b. Radiolucent Media and Their Uses. Oxygen, carbon dioxide, and room air
are commonly used as radiolucent media. These common gases are utilized in digital
subtraction angiography and joint spaces, as well as for double contrast in certain
examinations. Radiolucent media are sometimes used in arthrography. Although the
contrast provided this way is quite subtle, some physicians prefer the radiolucent media
since it causes less irritation and is quickly absorbed. A radiolucent medium is also
commonly used as a double-contrast enema to evaluate the colons for polyps.
1-26. REACTIONS TO CONTRAST MEDIA
a. Introduction. Intravenously injected contrast media can produce a reaction
similar to anaphylactic shock. True anaphylaxis is the result of hypersensitivity to a
drug to which the patient previously was not sensitive. For example, the first time a
patient receives penicillin, he may show no side effects; yet, following a second dose of
the same drug, this patient may have an anaphylactic reaction. This is thought to be
due to the formation of antibodies against a drug. Reactions to contrast media can
occur in patients who may or may not have had the particular radiopaque before.
Although the causes of contrast media reactions seem to be different, the symptoms are
so similar that they are generally thought of as a variety of anaphylaxis. Basically, two
types of reactions can occur following the injection of contrast media. They are
classified as histamine imbalance and hemodynamic reactions.
b. Histamine Imbalance Reaction. Histamine, a substance found in all
humans, has several functions, one of which is the release of blood plasma through
capillary walls to body tissue. This release of fluids produces swelling of tissue called
edema. Histamine is held in check by a histamine inhibitor and, in some people, the
balance between the two is delicate and rather easily upset. These people are the
unfortunate individuals who suffer from allergies such as hay fever.
(1) One of the theories about histamine imbalance is that iodinated contrast
media damage the histamine inhibitor sufficiently to cause an overabundance of
histamine. This imbalance causes the release of fluids into tissue, producing a set of
distinct, easily recognizable signs and symptoms.
(2) Among the signs and symptoms of a histamine imbalance reaction are
itching, a flushed appearance, watery eyes, faintness, hives, nausea, and breathing
difficulties. In a mild response of this type, the patient may feel hot, faint, or nauseated.
A moderate reaction might evoke watering of the eyes, localized swelling (especially of
the face and hands), and a flushed appearance. In its most serious form, breathing
problems due to swollen bronchial passages appear. Untreated severe bronchial
constriction can produce death by suffocation. Generally, this reaction occurs quickly,
but the specialist must be watchful for possible delayed reactions.