Breakthrough Anaphylaxis After Premedication
Ms. L is a 54-year-old woman with asthma undergoing CT-guided biopsy of a lung nodule under moderate sedation. Six years ago she developed hives after iodinated contrast. Per protocol she received prednisone 50 mg PO at 13, 7, and 1 hour before the scan and diphenhydramine 50 mg IV one hour before. Ten minutes after the contrast injection she says her throat feels tight. You hear inspiratory stridor, see diffuse hives on her chest and neck, and note a wheeze. Blood pressure is 84/52 mmHg, heart rate 130, SpO₂ 89% on room air. She is supine on the CT table; the radiologist is at the console and one technologist is in the room.
1. What are your first actions in the next 60–90 seconds, and in what order?
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Recognize and call for help first. Stridor plus hypotension plus hives after contrast is anaphylaxis. Press the emergency call or rapid response button and state clearly: "Anaphylaxis in CT, I need help now." Ask the technologist to bring the emergency cart and get the radiologist to the bedside.
Then, in order:
- Stop the injection if any remains.
- Airway: stridor means upper airway involvement. High-flow oxygen by nonrebreather, suction ready, bag-valve-mask at hand, prepare for possible intubation.
- Position: supine with legs elevated to support preload (if she vomits, roll to the side; if she is in severe respiratory distress, allow her to sit).
- Monitor: continuous SpO₂ and ECG, blood pressure every 2–3 minutes, reassess airway continuously.
- Confirm a patent IV line for drugs and fluid.
Do not give diphenhydramine or a steroid first and wait to see what happens. Antihistamines do not treat airway edema or shock and will delay the drug that does.
2. Which medication is first-line, at what dose and route, and what do you do if there is no response?
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Epinephrine is first-line. Give 0.3 mg IM (1 mg/mL concentration) into the mid-outer thigh; repeat every 5–15 minutes if symptoms persist.
If there is no improvement after IM dosing, or the patient deteriorates, escalate to IV epinephrine under close monitoring, typically 0.1 mg of a 1:10,000 dilution given slowly, or an infusion per protocol, with continuous cardiac monitoring.
Adjuncts, after epinephrine:
- Nebulized albuterol for persistent bronchospasm.
- Isotonic crystalloid bolus (roughly 1–2 L in an adult) for hypotension.
- H1/H2 blockers and corticosteroids later - they do not reverse shock.
Key reasoning point: premedication reduces but does not eliminate risk, and breakthrough reactions can be severe. The presence of a premedication record can create false reassurance; treat the patient in front of you.
3. After she stabilizes, what are the disposition, documentation, and future-planning steps?
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Disposition: transfer to the emergency department or a monitored observation area for at least 4–6 hours, because biphasic reactions can occur hours after apparent recovery. Do not discharge from the radiology suite.
Documentation: times of onset, each drug with dose/route/time given, response, vital signs, personnel involved, and the fluids given.
Allergy record and future planning:
- Update the allergy list to read "iodinated contrast - anaphylaxis (stridor, hypotension)," not "hives."
- Flag the record so no one gives iodinated contrast again without allergy/immunology input.
- Advise avoiding iodinated contrast when alternative imaging (ultrasound, MRI) can answer the question.
- If iodinated contrast is ever essential, plan premedication with a different agent and steroid regimen, in a setting with immediate resuscitation capability and supervision.
- Give written information, recommend a medical alert identifier, and notify the ordering provider.
- File a safety report per policy.