CO2, Gadolinium Provide Alternative Contrast Options

B Mitchel L. Zoler

PONTE VEDRA BEACH, FLA. -- Carbon dioxide and gadolinium are alternative contrast agents for imaging blood vessels in patients who can't tolerate conventional, iodinated contrast.

Although not ideal, for patients with renal insufficiency or a life-threatening iodine allergy, or who will soon receive a large iodine load (e.g., from iodine radiotherapy), these alternative agents can produce reasonably good x-ray images, especially when used together, J. Jeffrey Marshall, M.D., said at the annual meeting of the Society for Cardiovascular Angiography and Interventions.

A glomerular filtration rate of less than 30 mL/min certainly precludes the use of iodinated contrast; a GFR of less than 50 mL/min precludes iodinated contrast in some patients.

A combination of imaging with CO2 and gadolinium works well for digital subtraction angiography, said Dr. Marshall, medical director of the cardiac catheterization laboratory at the Northeast Georgia Heart Center in Gainesville.

CO2 has the advantages of being nonnephrotoxic and, generally, nontoxic. It's also nonallergenic and very inexpensive. A tank can be filled with medical-grade CO2 for about $5. Because the gas is about 400 times less viscous that conventional contrast dye, it can make visible tight lesions that iodinated contrast can't penetrate. It's quickly absorbed by tissues and metabolized by carbonic anhydrase.

CO2 produces contrast by displacing blood instead of mixing with it, as other contrast agents do. Because it is rapidly absorbed, imaging must be done very quickly--much faster than conventional imaging. Injected CO2 appears as a bubble in the blood vessel that quickly expands into the adjacent vasculature before it disappears. Several sequential images must be stacked to visualize a length of vessel in order to obtain an interpretable angiogram.

The quality of the image is inferior to that obtained with iodinated contrast, but this is balanced by CO2's safety. The main danger is in trapping bubbles of gas in small arteries and producing a vapor lock. Care must also be used to prevent air from mixing with the CO2 before it's injected.

Several gadolinium contrast dyes are on the market, but they are only approved for use with MRI.

Although all these dyes are much less nephrotoxic than iodinated contrast, they are not risk free. A rise in serum creatinine of more than 0.5 mg/dL occurs in about 5% of patients who receive a gadolinium dye, and anaphylactic reactions occur in about 1 in 100,000-500,000 treated.