Carbon dioxide and air are the most frequently used negative contrast media. They may be used singly or in combination with a positive contrast medium for studies of the GI system. Complications associated with the use of negative contrast media can result from inadvertent injection of air into the bloodstream, producing an air embolus. Although the injection of negative contrast media into the bloodstream during GI procedures is unlikely, it is important to remember the complications associated with this.

Positive contrast media create a density difference by attenuating and absorbing the ionizing radiographic beam. This reduces or stops the x-ray beam from hitting the image receptor, thereby creating a white or opaque area on the image. There are two types of positive contrast media: barium and iodinated contrast. While barium is relatively nontoxic, iodinated contrasts present a greater danger to the patient. The choice of which positive contrast medium to use is generally the decision of the radiologist; however, the radiographer must be familiar with all types of positive contrasts and their use.

Barium sulfate is the most frequently chosen contrast medium for radiologic examination of the GI tract. It is a white (or pink), crystalline powder that is mixed with water to make a suspension. It may be administered by mouth for examination of the upper GI tract, by rectum for examination of the lower GI tract, or by infusion of a thin suspension through a duodenal tube to visualize the jejunum and ileum. The use of high-contrast barium solution in the alimentary canal reveals organ outlines and demonstrates pathologic conditions of the visceral walls. By using double contrast of barium and air, the ability to detect small lesions is improved.

The toxic effects of barium are negligible if the suspension remains within the GI tract; however, if there is a break in the gastric mucosa caused by injury or disease, the barium sulfate may pass into the respiratory tract, the peritoneal cavity, or the bloodstream and cause adverse effects. Barium has been known to get into the bronchial tubes if there is a fistula from the esophagus to the trachea or if the barium is aspirated. Patients who are more prone to aspirating barium during a barium swallow study are the elderly and those with swallowing difficulties. Barium in the bronchi is dangerous from the standpoint that it will reduce the amount of air that will be taken into the alveoli. Barium will be retained in the lungs for a lengthy period and will interfere with subsequent chest radiographs (Fig. 12-1). Aspiration of oral or gastric contents into the tracheobronchial tree of older patients may represent a life-threatening situation. Although barium is not inherently toxic to lung tissue, patients will later exhibit a productive cough, fever, and elevated white blood cell as the body tries to rid the lungs of the foreign substance. Aspiration pneumonia is also a complication when barium sits in the bronchial tree.

If barium leaks into the peritoneal cavity, peritonitis may result. This possibility increases if the barium is mixed with fecal material. Fibrosis or formation of a barium granuloma may be a further complication. The possibility of leakage of the barium into the venous circulation through a perforation in the gastric mucosa as a result of trauma or disease must also be considered. This would produce an embolus that might be fatal. When perforation of the GI tract is suspected, an absorbable water-soluble iodinated contrast medium is used in place of barium.

Barium sulfate is often constipating. If the patient is not properly instructed following a procedure that involves its use, there may be a tendency to ignore the condition rather than have it treated, and fecal impaction or a bowel obstruction may result.

If the patient reports that a previous administration of oral barium suspension produced a sensation of nausea, the radiologist should be notified before it is administered again, because if the patient vomits, aspiration of barium may result. Although barium itself is rarely a cause for nausea, the patient may be convinced that it is the barium that is causing the nausea.

The type of cleansing enema to be used is always ordered by the physician. The most frequently used cleansing enemas include the saline enema, hypertonic enema, oil-retention enema, tap water enema, and the soapsuds enema.

Cleansing enemas can influence fluid and electrolyte balance in the body to varying degrees because they each
have a different degree of osmolarity, which influences the movement of fluids between the colon and the interstitial spaces beyond the intestinal wall. This means that, in the presence of some cleansing solutions that are instilled into the lower bowel, the bowel extracts fluid from the surrounding interstitial spaces. This happens because the higher osmolarity of the cleansing solution (hyperosmolar) induces the fluid to move across the semipermeable membranes of the intestinal wall. The body fluid that has moved into the large intestine is then excreted from the body along with the enema solution. Dehydration occurs if an excess of body fluid is excreted. This situation can be reversed.

Another situation occurs when an excess of fluid with low osmolarity (hypo-osmolar) may be instilled into the lower bowel and absorbed into the interstitial spaces surrounding the colon, thus creating fluid excess in the body. This is called fluid toxicity. These potential hazards must be considered when the physician orders a particular solution for use as a cleansing enema.