4.2 Pharmacology & Contrast Media

Drug Categories & Routes of Administration

Radiographers do not prescribe drugs, but they must recognize categories, routes, and the drugs used to treat contrast reactions. Routes of administration fall into two families:

• Enteral (via the GI tract): oral (PO), sublingual, and rectal. These are convenient but slower and subject to first-pass metabolism in the liver.
• Parenteral (bypassing the GI tract): intravenous (IV), intramuscular (IM), subcutaneous (SC), intradermal, and intrathecal. IV has the fastest onset and 100% bioavailability, which is why it is used for iodinated contrast and emergency drugs.

Other routes include topical, transdermal, and inhalation. Drug categories a radiographer should recognize include antihistamines (diphenhydramine/Benadryl), corticosteroids, bronchodilators (albuterol), vasopressors/adrenergics (epinephrine), antiemetics, analgesics, and contrast media.

The Rights of Medication Administration

The "rights" of medication safety are frequently tested: right patient, right drug, right dose, right route, right time, plus right documentation and the patient's right to refuse. Verifying the patient with two identifiers and confirming the correct contrast agent and dose is a direct application of these rights.

Iodinated Contrast Media

Iodinated contrast is a positive contrast agent—iodine's high atomic number increases x-ray attenuation, making vessels and organs appear white. Iodinated agents are classified by whether they dissociate in solution:

Osmolality is the key driver of adverse reactions—the higher the osmolality relative to blood, the greater the reaction rate. Because nonionic low-osmolality contrast media (LOCM) cause far fewer reactions, they are now the standard for intravascular use.

Barium Sulfate

Barium sulfate is a positive contrast agent for the GI tract. It is chemically inert and not absorbed, giving excellent mucosal coating. However, barium is contraindicated when perforation of the GI tract is suspected or before surgery, because leaked barium in the peritoneal cavity causes barium peritonitis. In those cases a water-soluble iodinated agent (e.g., Gastrografin/diatrizoate) is substituted, since it is safely absorbed. Water-soluble agents carry their own caution: if aspirated into the lungs they can cause pulmonary edema, so barium may be preferred when aspiration is the concern.

Contrast Reactions & Management

Reactions to iodinated contrast are graded by severity:

Epinephrine is the first-line drug for severe anaphylactoid reactions. A vasovagal reaction is distinguished by bradycardia with hypotension (versus the tachycardia of anaphylaxis) and is treated with leg elevation, fluids, and atropine if severe. Contrast extravasation (infiltration into soft tissue) is managed by stopping the injection, elevating the limb, and applying warm or cold compresses, then monitoring for compartment syndrome.

Patient Prep: Renal Function & Metformin

Before iodinated IV contrast, the most important laboratory screen is renal function, because contrast is nephrotoxic and cleared by the kidneys. Key values:

• Blood urea nitrogen (BUN): normal ~7–20 mg/dL.
• Creatinine: normal ~0.6–1.2 mg/dL.
• eGFR (estimated glomerular filtration rate): the best single indicator; > 60 is normal, < 30 signals high risk of contrast-induced nephropathy.

Metformin is an oral antihyperglycemic for type 2 diabetes. If renal function drops after contrast, metformin can accumulate and cause lactic acidosis. Guidelines direct that metformin be held for 48 hours after contrast when renal function is impaired, restarting only after renal function is confirmed stable.

The iodine/shellfish allergy myth

A reported "shellfish or iodine allergy" does not predict contrast reactions—there is no cross-reactivity with elemental iodine. The strongest predictor of a future reaction is a prior reaction to iodinated contrast, along with asthma and significant atopic history.

Premedication & Informed Consent

Patients with a documented prior contrast reaction or high-risk atopic history may receive a corticosteroid–antihistamine premedication regimen (e.g., prednisone plus diphenhydramine) beginning several hours before the exam to blunt an allergic-like response. The radiographer confirms the regimen was completed but does not order it. Before any iodinated injection, informed consent is verified: the patient must understand the procedure, benefits, risks (including reaction and nephrotoxicity), and alternatives. The radiographer documents the injection site, needle gauge, contrast type, volume, and any reaction.

Onset by Route & a Worked Scenario

Route determines how fast a drug acts, which matters in an emergency. IV acts within seconds, sublingual within 1–3 minutes (bypassing first-pass metabolism), IM/SC in minutes, and oral in 30–60 minutes. Consider a scenario: two minutes after an IV contrast injection a patient reports itching with scattered hives but normal breathing and stable vital signs. This is a mild reaction—the radiographer stops, reassures the patient, monitors vitals, and an antihistamine may be given; epinephrine is not indicated. If the same patient then develops wheezing, throat tightness, and falling blood pressure, the reaction has escalated to anaphylactoid, and the team activates the emergency response with epinephrine, oxygen, and IV fluids. Recognizing this progression—and matching the drug to the severity—is exactly what ARRT patient-care items test.