Pharmacology and contrast media

Pharmacology and contrast media

Exam category: Patient Care → Pharmacology
Why it matters: The most clinically dense Patient Care topic — it blends pharmacology, basic renal physiology, and emergency response. It rewards understanding why rules exist (why check kidney function at all) rather than pure memorization.

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The big picture (plain English)

Contrast media makes soft tissue and vasculature visible on x-ray, but it's still a drug — it has indications, contraindications, and possible side effects. Your job is to gather the right history, use the right contrast for the right patient, and recognize and respond correctly if something goes wrong.

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1. Patient history and administration

Confirm current medications (some, like metformin, affect contrast decisions — see below), ask about allergies (iodine, shellfish, medications generally), and check scheduling order — a barium study should generally come after other abdominal imaging (CT, ultrasound), since residual barium can obscure anatomy on later exams.

Venipuncture basics: select an appropriate vein (commonly antecubital), maintain aseptic technique, confirm blood return before injecting.

Premedication — what a risky history buys the patient

When the history flags a patient as likely to react, the answer isn't simply "proceed carefully." Two things change: a nonionic medium is indicated, and the procedure may be preceded by premedication to blunt the reaction.

The regimen is built from an antihistamine — diphenhydramine is the usual one — sometimes supplemented by an H2 blocker, with a corticosteroid added as the anti-inflammatory component.

The decision point is what gets tested. If a patient has a history of iodine allergy and has not been premedicated, the procedure may be canceled and another modality substituted — ultrasound or a non-contrast CT. Premedication is not a formality you can skip and make up for with vigilance; without it, the appropriate answer can be don't do this study today.

And premedication changes the timing you watch, not just the risk. Most reactions arrive almost immediately, but a reaction can still show up as much as 30 minutes out — occasionally hours later — which is why the patient is monitored after the exam rather than released at the end of the last image.

2. Routes of administration

How a drug or contrast agent enters the body sets how fast it acts and where it goes. The routes split into two families: enteral (into the GI tract) and parenteral (everything that bypasses the GI tract — almost always by injection).

Two exam-favorite points: iodinated contrast for CT and angiography goes IV — immediate and 100% delivered, which is exactly why a severe reaction can hit within seconds. And sublingual is the classic fast-mucosal example (nitroglycerin for angina acts in seconds because it bypasses the gut).

3. Dose calculation

A dose is usually ordered in weight units (mg), but the drug comes as a liquid concentration (mg/mL), so you convert in three moves:

1. Weight into kilograms: pounds ÷ 2.2 = kg.
2. Total dose from a weight-based order: dose per kg × weight (kg) = total dose.
3. Volume to draw up: total dose ÷ concentration (mg/mL) = mL.

Useful equivalences: 1 mL = 1 cc; solid-in-liquid strength is stated as mg/mL (iodinated contrast is labeled in mg iodine/mL); a ratio like 1:1000 means 1 part drug per 1000 parts solution.

Worked example: a child weighs 44 lb and the order is 4 mg/kg of a drug supplied at 10 mg/mL. Convert: 44 ÷ 2.2 = 20 kg. Total dose: 4 mg/kg × 20 kg = 80 mg. Volume: 80 mg ÷ 10 mg/mL = 8 mL.

Pediatric doses are set by the child's weight (and the package insert), not by an age-based shortcut formula — those are avoided because they're error-prone.

4. Contrast media types

Matching the agent to the patient, not just the exam

Choosing a contrast agent is judged on three things about the patient, and the perforated-bowel rule above is only the first of them:

1. Patient condition — the perforation rule, and any other reason the agent could escape where it shouldn't.
2. Age and body weight — the volume given to a child is scaled to how old and how heavy that child is, following the manufacturer's package insert, which states both a usual per-kilogram dose and a maximum not to be exceeded. That ceiling is the part students forget: a weight-based calculation can still produce an over-dose if it runs past the insert's cap. (The arithmetic itself is in §3 — the point here is that weight is a selection criterion, not only a math input.)
3. Laboratory values — BUN, creatinine and eGFR, covered next.

5. Kidney function and metformin

The eGFR 30 threshold, per the ACR Manual on Contrast Media: patients on metformin with eGFR ≥30 and no acute kidney injury — no need to discontinue metformin before or after contrast, and no obligatory renal reassessment afterward. Patients with eGFR <30 (or known AKI, or severe CKD stage IV/V) — metformin should be temporarily discontinued around the procedure. Metformin itself doesn't raise contrast-related kidney injury risk directly; the concern is that if contrast does cause acute kidney injury, reduced metformin clearance could lead to lactic acidosis.

6. Local complications

Swelling, pain, and tightness at the injection site during injection means stop immediately and notify the radiologist — never continue "to finish the study."

7. Systemic reactions — severity and timing

Most adverse reactions occur immediately after administration, which is exactly why patients are monitored closely right after injection, not just at the end of the exam.

Emergency medication categories (administered by the radiologist/emergency team, but a technologist should know them): epinephrine (first-line for severe/anaphylactic reactions), diphenhydramine (mild-moderate allergic symptoms), IV fluids (hypotension), oxygen (respiratory symptoms).

Your role during a reaction: stop the injection → call for help → stay with the patient, monitor vitals, reassure → document thoroughly (what was given, symptom timeline, treatment administered, patient response).

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Registry must-know facts

• Enteral routes (oral, rectal, NG) go through the GI tract; parenteral routes bypass it — intradermal, SC, IM, IV, plus intrathecal (myelography) and intraarterial (angiography).
• Sublingual/buccal absorb through the mucosa straight into the blood (fast, skips digestion). IV is the route for iodinated contrast — immediate and complete, which is why severe reactions can be immediate.
• Convert weight: lb ÷ 2.2 = kg. Weight-based dose: mg/kg × kg = total mg. Volume to give: total dose ÷ concentration (mg/mL) = mL. 1 mL = 1 cc; contrast strength is labeled in mg iodine/mL.
• Pediatric dosing is by weight and the package insert, not an age-based formula.
• Contrast is matched to the patient on three things: condition, age and body weight, and lab values. Pediatric volume is scaled to the child's age and weight, per the package insert, which sets both a usual per-kg dose and a maximum not to exceed — a correct per-kg calculation can still breach that cap.
• Premedication for a patient flagged as likely to react: antihistamine (diphenhydramine), sometimes plus an H2 blocker, plus a corticosteroid — alongside choosing a nonionic medium. If the patient has an iodine-allergy history and was not premedicated, the study may be canceled and ultrasound or non-contrast CT substituted.
• Most reactions are immediate, but a delayed reaction can still arrive up to 30 minutes out (rarely, hours). Monitor across that window.
• Barium contraindicated if perforation suspected — use water-soluble iodinated contrast instead.
• Non-ionic contrast = lower osmolality, better safety profile, standard for IV use.
• eGFR ≥30 + no AKI: no need to hold metformin. eGFR <30 (or AKI/severe CKD): hold metformin around the procedure.
• Extravasation/infiltration/phlebitis are local complications — stop injection immediately if suspected, regardless of how much of the study remains.
• Reaction severity: mild (self-limited skin/nasal symptoms) → moderate (diffuse urticaria, wheezing) → severe (laryngeal edema, cardiopulmonary arrest).
• Technologist's role in a reaction: stop, call for help, stay and monitor, document — not independently treat beyond scope.

Common trap

Older exam-prep material describes a blanket "hold metformin for 48 hours for everyone on contrast" rule. Current guidance ties the hold decision specifically to renal function (eGFR <30), not a universal rule for every patient taking metformin. Two more: students swap enteral (into the GI tract) and parenteral (bypasses it) — remember parenteral means injection. And on a weight-based dose, forgetting to convert pounds to kilograms first builds a 2.2× error into every calculation.

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Source: Ehrlich, Patient Care in Radiography, 10th ed. — premedication of patients suspected of being sensitive (an antihistamine such as diphenhydramine, sometimes supplemented by an H2 blocker, with a corticosteroid included in the regimen), the nonionic-medium indication for a high-risk history, the statement that an iodine-allergy patient who has not been premedicated may have the procedure canceled and another modality such as ultrasound or non-contrast CT substituted, and its instruction to keep anticipating a reaction across the 30 minutes following injection; and, for the age/weight criterion, that paediatric dosing is scaled to the child's age and weight as directed by the manufacturer's package insert, which states a usual per-kilogram dose and a total not to be exceeded. (Specific product dosing figures in that passage are a single manufacturer's insert values and are deliberately not reproduced here, since they do not generalize across agents. The text pairs a corticosteroid brand name with a drug name inconsistently in one sentence, so no brand/drug pairing is carried into this lesson.) Routes of administration and dose calculation grounded in the pharmacology / medication-administration chapters — lb÷2.2, dose/kg × kg, and mg/mL volume conversion verified against the ingested text); American College of Radiology, ACR Manual on Contrast Media (metformin/eGFR guidance verified directly against the ingested manual text — Category I: eGFR ≥30, no hold needed; Category II: eGFR <30/AKI/severe CKD, temporary hold).