Hemodynamic Monitoring in ECMO: Avoid Both Fick and Thermodilution Methods
Do not rely on either the Fick method or thermodilution for guiding hemodynamic management in patients on VA or VV ECMO—both techniques produce erroneous measurements due to the extracorporeal circuit. 1
Why These Methods Fail During ECMO
Thermodilution is Unreliable
Continuous thermodilution-based cardiac output monitoring is explicitly not recommended in patients under ECMO because indicator loss into the extracorporeal circuit leads to erroneous results. 1
Thermodilution overestimates cardiac output during VV ECMO, with the degree of overestimation directly proportional to the ECMO flow-to-cardiac output ratio and the recirculation fraction. 2
In VV ECMO patients, transpulmonary thermodilution (TPTD) measurements show marked increases in global end-diastolic volume index (GEDVI) and extravascular lung water index (EVLWI) after ECMO initiation—these increases are artifactual and more pronounced with femoral versus jugular indicator injection. 3
The site of indicator injection matters critically: femoral injection during VV ECMO causes substantial overestimation of volumetric parameters (GEDVI increased by 378 ± 247 mL/m² vs. only 36 ± 210 mL/m² with jugular injection). 4, 3
Even in non-ECMO patients, thermodilution shows poor agreement with the gold-standard Fick method, with median absolute percent error of 17.5% and over 30% of patients showing >25% error. 5
Fick Method Has Limitations
While the Fick method using measured oxygen consumption is considered the gold standard for cardiac output measurement, it has potential pitfalls in critically ill patients, particularly when there is increased oxygen consumption within the lungs. 6
The agreement between Fick and thermodilution is only excellent at cardiac output values <5 L/min; above this threshold, the concordance becomes too loose for clinical interchangeability. 6
What to Use Instead: The Recommended Monitoring Algorithm
Primary Monitoring Parameters 1, 7, 8
- Mean arterial pressure (MAP): Maintain ≥70 mmHg continuously via arterial line monitoring
- ECMO flow: Target 3-4 L/min initially (60-80 mL/kg/min for adults), with continuous recording
- Echocardiography: Perform repeated assessments to evaluate ventricular function, loading conditions, and cannula position
- Central venous oxygen saturation (SvO₂): Monitor continuously as part of tissue perfusion assessment
- Lactate: Serial measurements prioritizing absolute values over clearance rates
- Daily fluid balance: Positive fluid balance is an independent predictor of poor outcomes
Additional Monitoring for VA ECMO 1, 8
- Pulse pressure: Values <20 mmHg in the first 24 hours are associated with acute brain injury and poor outcomes
- Continuous cerebral oximetry (rSO₂): Mandatory for all VA ECMO patients, especially peripheral cannulation, to detect differential hypoxia
- Arteriovenous oxygen difference: Target 3-5 cc O₂ per 100 mL of blood as the most reliable metric for setting flow goals 7
Serial Blood Gas Monitoring 1
- Perform serial arterial blood gas sampling in the first 24 hours of ECMO
- Avoid arterial hypoxemia (PaO₂ <70 mmHg) for 24-48 hours in VA ECMO
- Avoid rapid changes in PaCO₂ (>50% decrease) in the peri-cannulation period, as this is independently associated with increased risk of intracranial hemorrhage
Critical Pitfalls to Avoid
Never use pulse contour analysis-based cardiac output monitoring during ECMO—it may underestimate cardiac output due to altered arterial waveform characteristics. 1
Do not assume volumetric parameters from TPTD are accurate during ECMO—GEDVI and EVLWI measurements are artificially elevated and unreliable for clinical decision-making. 4, 3
Recognize that hypovolemia can induce venous collapse during extracorporeal blood extraction, causing cannula "suctioning" and "chatter" that result in flow reduction and hemolysis—but do not use thermodilution to guide volume status. 1
Volume overload independently predicts poor outcomes in ECMO patients, so accurate fluid balance recording (not thermodilution-derived preload estimates) is essential. 1
The Bottom Line for Clinical Practice
Base your hemodynamic management on the combination of MAP monitoring, ECMO flow rates, repeated echocardiography, SvO₂, lactate trends, and daily fluid balance—not on Fick or thermodilution measurements. 1, 7, 8 The extracorporeal circuit fundamentally invalidates both traditional cardiac output measurement techniques, requiring a shift to these alternative monitoring strategies that have been validated in the ECMO population.