bowel perforation (including trauma or abscess), (2) administration before gastrointestinal surgery or endoscopy, and (3) evaluation of the position of percutaneously placed bowel catheters. Iodinated contrast agents are appropriate in these situations because they are rapidly reabsorbed from the peritoneal and interstitial spaces. 4 If a fluoroscopic study with iodinated contrast material does not demonstrate a suspected perforation, many radiologists repeat the study using barium to look for small leaks, which are often more apparent with barium than with iodinated contrast agents.
For gastrointestinal studies involving iodinated contrast material, low-osmolality agents are indicated in the following situations: (1) oral administration in patients who are at risk for aspiration, because contrast-induced pulmonary edema is less with low-osmolality agents; and (2) patients with fluid or electrolyte imbalances, because low-osmolality agents cause less dilution and fluid shifts into the bowel lumen. 4
Following oral or rectal administration, a small amount of iodinated contrast agent (approximately 1 to 2 percent) is absorbed by the bowel and excreted into the urine.4 Therefore, the risk of developing an anaphylactoid contrast reaction is theoretically the same as it is for intravascular administration, and the same preprocedural evaluation and precautions apply. However, moderate or severe contrast reactions are only very rarely reported following oral or rectal administration of iodinated contrast agents. Therefore, the actual risk may be lower than that for intravascular administration. These comments are also applicable to administration of iodinated contrast material into the bladder or male urethra for the evaluation of trauma. In such examinations, it is likely that absorption of the contrast agent is even less than in the gastrointestinal tract.
The administration of oral or rectal contrast material is important in a number of indications for abdominal CT. Unlike fluoroscopic studies, gastrointestinal CT studies entail no significant difference in diagnostic quality between barium and iodinated contrast agents because only very dilute gastrointestinal contrast material is required. Therefore, high-osmolality iodinated contrast agents may be routinely used for emergent CT evaluation of the gastrointestinal tract.
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