Contrast media

Barium sulfate is the most commonly used contrast medium for gastrointestinal contrast radiography of the vomiting patient. Oral barium sulfate is commercially available in powder form, which has to be reconstituted with water, or in liquid form, where addition of water usually is necessary to obtain the desired concentration.

Barium sulfate liquid preparations are recommended over the powder form and have a wide range of ingredients; their exact composition often remains a secret of the manufacturer. Additives include deflocculation, suspending, dispersing, wetting, anti-drying, anti-foaming and flavoring agents and magnesium.

They are important in providing one of the most important characteristics of a barium sulfate contrast agent: good adherence and coating of the GI mucosa. The pH of the barium sulfate suspension often is mentioned as an important factor for good mucosal coating and a tendency to fall out or "flocculate." However the pH is not just determined by the product on the shelf, but by the pH of the tap water added. Therefore, this can be quite variable.

Advantages and disadvantages of barium sulfate

Advantages
Excellent mucosal coating
No absorption
No mucosal irritation
High contrast
Low cost

Disadvantages
Causes irritation if peritoneal leakage
Aspiration pneumonia if inhaled

Iodinated contrast media are water-soluble and can be divided into two main groups: ionic and non-ionic. Ionic contrast media are cheaper, but have the disadvantage of a high osmolality. High osmolality leads to interstitial fluid being drawn into the GI tract; ionic contrast media should therefore not be administered orally to hypovolemic patients. Similarly, aspiration of ionic contrast media leads to pulmonary edema. In addition to the potential complications, the contrast is diluted, compromising diagnostic quality of the study.

Advantages and disadvantages of iodinated contrast media

Advantages
Rapid transit time
No irritation if peritoneal leakage

Disadvantages
Poor mucosal coating
Absorption
Hyperosmolality (ionic)
Expensive
Bitter taste

Gastric evaluation

Investigation of gastric disease is the most common indication for contrast studies in the vomiting patient. Different contrast methods for evaluation of the stomach in a vomiting patient are available. Placement of a gastric tube or nasoesophageal tube is indicated in patients with swallowing difficulties. If necessary, sedation with acepromazine or a light and short anesthesia with ketamine may be used.

Positive-contrast gastrography using barium sulfate usually is the first choice, based on good mucosal adherence. A simple technique that often is forgotten but may give excellent results, especially in patients with gastric masses or foreign bodies, is negative-contrast gastrography, using air from a carbonated beverage.

Double-contrast gastrography is the best method to evaluate the gastric mucosa, but is not often used in veterinary medicine compared with human medicine. It is more time-consuming and technically complicated than positive- or negative-contrast gastrography. The patient has to be anesthetized using an anesthetic agent which paralyzes the gastric wall (barbiturates, gas anesthetics).

Alternatively, glucagon at a dose of (0.10 to 0.35 mg IV) may be used to induce paralysis of the gastric wall. Administration of glucagon is contraindicated in patients with pheochromo-cytoma and uncontrolled diabetes mellitus. because glucagon can cause catecholamine release and hyperglycemia. Contrast studies of the small intestines often are low-yield procedures and should be reserved for select patients, such as those with persistent vomiting, negative findings on survey radiographs or unusual abnormalities seen on survey films.