Radiographic Contrast Agents and Contrast Reactions
Radiopaque contrast agents are often used in radiography and fluoroscopy to help delineate borders between tissues with similar radiodensity. Most contrast agents are iodine-based.
Iodinated contrast agents may be
Ionic contrast agents, which are salts, are hyperosmolar to blood. These agents should not be used for myelography or in injections that may enter the spinal canal (because neurotoxicity is a risk) or the bronchial tree (because pulmonary edema is a risk).
Nonionic contrast agents are low-osmolar (but still hyperosmolar relative to blood) or iso-osmolar (with the same osmolarity as blood). Newer nonionic contrast agents are now routinely used at nearly all institutions because they have fewer adverse effects.
The most serious contrast reactions are
Reactions vary in severity:
The mechanism is anaphylactoid (see Anaphylaxis); risk factors include the following:
Treatment begins by stopping contrast infusion.
For mild or moderate reactions, diphenhydramine 25 to 50 mg IV is usually effective.
For severe reactions, treatment depends on the type of reaction and may include oxygen, epinephrine, IV fluids, and possibly atropine (for bradycardia).
In patients at high risk of contrast reactions, imaging tests that do not require iodinated contrast should be used. If contrast is necessary, a nonionic agent should be used, and patients should be premedicated with prednisone (50 mg po 13 h, 7 h, and 1 h before injection of contrast) and diphenhydramine (50 mg po or IM 1 h before the injection). If patients require imaging immediately, they can be given diphenhydramine 50 mg po or IM 1 h before injection of contrast and hydrocortisone 200 mg IV q 4 h until imaging is completed.
In contrast-induced nephropathy, serum creatinine typically begins to increase within 24 h after administration of IV contrast; it peaks between days 3 and 5 and returns to baseline within 7 to 10 days.
Common risk factors include the following:
In patients at risk of acute kidney injury after receiving iodinated intravascular contrast, the following measures should be considered:
Many hydration regimens exist; one example is IV administration of 0.9% normal saline at 1 mL/kg for 24 h beginning a few hours before the procedure.
Acetylcysteine may be given as premedication for patients at risk of developing nephrotoxicity, but its efficacy is uncertain. Oral antihyperglycemic drugs, such as metformin, should be withheld for 48 h after IV contrast administration to avoid drug accumulation if contrast-induced nephrotoxicity occurs.
Because many protocols dealing with contrast agents and reactions are specific and continually updated, it is important to discuss such details with the imaging department.