When utilizing gas as an imaging agent the biggest concern is the possibility of embolic occlusion and secondary ischemia. The rapid solubility of CO2 allows for intravascular use but the coronary, thoracic aortic, and cerebral vessels are less forgiving and delivery into these vessels should be avoided. Likewise, CO2 should never be delivered in or adjacent to the thoracic aorta especially in the prone position. Theoretically the buoyancy of CO2 can dissipate into spinal vessels and cause ischemia from embolus or trapping. Although there are literature discussions to the contrary, most operators will avoid direct injections in these sensitive vessels.21-25 Additionally, because of the tendency of CO2 to reflux, it is prudent to avoid intra-arterial injections above the diaphragm. Similarly, when imaging dialysis interposition grafts or fistulas, the arterial limb should be examined cautiously. To reduce the possibility of central cerebral reflux the patient can be placed in the Trendelenburg position or a microcatheter can be inserted into the artery and a gentle antegrade angiogram can be performed. In fact it is good practice, regardless of the procedure, to refrain from arterial delivery with the patient’s head in the elevated position. Again, this reduces the potential for central reflux into the arterial cerebral and thoracic circulation.
Other clinical scenarios that theoretically predispose a patient to untoward embolization include right-to-left shunts and the combination of pulmonary artery hypertension and a patent foramen ovale. These instances are extremely rare and mostly hypothetical but operators should be knowledgeable of this possibility. In the patients with known right to left shunts, CO2 can be safely injected into the venous system with the patient’s position right side up (left lateral decubitus) to trap the gas in the right atrium away from the septal defect.
An additional rare contraindication is the use of nitrous oxide general anesthesia when using intravenous CO2. When using this anesthetic there is the potential for N2 residing in the soft tissues to diffuse into the CO2 bubble, causing it to be 5 times to 6 times more occlusive. This potential scenario arises in transjugular intrahepatic portosystemic shunt (TIPS) patients in which CO2 is used in general anesthesia for the procedure.
A recurrent concern for novice operators is the use of CO2 in patients with chronic obstructive pulmonary disorder (COPD). Considering the small amounts of CO2 necessary for imaging compared to endogenous production, it is unlikely that a clinical dose will cause a problem as long as the patient is breathing spontaneously. As a precautionary measure in COPD patients, it is suggested to allow more time between injections. Instead of the recommended 30 seconds to 60 seconds between injections in most routine patients, those with COPD should be increased to 2 minutes to allow for definitive dissolution.
It is important to remember the potential increase in radiation exposure to the operator and the patient when using CO2 DSA. It is recommended that the frame rate for acquiring CO2 images approximate 6 per second or more. This is double the frame rate for typical iodinated contrast imaging, which could potentially result in an increase in radiation by a factor of 4. However, this number is difficult to measure exactly because each scenario is different and finding the diagnosis or accomplishing the desired result using CO2 may decrease the overall necessity for more runs. Finally, the consequences of not using CO2 DSA may far exceed the potential adverse effects of a slight increase in radiation from CO2 DSA use. Considering the potential to increase radiation exposure when performing CO2 DSA, typical radiation protection steps should be employed.
In addition to the above contraindications, there are a few minor disadvantages that exist when changing from a fluid-based vascular contrast to carbon dioxide. The primary disadvantage is learning how to employ a gas-based delivery system as opposed to iodinated contrast. CO2 is invisible, colorless, odorless, and cannot be seen or felt so the comfort level for use is much less than iodinated contrast. The operator must learn and feel confident in the type of delivery system.
Additionally, CO2 vascular imaging is typically not as dense, and patient motion can seriously affect the final product. Likewise, bowel gas motion can deteriorate abdominal images. For these reasons and because of the fact that gas properties are much different than traditional liquid agents, rendering diagnostic images and post-processing may be more labor intensive when using CO2. Reducing patient motion by injecting CO2 gently without explosive delivery is extremely beneficial. To reduce bowel gas motion some operators use glucagon 1 mg/mL intravenously prior to injection. Using a gastrointestinal compression device to assist in displacing the bowel has been suggested.25