CT Imaging

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

  • Ascending aorta
  • Aortic arch
  • Thoracic descending aorta
  • Abdominal aorta

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

  • Ascending aorta
  • Aortic arch
  • Thoracic descending aorta
  • Abdominal aorta

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

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