Iodinated Contrast

Introduction

• The purpose of contrast is to provide opacification of tissues to better illustrate organs, blood vessels, and other structures.
• Two types of contrast are used in imaging. Iodinated contrast is used with CT imaging, while gadolinium is used for magnetic resonance imaging.

Chemical Properties

• To minimize toxicity, most iodinated contrast in contemporary practice is nonionic and low osmolality.
• The nonionic compounds are organic agents that covalently bind to the iodine. Their side effect profile is better than ionic contrast as they do not dissociate into the relative component molecules.
• The most commonly used organic molecules found in contrast media are iohexol, iodixanol, and ioversol.¹

Mechanism of Action

• Iodinated contrast absorbs X-rays, which enhances the visualization of the structures.
• Iodine has a high atomic number compared to most tissues and thus absorbs X-rays differently from most body tissues due to the photoelectric effect.¹

Pharmacokinetics

• Iodinated contrast has a peak plasma concentration at 2 minutes after intravenous (IV) injection.
• The contrast rapidly diffuses from the plasma to the interstitial compartment with a half-life of approximately 7 minutes.
• The elimination half-life is around 1-2 hours, but can increase to 40 hours or more in patients with renal dysfunction.
• Iodinated contrast is not metabolized and is excreted in an unchanged form by the kidneys.¹

Indications

• IV contrast provides opacification of the vasculature and abdominal/pelvic organs.
• CT with IV contrast can be used to elucidate abdominal pathologies such as inflammation, abscesses, active bleeding, and neoplasms.²
• It can be used to detect vascular injuries such as active extravasation, dissections, and pseudoaneurysms.³
• For brain imaging, IV contrast is most useful in cases when the blood-brain barrier has been compromised, such as infection, inflammation, and neoplasms.⁴
• Oral contrast is used to evaluate intraluminal gastrointestinal pathologies such as intestinal abscesses, postoperative bowel anastomotic leaks, tumors, and fistulas.⁵
• While barium contrast is more commonly used for oral contrast, iodinated contrast should be used when there is concern for bowel perforation, as it is less likely to cause an inflammatory reaction if leaked into the extraluminal space.⁵
• Rectal contrast can be used to evaluate suspected penetrating injuries of the colon or anastomotic leaks of the distal gastrointestinal tract.^2,6

Safety

• Iodinated contrast is generally considered safe for most patient populations, but special attention should be given to those with risk factors (age, comorbidities, and medications).
• Contrast reactions can be physiologic, such as flushing, hypertension, nausea, and vomiting.
• Allergic-type reactions can be mild, such as pruritus, but can escalate to severe and life-threatening conditions, including hypotension, hypoxia, and significant facial and laryngeal edema.
• The rate of acute adverse events is between 0.2% and 0.7%.
• The rate for severe acute reactions is 0.04%.⁷

Risk Factors for Contrast Reactions⁸

• Age: infants and those older than 60 years old
• Comorbidities: renal disease, asthma, heart disease, dehydration
• Medications: non-steroidal anti-inflammatory drugs, beta blockers, recombinant interleukin-2
• Contrast-related: dose greater than 20 mg iodine, faster injection rate, intra-arterial injection, history of contrast reactions.

Contraindications⁶

• Absolute: history of severe or anaphylactic reaction to iodinated contrast
• Relative:
  • Pregnancy
  • Ongoing radioactive iodine treatment for thyroid disease (may interfere with therapy)
  • Uncontrolled hyperthyroidism (risk of precipitating thyrotoxic crisis)
  • Renal failure

Contrast-Associated Acute Kidney Injury

• Contrast-associated acute kidney injury (AKI) is a clinical diagnosis.
• Definition:⁹
  • Serum creatinine has increased by 1.5-fold or more from baseline within 7 days after contrast exposure or
  • Serum creatinine level has increased by at least 0.3 mg/dL within 48 hours or
  • Urinary output less than 0.5 ml/kg/hr that persists for at least 6 hours following administration
• The management of contrast-associated AKI is supportive and includes elimination of other potential kidney insults, hemodynamic and electrolyte assessment and management, appropriate dose adjustment of medications, and monitoring for uremic signs and symptoms.
• Patients with an eGFR >30 do not have an increased risk of postcontrast AKI.^10,11
• Patients with end-stage renal disease with anuria can receive IV contrast without the risk of further renal damage because the kidneys are no longer functioning.
• Patients receiving hemodialysis or continuous renal replacement therapy should not change their schedule based on contrast administration.
• IV volume expansion is the preferred prevention method. Typical regimens include infusion of isotonic saline 1 hour prior to the exam and continued 3-12 hours after.¹¹

Hypersensitivity Reactions

• Allergic reactions are uncommon, with an average of occurrence rate of 0.6% for all allergic reactions and 0.04% for severe.
• A prior allergic-like reaction to iodinated contrast is the strongest risk factor for a recurrent reaction.¹¹
• Oral premedication regimens:¹¹
  • Option 1:
    • Prednisone 50 mg PO at 13 hours, 7 hours, and 1 hour before contrast administration and
    • Diphenhydramine 50 mg PO/IV 1 hour before contrast administration
  • Option 2:
    • Methylprednisolone 32 mg PO 12 hours and 2 hours before contrast administration
    • optional Diphenhydramine 50 mg PO/IV 1 hour before contrast administration
• IV premedication regimens:¹¹
  • Option 1:
    •  Methylprednisolone 40 mg IV or hydrocortisone 200 mg IV immediately then every 4 hours until contrast administration and
    • Diphenhydramine 50mg IV 1 hour before contrast administration
  • Option 2:
    • Dexamethasone 7.5 mg IV immediately then every 4 hours until contrast administration and
    • Diphenhydramine 50 mg IV 1 hour before contrast administration
  • IV premedication regimens administered for less than 4 hours have not been shown to be effective, therefore a minimum of 4-5 hours duration of pretreatment is recommended.