Overview
CT imaging has considerable logistic implications in ECMO patients. It requires allocation of additional staffing and is preferentially done during day time with the assistance of the support consultant or the ECMO fellow (see intrahospital transport). If an emergency scan is indicated overnight the support consultant should be called to facilitate this. The following describes the process of acquiring optimal scans.
Steps to be undertaken with each CT scan
This is a joint exercise between the ICU team, radiologist and radiographers in planning the best way to approach a contrast enhanced ECMO CT scan. The steps are based on the collaborative work undertaken between ICU and Radiology and should be followed.
(If the patient is on VV ECMO or durable VAD you can proceed with the examination as usual with only slight modification for VV ECMO provided the CT request is correct).
Step 1 CT request
Ensure the CT request is correct with all details included 1) type of ECMO (VA, VV, temporary VAD or other mechanical support), 2) specify the access and return and 3) area of interest/ clinical question. This should be clarified before leaving the ICU cubicle to determine the correct scan, time estimate and allow for planning of the examination.
Step 2 Clinical question – area of interest
Determine the exact anatomy of interest in the scan, this requires careful interpretation of the clinical details and will require a discussion between the radiologist, the ECMO team and the CT radiographer. Note any clinical question regarding the aorta needs to be specified in detail.
Step 3 Decision on best contrast delivery and ECMO modification
Generate a joint understanding between CT radiographer and ICU ECMO team at the time of the scan to determine which type of flow (native or retrograde) would be optimal for delivery of contrast to the anatomical area of interest. An educated estimate of the native cardiac output and the blood flow direction in the area of interest is made and the ECMO blood flow modified accordingly. If uncertain, aim to use native flow and reduce the ECMO blood flow (see available charts below to assist decision making).
Step 4 Contrast rate and connection
Determine rate of contrast injection after the modification of the ECMO blood flow
Step 5 Observation run
Perform the observation run, if required, with the modified ECMO blood flow and compare the flow pattern seen to those on the attached diagram to confirm if native or retrograde flow is supplying your area of interest.
Step 6 Scanning
If the observation run confirms your plan, continue with the scan. If the flow pattern suggests an alternative flow should be used then make the necessary modifications and proceed with the scan or perform a second observation run with new modifications.
Step 1 CT request
Request of CT scans on mechanical support devices
It is essential to include basic details of the extracorporeal support to ensure clear communication regarding the configuration and the clinical question.
Essential
- ECMO mode VA or VV or temporary VAD or other mechanical support
- Specify access and return cannulae
- Clarify region of interest/ specific clinical question
ECMO mode | Access & return of blood | |
Temporary VAD | Temporary RVAD (tRVAD) Access RA/IVC with return into the pulmonary artery oxyRVAD (temporary RVAD with oxygenator) Temporary LVAD – indicate return Access always from the LV apex, return options below: ascending aorta subclavian artery R/L descending aorta femoral artery R/L Temporary BiVAD – list separately tRVAD and tLVAD | |
VA ECMO | Access IVC/SVC/RA 2nd access if applicable IVC/SVC LV vent apical or trans-aortic | Return options femoral artery L/R subclavian artery L/R descending aorta ascending aorta (central ECMO) |
Note: Temporary RVAD and/ or LVAD are scanned as per standard protocol without specific considerations if the return of the LVAD is in the ascending aorta. This is still important to mention to avoid confusion since the patient will still come with an ECMO console. Durable VAD patients have NO ECMO console attached.
VV ECMO
In VV ECMO the systemic blood flow is determined by the native cardiac function. Contrast is preferentially injected via a central access. There is potential for recirculation within the ECMO circuit, the ECMO blood flow therefore should be set to the lowest tolerated blood flow maintaining oxygenation for the period of the scan.
Key points
- Reduce ECMO blood flow if tolerated to minimise circulation of contrast in ECMO circuit
- Use CVC for contrast injection
- An observation run can be performed but is generally not required
Step 2 Area of interest – type of scan
Should be specified in detail with the clinical question. Any request for chest and abdominal imaging are considered as separate scans, routine protocols can generally not be applied. This includes aortic pathology, see below.
Note: VV ECMO scans can be protocolled and performed in the usual way with the modifications listed under VV ECMO above.
Imaging with arterial return
Clinical question | Scan |
General | |
Severe sepsis – unclear focus | CT abdomen portal venous phase combined with a CT chest without additional contrast |
Bleeding – need to specify likely origin territory
| See below for specific scans. Note: there is no simple aortogram, if required please discuss and perform as separate thoracic and abdominal scans |
Post ECPR (ECMO facilitate CPR/ cardiac arrest) – indicate cardiac/ or potential non cardiac cause | CTB CTPA (if non-cardiac) otherwise CT chest post abdominal scan CT abdomen arterial phase/ portal venous phase |
VAD or transplant work up* | CTB if intubated or GCS <15 CT abdomen portal venous phase followed by CT chest without additional contrast |
CT angiogram | |
Aortic pathology (CT aortogram) – need to specify
| Arterial phase CT chest or abdomen with focus on the area of interest. Achieve clear agreement on the range of the aorta prior to scanning. A full aorta will require two scans. With timing run as indicated below |
CT angiogram upper limb | Consider ultrasound in first instance |
CT angiogram lower limb | Consider ultrasound in the first instance. Further options CT leg angiogram or in very selected cases angiography in IR may be considered. |
For other body regions see below | |
Head & neck | |
CT carotid, circle of Willis | Ensure both carotids (and vertebral) arteries are receiving either antegrade or retrograde aortic flow. Can be difficult at times to image in one scan. |
Chest | |
Pulmonary embolism or pulmonary vein pathology | CTPA or CT pulmonary venogram both entirely dependent on native cardiac output |
Lung pathology – pulmonary haemorrhage/ pulmonary infiltrates or abscess/ complex pleural effusion | If in conjunction with abdominal contrast CT, chest CT scan at timing of portal venous phase without additional contrast |
Abdomen | |
Abdominal pathology (except bleeding – see above) | Routine use of ECMO flow used for contrast delivery. No delay in femoral return given distal vessels already enhanced |
Other more specific scans should be discussed ideally between the intensivist, radiologist and CT radiographer before the patient leaves the ICU cubicle for the scan.
*VAD work up
Explanation: Aim is the exclusion of any obvious malignancy that would represent a contra-indication to a durable VAD implantation. If there is no additional radiological question this requires a portal venous phase abdominal CT followed immediately by a chest CT without additional contrast. A CTB should be included if the patient is either intubated or not fully assessable neurologically.
Step 3 Joint decision on the contrast delivery and modification of ECMO flow
IMPORTANT
Establish whether the region of interest is likely to be perfused by anterograde flow via the native cardiac output or retrograde by the ECMO blood flow. This should be a team discussion at the time of the scan between the CT radiographer and the intensivist.
Use chart to assist communication/ decision making for VA ECMO with femoral return
The decision is largely dependent on the patient’s native cardiac output which is commonly difficult to judge with mid-range pulsatility >15 and depends on the region of interest. In case of uncertainty the ECMO flow should be reduced and an observation run performed. Abdominal scans are always scanned using the ECMO blood flow to deliver the contrast..
ECMO modification
Area of interest likely perfused by | ECMO blood flow | IV contrast | Observation run |
Native (antegrade) blood flow | Reduce* | CVC | Confirm blood flow |
ECMO (retrograde) blood flow | Increase** | via ECMO | Confirm blood flow |
Uncertain | Reduce* | CVC | Reassess post run |
*Reducing ECMO blood flow
ECMO blood flow is reduced as low as acceptable to the clinician for the time of the scan. Not lower than 1.5L/min if not anticoagulated.
**Increasing ECMO blood flow
ECMO blood flow is increased as high as tolerated for the time of the scan without access insufficiency occurring.
In case of uncertainty whether the contrast to the area of interest is delivered by the ECMO circuit, the observation run should be done with contrast delivery via the CVC with minimal ECMO blood flow.
Specific circumstances
Temporary LVAD with return to subclavian artery or descending aorta
There is no injection port on the temporary LVAD contrast delivery will therefore depend on native delivery from the right to left heart.
Step 4 Contrast rate and connection
Contrast connections to the CVC are made as usual. A standard contrast injection rate of 5 ml/s is used.
Contrast connections to the ECMO circuit is the responsibility of the ICU consultant. Link to practical process of connection.
Retrograde contrast delivery by direct injection into the ECMO circuit leads to a reliable contrast delivery into the patient’s arterial system. The time from injection until the contrast will appear in the patient’s circulation is very short and approximately 3 seconds. There will be only a small variation depending on the ECMO flow settings and the distance required to travel within the body. By using retrograde flow it is important to note that more caudal to the monitoring region the arterial vessels will already be contrast filled hence no further delay when triggering the CT scan is required.
The monitoring delay for observation run and the scan should be 0 seconds for direct injection into the ECMO circuit.
Rate of contrast for direct injection into the ECMO circuit
Note: any adjustments to the ECMO flow should be made before calculating the injection rate.
ECMO return | Desired contrast dilution | Correction factor | Rate of contrast injection dependent on ECMO blood flow |
Only aortic or subclavian returns | 1:10 | 1.66 | 1.66 x ECMO blood flow (L/min) = rate of contrast in ml/s |
All femoral VA ECMO | 1:15 | 1.11 | 1.11 x ECMO blood flow (L/min) = rate of contrast in ml/s |
The table demonstrates the variables for the contrast injection. The ECMO blood flow can be read from the ECMO console at the time after potential adjustments. The desired dilution is determined by the radiographer according to experience and patient factors such as size. The standard dilution is 1:15 for femoral VA ECMO and 1:10 only for ECMO with aortic or subclavian returns.
Example: a desired dilution of 1:15 is selected, the ECMO blood flow is at 4.2L/min. This results in a contrast rate of 4.7 ml/min (4.2 x 1.11).
In case of uncertainty whether the contrast to the area of interest is delivered by the ECMO circuit, the observation run should be done with contrast delivery via the CVC with minimal ECMO blood flow.
Practical steps for contrast injection into the ECMO circuit
Once the decision is made to use retrograde ECMO flow to deliver the IV contrast. The ICU consultant transporting the patient is responsible for the connection of the contrast line to the ECMO pre-oxygenator port. Utmost caution needs to be taken given the potential risk of air embolism. Contrast Injections should only occur into the ECMO pre Oxygenator port and not into the Distal Perfusion Cannula.
If there is uncertainty about the type of flow (native or retrograde) to the area of interest then an observation run using native blood flow should be attempted before utilising direct injection.
ICU consultant
- Check the contrast syringe jointly with the radiographer for bubbles (see image 2 below)
- Minimal sterile set up with sterile gloves (see image 1 below)
- Aspirate from ECMO circuit first then connect contrast line – caution positive pressure
- Ensure the connection is free of air bubbles (see images 2 below)
- Aspirate any generated air bubble
- Test injecting of normal saline by the radiographer
- See exemplary video https://youtu.be/6iEKN3YXuL8
- Disconnect contrast line after use
Radiographer
- Prior to patient arrival Radiographer can draw up contrast and saline using no touch technique as usual.
- Take extra care to remove the bubbles as much as possible and jointly with the ICU consultant check this.
- Assist ECMO Doctor when required by winding contrast syringes forward as they try and remove air bubbles that are caused during the connection process.
- It is normal that some blood will be pushed back up the injector tubing giving the appearance of arterial cannulation which this essentially is.
- Once the connection has been made the circuit should then be turned up to the maximum safe level and contrast injection rate calculated as per instructions.
- Perform saline test injection, Radiographer to monitor injection graph while ECMO Doctor monitors connection.
- Perform Observation Run at the required level with no monitoring delay.
- Perform Scan with appropriate contrast bolus size and injection rate as calculated below.
Sterile set up
Image 1. Minimal sterile field to prepare, with betadine, syringe to aspirate and red cap when disconnecting
Video link https://youtu.be/6iEKN3YXuL8
Check for air bubbles
Image 2. A Check the injector for air bubbles which is later turned with its black base to the ceiling. B Check for air bubbles post connection (required to be aspirated), the positive pressure pushes blood into the injection line.
Step 5 Observation run
Key points
- The contrast bolus is 20ml contrast and 20ml of saline
- If using contrast delivery via ECMO circuit
- Perform observation run at level of T12 for abdominal scans ensuring that the celiac axis will enhance
- Perform observation run at level of carina for thoracic scans
- Perform observation run at mid-neck level for carotid/COW scans
- Calculate injection rate dependent on the ECMO blood flow after any modification (see table above – mostly ECMO flow Rate in L/MIN x 1.1 = Injection rate in ml/s)
- If using contrast injection via CVC
- Perform observation run at the level of the carina (unless specific application) and interpret the flow pattern
- For abdominal scans if contrast not seen on observation run move monitor to L2
- If the flow pattern of the observation run suggests an alternative blood flow then expected, make the necessary (reverse) modifications to the ECMO circuit and use the alternate way to deliver the contrast and perform a second observation run.
Pattern recognition at level of carina to interpret observation run
Illustrated are examples during the observation run to confirm or negate the predicted flow pattern (native versus retrograde flow) when performing a patient injection.
Step 6 Summary & Scanning tips
Summary table for VA ECMO with femoral return cannula ONLY
Scan Key area of interest | Pulsatility on arterial line | Decision on contrast delivery* | ECMO blood flow | Level of Monitor | Observation run All with 20ml contrast | Scan instruction |
CT carotid/COW | >10 | Native flow – CVC 80ml contrast | down | Mid-neck | Ensure Left carotid enhances | Trigger when Left carotid enhances. Minimum delay |
<10 | Retrograde – ECMO circuit 80ml contrast | up | Mid-neck | Ensure Right carotid enhances | Trigger when Right carotid enhances. Minimum delay | |
CTPA | Proceed even if no pulsatility with organised rhythm | Native flow – CVC 80ml contrast | down | Carina | 5s monitoring delay If low cardiac output extend length of observation run beyond 30s | Only trigger scan once contrast in lung periphery |
CT chest – lung pathology (post abdominal scan) | irrespective | Residual contrast | Unchanged from abdominal scan | N/A | No additional observation run required | Some arterial/venous contrast sufficient to interpret scan |
CT chest – lung pathology (only chest scan) | irrespective | Native flow – CVC 80ml contrast | down | Carina | No observation run | Contrast monitoring with extended length. Trigger when contrast seen in ascending aorta |
CT abdomen arterial phase | irrespective | Retrograde – ECMO circuit 90ml contrast | up | T12 | Ensure contrast reaching celiac trunk. If not seen proceed with contrast monitoring at L2 | No added delay to scanning, trigger once contrast at monitoring level |
CT abdomen portal venous phase | irrespective | Retrograde – ECMO circuit 80ml contrast | up | T12 | Ensure contrast reaching celiac trunk. If not seen proceed with contrast monitoring at L2 | Standard delay of 35s post contrast triggering (extend to cover chest if required) |
CT aortogram (need to define area of interest and exact range of aorta required)** | ||||||
CT ascending aorta | Any pulsatility even <10 | Native flow – CVC 100ml contrast | down | Carina | If low cardiac output extend length of observation run beyond 30s | Minimum delay |
None | Retrograde – ECMO circuit 100ml contrast | up | Carina | Retrograde filling all along the aorta, descending first | Minimum delay | |
CT aortic arch | >10 | Native flow – CVC 100ml contrast | down | Mid-neck | Ensure Left carotid enhances | Trigger when Left carotid enhances. Minimum delay |
<10 | Retrograde – ECMO circuit 100ml contrast | up | Mid-neck | Ensure Right carotid enhances | Trigger when Right carotid enhances. Minimum delay | |
CT thoracic descending aorta | >15 | Native flow – CVC 100ml contrast | down | Carina | Contrast in descending aorta | Difficult to predict correct contrast delivery, may need to change to alternate flow. Start with native flow if unsure. |
<15 | Retrograde – ECMO circuit 100ml contrast | up | Carina | Contrast in descending aorta | ||
CT abdominal aorta (focus on aorta) | irrespective | Retrograde – ECMO circuit 90ml contrast | up | T12 | If contrast does not reach repeat at L2 | No added delay to scanning, trigger once contrast at monitoring level, |
CT angio legs | irrespective | Retrograde- ECMO circuit 80ml contrast | up | Timing runs at top of scan range and knees | Replaced by timing runs as per usual leg angiogram protocol | Usual trigger and scan as non ECMO protocol Direct injection into distal perfusion cannula not indicated |
*Decision on using native or retrograde blood flow based on team discussion, pulsatility and clinical gestalt
**A complete thoracic aortogram, aortic arch or carotid may not be possible, if desired though start with the native flow for the proximal portion followed by a scan with maximum ECMO blood flow and retrograde delivery into the ECMO circuit for the distal part.
All ECMO Contrast Exams VV, VA or Temporary VAD
- Position patient head first in scanner
- Connect contrast to central access or via ECMO circuit pending prior decision making
- Perform scout that will include observation run location plus required anatomy
- Make necessary changes to ECMO circuit flow rates
- Patient Injections = Observation run 20ml Contrast 20ml Saline 5ml/s, monitoring delay 5 seconds
- ECMO Injections = ALL runs with calculated injection rate into pre-oxygenator ECMOport, monitoring delay 0 seconds.
CTPA VV and VA ECMO
Radiographers may use an observation run as a timing run to calculate time to enhancement of peripheral pulmonary arteries Use manual delayed CTPA scan rather than smart prep
Use 80ml contrast at 5ml/s with saline flush through central access
VA ECMO CAP (chest/abdo/pelvis)
A C+ CAP for femoral VA ECMO is a single portal venous phase scan UNLESS the clinical question is regarding PE, bleeding or aortic pathology.
All Other VV ECMO Scans
Should be able to use normal scan protocols with injection into central access. No Observation Run required or contrast bolus increase.
Extra
To follow