Intravenous Contrast

Intravenous contrast may be divided into:

• X-ray Contrast
  These agents are all based on a tri-iodinated benzene ring, which absorbs x-ray radiation. Alterations to this ring alter toxicity, lipophilicity, and elimination.
  • Agents are classified by these structural differences into:
    • Ionic
      Ionic substances are strong acids and are water soluble due to ionisation. They are further divided into:
      • Monomers
        Typically high molecular weight.
      • Dimers
    • Non-Ionic
      Water soluble due to hydrophilic side chains. Lower molecular weight than ionic contrast agents.
      • Monomer
        Agent of choice for angiography.
        • Easy to inject
        • Water soluble at physiologic pH
      • Dimer
        Harder to inject than monomers due to higher viscosity. Typically used for urography.
  • All are renally eliminated, and may be retained in renal dysfunction
• Gadolinium Contrast
  Gd³⁺, due to its seven unpaired electrons, is paramagnetic and will alters the magnetic field of an MRI machine.
  • Free gadolinium is nephrotoxic and must be chelated
  • This increases its solubility and allows it to be renally eliminated
  • Gadolinium also attenuates x-rays, but is not used as x-ray contrast as doses required would be toxic

Adverse Reactions

Adverse reactions to low-osmolarity agents are uncommon (3%), with severe reactions being very rare (0.04%) and fatal reactions being extremely rare (1:170,000).

General Adverse Reactions

Adverse reactions include:

• Chemotoxicity
  • Platelet inhibition
  • Increased vagal tone
    • Negative inotropy
    • Negative chronotropy
• Ionic toxicity
  • Cellular membrane dysfunction
    May worsen myasthenia gravis.
• Osmotoxicity
  • Pain
  • Emesis
  • Increased PAP
  • Decreased PVR
• Hypersensitivity reaction
  Typically occur within 20 minutes of injection.

Risk factors include:

• Asthma or atopy
• Critically ill
• Cardiac disease
• Renal disease

Contrast Nephropathy

Defined as an increase in creatinine by 25% above baseline within three days of IV contrast administration.

• It is theorised that osmotic stress and direct tubular toxic effects lead to renal tubular injury, and may cause acute tubular necrosis
• Typically is benign, with creatinine returning to baseline within 10-14 days
• Significant uncertainty as to whether contrast media do cause acute kidney injury
  IF this risk is present, it is probably only relevant in patients who have:
  • Impaired renal function
  • Arterial contrast
• Rehydration and volume correction are effective in preventing a rise in creatinine

References

1. Dickinson MC, Kam PC. Intravascular iodinated contrast media and the anaesthetist. Anaesthesia. 2008 Jun;63(6):626-34.
2. The Royal Australian and New Zealand College of Radiologists. Iodinated Contrast Media Guideline. Sydney: RANZCR; 2016.