Calculating Shunt Fraction During ECMO
The shunt fraction during ECMO is calculated using indicator dilution techniques, most commonly the lithium dilution method, which involves injecting lithium chloride into the ECMO circuit and simultaneously measuring concentration-time curves in both arterial blood and the ECMO circuit to derive both cardiac output and recirculation/shunt fraction. 1
The Lithium Dilution Method (Gold Standard for VV ECMO)
The most established technique uses isotonic lithium chloride (0.15 mmol or 150 mmol/l solution) as the indicator 1, 2:
Injection and Measurement:
- Inject lithium chloride into the ECMO flow returning to the patient (just downstream of the membrane oxygenator)
- Use two lithium sensors simultaneously to record lithium dilution curves:
- One in arterial blood (systemic circulation)
- One in the ECMO circuit (just upstream of the membrane oxygenator)
- The shunt fraction is derived from analyzing these paired concentration-time curves 1
Accuracy: In vitro validation demonstrates excellent precision with the formula: Measured shunt flow = 0.008 + 1.09 × actual shunt flow (R = 0.997) 2. This method allows calculation of both cardiac output and shunt fraction simultaneously.
Alternative Methods
Modified Thermodilution (VV ECMO)
For veno-venous ECMO specifically, a modified thermodilution technique can measure recirculation fraction and cardiac output simultaneously 3:
- Inject cold bolus into the arterial ECMO limb
- Place thermistors in both ECMO limbs and the pulmonary artery
- Calculate recirculation fraction from the ratio of areas under the temperature curves (AUC method) or from flow-based partitioning of bolus volumes
- Accuracy: Recirculation fraction bias of -5.4% with limits of agreement -18.6% to 7.9% for AUC method 3
Novel Sweep-Gas Control Method
A newer, less invasive approach temporarily ceases sweep gas and measures oxygen saturation changes 4:
- Temporarily stop sweep gas flow
- Monitor oxygen saturation at oxygenator inlet (SpreO₂) and outlet (SpostO₂)
- The ratio of temporal changes (ΔSpreO₂/ΔSpostO₂) converges to the recirculation ratio
- Prediction time: 3-4 seconds for recirculation ≤50%, longer for higher recirculation
- Accuracy: Relative prediction error of 7.0% ± 5.2% in circuit experiments 4
Fick Principle Application (Central VA ECMO)
For central VA ECMO (right atrial-to-aorta), a 'shunt fraction' method based on the Fick principle can derive native cardiac output during weaning 5. This represents the newest adaptation for VA ECMO configurations.
Critical Clinical Considerations
Recirculation fraction is flow-dependent: As ECMO blood flow rate decreases, recirculation fraction decreases significantly 6. This affects the accuracy of perfusion assessments and must be accounted for when interpreting measurements.
Common pitfall: Standard thermodilution is unreliable in VV ECMO without modification 3. Do not use conventional cardiac output monitoring techniques without adapting the methodology for the ECMO circuit.
Practical utility: These measurements allow optimal adjustment of ECMO flow rates and cardiovascular support, potentially improving patient management and informing cannula positioning decisions 1, 2.