An ECMO Earthquake?

Author: Chris Nickson
Reviewer: Aidan Burrell

Everything ECMO 003

The 26 year-old man from Everything ECMO 001 is still on VV ECMO for severe ARDS secondary to influenza. He is not yet ready to be weaned off ECMO (see Everything ECMO 002).

The ICU nurse is concerned about this:

Q1. What is shown in the video and what does it indicate?

“Kicking” of the ECMO lines.

Kicking is visible and palpable shaking of the circuit tubing draining to the pump. It indicates access insufficiency (not an earthquake).

Access insufficiency occurs when the suction pressure at the access cannula exceeds venous return. When this occurs inflow is interrupted due to partial or complete occlusion of the inlet ports of the access cannula by the walls of the collapsible vein. After a few seconds, ongoing venous return fills up the vein again and the cannula ports reopen to function once more. This cycle repeats itself resulting in unstable, fluctuating ECMO flows (shown in L/min on the ECMO console) despite a stable pump speed (rpm).

Q2. How can this be confirmed in subtle cases?

Suspect access insufficiency if any of the following are present:

  • unstable circuit flows
  • increasingly negative pressures (if using the HLS system)
  • hemolysis (e.g. plasma free-haemoglobin level is >0.10 g/dL)

Note that ‘kicking’ may not occur in subtle cases of access insufficiency, especially when using multi-stage cannula with multiple inlet holes.

Access insufficiency can be confirmed by performing a ‘ramp test’: increase the pump speed (rpm) in a stepwise fashion and record the ECMO blood flows that are achieved:

  • there is no access insufficiency if ECMO blood flow steadily increases with increasing rpm
  • there is access insufficiency if ECMO blood flow decreases once a threshold rpm is exceeded (kicking will worsen)

Q3. What are the causes?

Causes of access insufficiency include:

  • Hypovolaemia/ haemorrhage
  • Poorly sited access cannula (too low)
  • Excessive pump speed (rpm setting)
  • Patient coughing or straining
  • Positional (e.g. after turning the patient)
  • Acute vasodilatation (e.g. sedation bolus)
  • Increased intra-abdominal pressure
  • High output cardiac failure (e.g. septic shock)
  • Cardiac tamponade (e.g. may occur after sternotomy performed during cardiothoracic surgery)
  • Thrombosis at cannula access site
  • Worsening cardiac function (e.g. cardiac arrest or acute heart failure while on VV ECMO)

Access insufficiency is much more common in patients on VV ECMO than VA ECMO. It may occur in the setting of aggressive diuresis to improve respiratory function. If access insufficiency occurs in patients  on VA ECMO then obstructive causes of shock must be excluded (e.g. tamponade, tension pneumothorax). Severe aortic regurgitation and severe pulmonary haemorrhage are other potential causes of access insufficiency in patients on VA ECMO.

Q4. What immediate action is required?

Reduce the pump speed (rpm) until evidence of access insufficiency resolves, while still attempting to maintaining adequate oxygenation. The rpm can be decreased by 500 rpm every 10 seconds until kicking resolves.

Ensure that the patient is adequately sedated and consider neuromuscular paralysis if appropriate (this will prevent coughing and straining from causing access insufficiency).

A fluid bolus can be given as a temporising measure if pump speed settings cannot be re-established. However, recurrent fluid boluses should be avoided if possible as they will lead to fluid overload and oedema.

The problem continues despite your actions.

Q5. What next?

Ongoing access insufficiency requires a systematic approach that includes:

  • Exclusion of ongoing haemorrhage and/or hypovolaemia
  • Confirmation of adequate cannula position
  • Optimise cannula positioning (remember that ECMO cannula are not usually pushed in further once they have been inserted, although arterial cannulae may be re-advanced by a consultant if it is the site of bleeding)
  • Optimise patient position
  • Avoid vasodilators
  • Check intra-abdominal pressure, seek and treat underlying cause if increased
  • Exclude causes of obstructive shock (e.g. perform echocardiography)

If these measures are unsuccessful an additional access cannula may need to be inserted (high flow configuration).

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