Q56 / 110
A 42-year-old man with severe ARDS is placed on VV ECMO. Despite unchanged ECMO settings, his oxygen saturation suddenly drops from 92% to 84%. Which of the following is the most likely explanation?
Correct answer: B — Increase in cardiac output
VV ECMO oxygenation depends heavily on the ratio of ECMO blood flow to cardiac output. If cardiac output increases without a corresponding increase in ECMO flow, a smaller fraction of blood is oxygenated by the circuit. This leads to lower arterial oxygen saturation. Sweep gas flow mainly affects CO2 removal, not oxygenation. Increasing ECMO flow would improve oxygenation.
Board pearl: On VV ECMO, increases in cardiac output can worsen oxygenation by reducing the ECMO flow-to-cardiac-output ratio.
VV ECMO oxygenation depends heavily on the ratio of ECMO blood flow to cardiac output. If cardiac output increases without a corresponding increase in ECMO flow, a smaller fraction of blood is oxygenated by the circuit. This leads to lower arterial oxygen saturation. Sweep gas flow mainly affects CO2 removal, not oxygenation. Increasing ECMO flow would improve oxygenation.
Board pearl: On VV ECMO, increases in cardiac output can worsen oxygenation by reducing the ECMO flow-to-cardiac-output ratio.
Q57 / 110
A patient on VV ECMO has persistent hypoxemia despite maximal ECMO settings. Which intervention is most likely to improve oxygenation?
Correct answer: B — Increase ECMO blood flow
The key determinant of oxygenation on VV ECMO is the ratio of ECMO blood flow to cardiac output. Increasing flow increases the proportion of blood that is oxygenated extracorporeally. Sweep gas primarily affects CO2 removal. Hemoglobin is important for oxygen content but decreasing it would worsen oxygen delivery. Ventilator FiO2 plays a secondary role compared with ECMO flow.
Board pearl: Increasing ECMO blood flow improves oxygenation by increasing the fraction of cardiac output passing through the circuit.
The key determinant of oxygenation on VV ECMO is the ratio of ECMO blood flow to cardiac output. Increasing flow increases the proportion of blood that is oxygenated extracorporeally. Sweep gas primarily affects CO2 removal. Hemoglobin is important for oxygen content but decreasing it would worsen oxygen delivery. Ventilator FiO2 plays a secondary role compared with ECMO flow.
Board pearl: Increasing ECMO blood flow improves oxygenation by increasing the fraction of cardiac output passing through the circuit.
Q58 / 110
A 55-year-old woman on VV ECMO for ARDS develops worsening hypercapnia. Which adjustment is most appropriate?
Correct answer: A — Increase sweep gas flow
CO2 removal during VV ECMO is highly efficient and primarily controlled by sweep gas flow. Increasing sweep increases the diffusion gradient for CO2 removal. Blood flow has a lesser effect once adequate flow is achieved. Ventilator changes have limited effect compared with sweep. Therefore, increasing sweep gas is the correct intervention.
Board pearl: Sweep gas flow is the primary determinant of CO2 removal on VV ECMO.
CO2 removal during VV ECMO is highly efficient and primarily controlled by sweep gas flow. Increasing sweep increases the diffusion gradient for CO2 removal. Blood flow has a lesser effect once adequate flow is achieved. Ventilator changes have limited effect compared with sweep. Therefore, increasing sweep gas is the correct intervention.
Board pearl: Sweep gas flow is the primary determinant of CO2 removal on VV ECMO.
Q59 / 110
A patient on VV ECMO has a central venous oxygen saturation of 85%, but appears clinically in shock. What is the most likely explanation?
Correct answer: B — Recirculation causing falsely elevated venous saturation
In VV ECMO, recirculation occurs when oxygenated blood returns to the drainage cannula without first passing through the systemic circulation. This can falsely elevate venous oxygen saturation despite poor tissue perfusion. Therefore, venous saturation must be interpreted cautiously. Clinical context and other perfusion markers are essential.
Board pearl: Recirculation can falsely elevate venous oxygen saturation on VV ECMO.
In VV ECMO, recirculation occurs when oxygenated blood returns to the drainage cannula without first passing through the systemic circulation. This can falsely elevate venous oxygen saturation despite poor tissue perfusion. Therefore, venous saturation must be interpreted cautiously. Clinical context and other perfusion markers are essential.
Board pearl: Recirculation can falsely elevate venous oxygen saturation on VV ECMO.
Q60 / 110
Which of the following is the most important consideration when assessing cardiac output in a patient on VV ECMO?
Correct answer: B — Indicator dilution and thermodilution are confounded by indicator loss into the circuit; a multimodal approach, often anchored by echocardiography, is preferred over any single modality
Indicator dilution and thermodilution techniques are unreliable on VV ECMO because indicator is lost into the extracorporeal circuit, which tends to overestimate cardiac output. Mixed venous oxygen saturation is confounded by ECMO return flow and recirculation. Pulse contour analysis has its own significant limitations and is not validated as a gold standard on ECMO — it is not simply "most reliable" by default. A recent review emphasizes that no single monitoring modality adequately captures hemodynamic complexity during ECMO and recommends a multimodal approach; transthoracic or transesophageal echocardiography is often the most practical anchor, with one study showing acceptable agreement between pulse wave analysis (PRAM) and TTE-derived cardiac output during VV ECMO, though this is based on limited data. To be precise about the relative reliability of methods: pulse contour analysis is better described as "less confounded" by the extracorporeal circuit than thermodilution — not as unconditionally reliable — and it mainly provides trend information rather than absolute accuracy. Echocardiography (particularly 3D echo or LVOT VTI-based methods) is generally regarded as the practical reference standard for cardiac output on VV-ECMO, with thermodilution methods showing a mean overestimation bias of roughly +2 L/min due to indicator loss into the circuit.
Board pearl: No single cardiac output monitoring modality is reliable in isolation on VV ECMO — dilution methods are confounded by circuit loss, and pulse contour analysis is not a validated gold standard; use a multimodal approach anchored by echocardiography. Pulse contour analysis is "less confounded," not definitively "most reliable" — echocardiography remains the practical reference standard.
Indicator dilution and thermodilution techniques are unreliable on VV ECMO because indicator is lost into the extracorporeal circuit, which tends to overestimate cardiac output. Mixed venous oxygen saturation is confounded by ECMO return flow and recirculation. Pulse contour analysis has its own significant limitations and is not validated as a gold standard on ECMO — it is not simply "most reliable" by default. A recent review emphasizes that no single monitoring modality adequately captures hemodynamic complexity during ECMO and recommends a multimodal approach; transthoracic or transesophageal echocardiography is often the most practical anchor, with one study showing acceptable agreement between pulse wave analysis (PRAM) and TTE-derived cardiac output during VV ECMO, though this is based on limited data. To be precise about the relative reliability of methods: pulse contour analysis is better described as "less confounded" by the extracorporeal circuit than thermodilution — not as unconditionally reliable — and it mainly provides trend information rather than absolute accuracy. Echocardiography (particularly 3D echo or LVOT VTI-based methods) is generally regarded as the practical reference standard for cardiac output on VV-ECMO, with thermodilution methods showing a mean overestimation bias of roughly +2 L/min due to indicator loss into the circuit.
Board pearl: No single cardiac output monitoring modality is reliable in isolation on VV ECMO — dilution methods are confounded by circuit loss, and pulse contour analysis is not a validated gold standard; use a multimodal approach anchored by echocardiography. Pulse contour analysis is "less confounded," not definitively "most reliable" — echocardiography remains the practical reference standard.
Source(s): Linden K, Schmandt M, Muders T, et al. Estimation of Cardiac Output Under Veno-Venous Extracorporeal Membrane Oxygenation: Comparing Thermodilution Methods to 3D Echocardiography. ASAIO J. 2025;71(1):75-81. doi:10.1097/MAT.0000000000002283. | Mirus M, Klinitzke P, Spieth PM. Hemodynamic Monitoring During ECMO. Minerva Anestesiol. 2026 Jul-Aug;92(7-8):750-761. doi:10.23736/S0375-9393.26.19888-5. | Greiwe G, Flick M, Hapfelmeier A, et al. Agreement Between Cardiac Output Measurements by Pulse Wave Analysis... and Transthoracic Echocardiography in VV-ECMO. Eur J Anaesthesiol. 2023;40(6):436-441. doi:10.1097/EJA.0000000000001828. | Greiwe G, Flick M, Hapfelmeier A, et al. Agreement Between Cardiac Output Measurements by Pulse Wave Analysis... and TTE in VV-ECMO. Eur J Anaesthesiol. 2023;40(6):436-441. doi:10.1097/EJA.0000000000001828.