Bad Bubbles and VV ECMO

Author: Pamela Eakin
Peer reviewers: Aidan Burrell, Emma King

Everything ECMO 019

A 56-year-old lady is on femoral-femoral VV ECMO for bacterial pneumonia and ARDS. She has been transferred to you from another institution. The nurse had been performing some cannula checks when there is a sudden loss of ECMO blood flow.

This is what you see:

Figure 1.

Q1. What is the cause?

Air has entered the pump, causing it to “air lock” or stop working.

Q2. What is the immediate management?

This is an emergency.  Many of the management actions will take place simultaneously. 

  • Clamp the circuit on the return line close to the patient (but not over the cannula) and stop the pump
  • With ECMO support stopped anticipate clinical deterioration
    • Increase Fi02 to 1.0, increase ventilation
    • Titrate vasopressors/inotropes as appropriate, prepare to commence CPR.
  • Minimise complications of air in the patient
    • Put the patient in the Trendelenburg position (head down), so air collects in the right ventricle
    • Attempt to aspirate air from the right heart using a central venous cannula or pulmonary  artery catheter
  • Restart ECMO support
    • If possible aspirate the air from the circuit
    • Identify the source of air.  For instance, if it is a defective cannula then this needs to be addressed by re-cannulation
      • If you can establish the cause and eliminate it AND aspirate ALL the air then it may be possible to restart the circuit

Aspirating air from the oxygenator

Figure 2. Aspirating air from the oxygenator.

Q3. Where can air enter the circuit?

Air is at risk of being entrained:

  • At any connection or 3-way tap, especially access points on the negative pressure side of the circuit. It is for this reason that it is routine at the Alfred ICU to remove all access points on the negative pressure side of the circuit prior to connection to the patient.
  • Arterial cannulae have a side port with a cap to secure it. If used for access (such as jugular access in high flow configurations) meticulous attention must be used to ensure the cap is not open to air
  • Damage to the circuit:
    • Suturing of lines is a risk factor for inadvertent and unrecognised puncture of the cannulae.  As the breach point is small it slowly allows air to enter the circuit, unrecognised for some time. Cannulae are also at risk of unravelling with minor trauma.
      • The cannula must be replaced if this occurs.
      • For this reason, cannulae at the Alfred ICU are secured using a non-suture-based approach involving a grip-LOK dressing (see Everything ECMO 017)
  • Accidental decannulation of access line whilst on ECMO
  • Via the oxygenator
    • Oxygenator rupture — frothy blood may be seen dripping out of the bottom of the oxygenator
    • If fresh gas flow pressure is greater than the pressure of the blood through the oxygenator
      • Maintain the oxygenator below the level of the patient
  • During interventions on ECMO:
    • ECMO cannulation
    • Circuit change or reconfiguration
      • Connections should be made using an underwater seal.
      • Care should be taken when adding a cannula to the venous system (eg IJV cannula) while the ECMO circuit is still running with access via the IVC.
    • Line insertion
      • Position patient appropriately
      • Consider transiently reducing ECMO blood flow (if able) to minimize the risk of air entrainment during the procedure
  • Tracheostomy placement
    • There have been case reports (Lother et al, 2016) of fatal air embolism during percutaneous tracheostomy placement in patients on VV ECMO with an Avalon cannula in situ.  These patients were on full support via the circuit in the supine position.  The air embolism was within seconds of dilatation.  The proposed mechanism was air entering the systemic circulation via the inferior thyroid vein.  The access point on the Avalon cannula is in the SVC.
  • Risk of air embolism can be minimised by
    • Positioning the patient in the Trendelenburg position (head down)
    • Reducing ECMO flows during tracheostomy placement
    • Using wet swabs to cover the puncture site during tracheostomy placement

Q4. How might air in the ECMO circuit manifest?

Air in the ECMO circuit may manifest as:

  • Air bubbles anywhere in the circuit
  • ‘Foam’ at the upper aspect of the oxygenator
  • Cessation of flow if the bubble intervention is switched on when using a Cardiohelp ECMO console with an HLS circuit
  • It may present as a sudden loss of ECMO flow if a large embolism is present – a so-called “air lock”.

Q5. What is the pathophysiology of a venous air embolism?

Consequences of air entering the venous system:

  • Small volumes of air, in the absence of a patent foramen ovale (PFO), can be filtered by the lungs and absorbed prior to reaching the arterial circulation.
  • Air in the lungs can increase pulmonary pressures resulting in right ventricular (RV) strain and RV failure
  • Large volumes of air essentially act as an ‘air lock’, resulting in reduced blood flow to the left ventricle with subsequent cardiac arrest. 
  • In the presence of a PFO, the air can traverse into the arterial circulation and may result in:
    • A stroke by air entering the cerebral circulation
    • Arrhythmias or a myocardial infarction by entering the coronary circulation
    • Other organ infarcts
  • The associated morbidity and mortality depends upon:
    • The volume and speed with which the air enters the circulation.
  • The position of the patient, which influences the location of the air embolus
    • The rationale behind placing patients in the Trendelenburg position is to allow the air to collect in the right ventricle, potentially allowing aspiration and also in an attempt to allow blood to flow into the pulmonary venous circulation.
  • This may be a fatal complication of ECMO

Q6. What definitive treatments may be required?

If initial methods to aspirate the air fail, or there has been a large volume of air entering the circuit, then the circuit must be changed. This should be done immediately and requires assembling a team, equipment, etc, as described in Everything ECMO 012. Ideally, there should be a circuit prepared for such an emergency in your hospital.

Note – hyperbaric oxygen is sometimes suggested when massive air embolism occurs on ECMO, but the ECMO equipment is not rated for diving, so this therapy is contraindicated.


  • Delnoij TS, Driessen R, Sharma AS, Bouman EA, Strauch U, Roekaerts PM. Venovenous Extracorporeal Membrane Oxygenation in Intractable Pulmonary Insufficiency: Practical Issues and Future Directions. Biomed Res Int. 2016; [article]

  • Lother A, Wengenmayer T, Benk C, Bode C, Staudacher DL. Fatal air embolism as complication of percutaneous dilatational tracheostomy on venovenous extracorporeal membrane oxygenation, two case reports. J Cardiothorac Surg. 2016;11(1):102. [article]

  • Mccarthy CJ, Behravesh S, Naidu SG, Oklu R. Air Embolism: Practical Tips for Prevention and Treatment. J Clin Med. 2016;5(11) [article]

  • Pellegrino V, Sheldrake J, Murphy D, Hockings L, Roberts L. Extracorporeal Membrane Oxygenation (ECMO). Alfred ICU Guideline, 2012.

One comment to “Bad Bubbles and VV ECMO”
  1. Excellent and to the point ! Practical for t’es Chong purposes.
    You Aussies got it down under !

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