Oh no, low flow!

Everything ECMO 040: Low flow alarm

Author: Dr Jeremy Smith
Reviewer: A/Prof Chris Nickson

A 5- year-old lady was placed onto veno-arterial (V-A) extracorporeal membrane oxygenation (ECMO) support during ECMO-cardiopulmonary resuscitation (ECPR) following an out of hospital cardiac arrest (OOHCA).  She is day 3 in ICU and being weaned off her sedation.

You have been asked to review her as there has been frequent low flow alarms over the last 15 minutes.

Q1. What is a low circuit flow alarm?

This alarm indicates that the circuit blood flow has fallen below the set alarm limit.

Q2. What should the low flow limit be set at?

0.5L below the ideal ECMO circuit blood flow is the setting used at the Alfred ICU.

Q3. How can a low circuit flow alarm affect the patient?

The amount of support provided to the patient is reduced which may lead to hypoxia or decreased perfusion.

Q4. What are the potential causes of a low circuit flow alarm?

  • Access insufficiency (see Everything ECMO 003)
    • Hypovolaemia, need to consider bleeding
    • Poorly sited access cannula (too low)
    • Cardiac tamponade (common post sternotomy)
    • Excessive RPM setting
    • increased intrathoracic pressure
      • Patient coughing or straining
      • tension hydrothorax
      • Positional (after turning the patient)
      • Acute vasodilatation (sedation bolus)
      • New onset of sepsis (High cardiac output cardiac failure)
      • Increased intra-abdominal pressure
      • Abdominal compartment syndrome  
      • Thrombosis at cannula access site
      • Severe aortic regurgitation / fatal pulmonary haemorrhage (VA ECMO)
      • Worsening cardiac function: cardiac arrest or acute heart failure (VV ECMO)
    • Speed setting change without adjusting the alarm limit
  • Obstruction to flow
    • Kinking of tubing
    • Thrombus in oxygenator or pump head
      • Pump head thrombosis is rare
    • Air emboli (rare)
  • Elevation in blood pressure (VA ECMO only) – increase in afterload leads to reduction in flow

Q5. How can we assess for the cause?

A coordinated team approach is useful in an emergency, with different team members performing the necessary checks concurrently:

  • Check patient
    • Check for recent patient position change
    • Signs of bleeding/hypovolaemia
    • Check for coughing or straining
    • Check for hypertension
  • Check cannula position
    • Check position hasn’t changed
  • Check tubing
    • Look for kinks in tubing
    • Check for “kicking” or “chattering” of tubing
    • Check for clots in tubing with torch, especially around connections
  • Check circuit
    • Check for thrombosis in pump head and oxygenator
      • With torch look for thrombi
      • Look for changes in delta P (pressure change over oxygenator)
        • Should be less than 10mmHg per 1L of flow, however a rise over time at the same speed is also concerning
      • Can check oxygenator function with pre and post oxygenator blood gas
        • Should have a PaO2 >300
      • Check bloods for signs of haemolysis/consumptive coagulopathy (rising plasma free Hb, LDH and D-dimer)
    • Check for air emboli
      • Especially in oxygenator

Q6. How can we troubleshoot a low circuit flow alarm?

If access insufficiency is the cause:

  • Immediate action is to reduce the pump speed (RPMs) until access insufficiency resolves, to allow for support to continue while troubleshooting the cause
    • Decrease RPMs by 500 every 10 seconds until kicking resolves
  • Then ensure patient is adequately sedated and consider neuromuscular blockade if appropriate
  • Fluid bolus can be considered to temporise the problem if RPMs can’t be re-established to initial settings.  However, recurrent fluid boluses will lead to fluid overload and should be avoided.
  • Ongoing access insufficiency requires a thorough search for the cause (as above) and management of the cause once identified
  • For more on access insufficiency see Everything ECMO 003:

If obstruction to flow that isn’t kinking or positional is the cause then consider the need for a circuit change.

References

  • Alfred ICU ECMO Guideline [Accessed 17 Aug 2022]. Available at URL: ecmo.icu
  • Sidebotham D. Troubleshooting Adult ECMO. The Journal of Extra Corporeal Technology 2011;43:27-32. [article]
  • Nickson, C 2015. Everything ECMO 003: An ECMO Earthquake?. Intensive Blog. [Accessed 17 Aug 2022]. Available at URL: https://intensiveblog.com/an-ecmo-earthquake/

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