Contrast-enhanced CT of patients undergoing extracorporeal membrane oxygenation (ECMO) presents unique technical and diagnostic challenges. Modifications made will depend on whether the ECMO is venovenous or venoarterial.
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Technical considerations
Several factors need to be considered to ensure that contrast-enhanced CT studies of ECMO patients are both successful and safe. The complexity of the study is dependent on the type of ECMO the patient is receiving, the examination requested, and the patient's stability. Transfer times from the ward to CT and back can be up to 80 minutes 1.
It is important to include all teams, including the radiologist and radiographer in the planning of the examination. Components that need clarifying (and ultimately should be noted in the request) include:
type of extracorporeal life support
cannulae position in particular the drainage and return
clinical question and area of interest
NB: This article is intended to outline some general principles of protocol design from current literature 1-5. The specifics will vary, each department will have stringent well-written guidelines to ensure these examinations are completed safely per local policy.
Venovenous (VV) ECMO
In venovenous ECMO (VV ECMO) the cardiac function will dictate the systemic blood flow and hence exams tend to be similar to standard protocols.
Contrast delivery
administration of contrast can be achieved via standard central access
often a higher bolus of contrast is required (10-20%) 5
if there is no central access, contrast can be injected via the pre-oxygenator port
ECMO blood flow
Due to the potential of contrast recirculation via ECMO, it is recommended to reduce the blood flow to the lowest possible (and safest) setting 5.
Venoarterial (VA) ECMO
Venoarterial ECMO (VA ECMO) is the more technically demanding scenario in CT imaging. Blood flow is significantly altered with deoxygenated blood bypassing the heart and pulmonary circuit and returns, most often, via the femoral artery. This creates retrograde flow and means that particular regions of interest will need to be imaged differently than non-ECMO patients.
If the perfusion of the region of interest is uncertain an antegrade injection should be trialled with a test bolus, if this does not work - the retrograde approach should be explored.
Contrast delivery - antegrade blood flow
This method is ideal when the region of interest is being perfused via the native blood flow, such as the pulmonary circulation, rather than the retrograde flow. A pressure-rated central venous catheter would be utilized.
If cardiac output is poor, a retrograde approach should be considered.
ECMO blood flow
The injection will be through a pressure-rated central line. The blood flow should be reduced to the lowest possible (and safest) setting.
A test bolus should be performed to ensure optimal enhancement is achieved, if examining the pulmonary circuit note the time to enhancement may take upwards of 30 seconds 5.
Contrast delivery - retrograde blood flow
This method is ideal for abdominal exams, exams with low cardiac output, or exams that did not produce a diagnosis text bolus using a central venous catheter. Note: exams of the aorta may need to be performed in two scans.
ECMO blood flow
The contrast will be injected via the ECMO circuit and will require the blood flow to be increased to the highest possible (and safest) setting.
A test bolus should be performed to ensure optimal enhancement is achieved.
Patient positioning
The patient should be position head-first to ensure equipment can be placed at the feet of the scanner. Scan direct does not need to be altered for a retrograde flow. A minimal delay should be set for angiography.
Venoarterial ECMO radiographic pitfalls
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pseudo filling defects
partially opacified aorta may be due to an early phase examination due to retrograde filling 1
there are cases (left heart failure) where the left heart does not enhance at all 1