In a patient on an Impella device and veno‑arterial extracorporeal membrane oxygenation (VA‑ECMO), why are both the Impella green motor screen and the aortic pressure waveform flat?

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Flat Impella Motor Screen and Aortic Pressure Waveform in ECPELLA

Both screens are flat because the VA-ECMO is providing such high flow that it has completely taken over the circulation, eliminating pulsatility and creating a state where the Impella cannot detect meaningful left ventricular pressure gradients—this represents either total LV unloading or, more concerning, inadequate Impella function in the setting of ECMO dominance.

Understanding the Physiology

When a patient is on both Impella and VA-ECMO (ECPELLA configuration), these devices have opposing effects on the left ventricle:

  • VA-ECMO increases LV afterload by pumping blood retrograde into the aorta, which can overload the left ventricle
  • Impella unloads the LV by actively removing blood from the ventricle and ejecting it into the aorta

Why Both Screens Are Flat

The flat waveforms indicate non-pulsatile flow, which occurs when:

  1. VA-ECMO flow is dominant (likely >4 L/min), completely overwhelming native cardiac output and creating continuous rather than pulsatile arterial pressure 1, 2

  2. The LV is in an "uncoupling" state where no blood is being ejected from the ventricle—the aortic valve remains closed throughout the cardiac cycle 2

  3. The Impella motor current amplitude is very low (potentially around 20 mA or less), reflecting minimal pressure gradient between the LV and aorta because the LV is completely decompressed 1

Immediate Troubleshooting Algorithm

Step 1: Verify ECMO Flow

  • Check ECMO flow is >2 L/min to confirm the device is functioning 3
  • Ensure flow sensor is correctly oriented and programmed interventions are reset 3
  • Visually inspect for color change between drainage and return tubes 3

Step 2: Assess Impella Function

  • Increase the P setting on the Impella if able and follow Impella-specific algorithms 3
  • Check Impella motor current amplitude—if <20 mA with non-pulsatile aortic pressure, this may represent total LV unloading 1
  • Verify Impella position across the aortic valve with echocardiography

Step 3: Determine Clinical Context

If MAP >55 mmHg, ECMO flow >2 L/min, and saturations >88%: This represents adequate circulation with total LV unloading 3. The flat waveforms may be physiologically appropriate in the early phase of ECPELLA support when maximal LV decompression is desired 1.

If MAP <30 mmHg, no recordable saturations: This is inadequate circulation requiring immediate intervention—restart CPR and maximize ECMO flows 3

Clinical Implications

Total LV Unloading (Potentially Therapeutic)

When both waveforms are flat with adequate systemic perfusion, this may represent the "LV uncoupling condition" where:

  • LV pressure is reduced to approximately 10 mmHg 1
  • Aortic valve does not open
  • Coronary perfusion is actually enhanced and becomes continuous throughout the cardiac cycle 2
  • This condition protects the injured myocardium in the early phase of cardiogenic shock 1

Device Malfunction (Concerning)

Flat waveforms with inadequate perfusion suggest:

  • Impella malposition or dysfunction—not effectively unloading the LV
  • Excessive ECMO flow without adequate LV venting, risking pulmonary edema
  • LV distension despite Impella presence

Management Strategy

In the first 24-72 hours of ECPELLA support:

  • Maintain total LV unloading condition by keeping Impella motor current amplitude ≤20 mA with non-pulsatile aortic pressure 1
  • Monitor for signs of inadequate perfusion (lactate, urine output, mentation)
  • Use echocardiography to confirm LV is decompressed without distension 3

As patient improves:

  • Gradually reduce ECMO flow to allow native cardiac function to resume
  • Impella motor current amplitude will increase (>100 mA) as pressure gradient returns 1
  • Pulsatility should return to both waveforms as the heart recovers

Critical Pitfalls

  • Do not assume flat waveforms always indicate device failure—in ECPELLA, this may be the intended therapeutic state 1, 2
  • Do not reduce Impella support prematurely—maintain LV unloading until ECMO flows can be weaned 3
  • Do not overlook RV failure—ECPELLA primarily supports the LV; assess for RV distension with echo 3
  • Monitor for hemolysis (increased with combined devices) and bleeding complications 4

The key is correlating the flat waveforms with clinical perfusion parameters (MAP, lactate, urine output, saturations) rather than assuming they represent malfunction 3.

References

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