ECMO - Extracorporeal Membrane Oxygenation ECMO Physiology and Principles Questions and Answers — Questions and Answers
Question 1: In a patient on Veno-Venous (VV) ECMO for severe ARDS, the primary determinant of carbon dioxide removal (decarboxylation) is the:
- ECMO blood flow rate (L/min)
- Fraction of delivered oxygen (FdO2) in the circuit
- Patient's hemoglobin concentration
- Sweep gas flow rate (L/min) (Correct answer)
Correct answer: Sweep gas flow rate (L/min)
Carbon dioxide removal in the ECMO circuit is primarily dependent on the sweep gas flow rate. The fresh gas (sweep gas) flowing through the oxygenator creates a concentration gradient that drives CO2 out of the blood. While blood flow and membrane surface area play a role, the sweep gas flow is the most direct and primary control for adjusting PaCO2.
Question 2: A patient is on peripheral Veno-Arterial (VA) ECMO via the femoral vessels for cardiogenic shock. The patient begins to show signs of myocardial recovery, and the team notes that the SpO2 on the right hand is consistently lower than the SpO2 on the foot. This phenomenon is best described as:
- Recirculation
- Harlequin Syndrome (Differential Hypoxia) (Correct answer)
- Oxygenator failure
- Systemic inflammatory response
Correct answer: Harlequin Syndrome (Differential Hypoxia)
This scenario describes Harlequin Syndrome, or differential hypoxia. It occurs in peripheral VA ECMO when the patient's heart recovers and begins to eject poorly oxygenated blood from the native lungs, which preferentially perfuses the upper body (including the heart and brain). The well-oxygenated blood from the ECMO circuit flows retrograde up the aorta but may not reach the aortic arch, leading to lower oxygen saturation in the upper extremities compared to the lower extremities.
Question 3: Which of the following physiological changes is an expected effect of initiating Veno-Arterial (VA) ECMO in a patient with profound cardiogenic shock?
- Increased left ventricular preload
- Decreased mean arterial pressure
- Increased left ventricular afterload (Correct answer)
- Decreased pulmonary artery pressure
Correct answer: Increased left ventricular afterload
Initiating VA ECMO involves returning oxygenated blood into the arterial system under pressure. This retrograde flow opposes the native cardiac ejection, thereby increasing the afterload that the left ventricle must pump against. While VA ECMO decreases preload by draining venous blood, it characteristically increases LV afterload, which can be detrimental if not managed properly.
Question 4: A patient on Veno-Venous (VV) ECMO for respiratory failure develops worsening hypoxemia despite a high FdO2 of 1.0. The pre-oxygenator saturation (venous blood entering the circuit) is noted to be rising, and the post-oxygenator saturation remains high. Which of the following is the most likely cause?
- Inadequate anticoagulation
- High patient cardiac output
- Significant recirculation (Correct answer)
- Membrane lung thrombosis
Correct answer: Significant recirculation
Recirculation occurs in VV ECMO when a significant fraction of the newly oxygenated blood returned to the patient is immediately drained back into the ECMO circuit without circulating systemically. This is often due to the proximity of the drainage and return cannulas. A key sign is a rising pre-oxygenator saturation, as the circuit is draining already oxygenated blood, which reduces the efficiency of systemic oxygen delivery and causes patient hypoxemia.
Question 5: In managing a patient on VV ECMO, increasing the ECMO blood flow rate (Qb) will have the most direct and significant impact on which of the following parameters?
- Patient's arterial pH
- Patient's PaCO2
- Patient's arterial oxygenation (PaO2) (Correct answer)
- Circuit recirculation fraction
Correct answer: Patient's arterial oxygenation (PaO2)
In VV ECMO, the patient's arterial oxygenation is primarily determined by the amount of oxygenated blood delivered by the circuit relative to the patient's total cardiac output. Increasing the ECMO blood flow rate (Qb) increases the delivery of oxygen (DO2) from the circuit, which has the most direct and significant impact on improving the patient's PaO2 and SaO2. While flow can influence recirculation, its primary therapeutic target is oxygenation.
Question 6: Which statement accurately describes a fundamental physiological difference between Veno-Venous (VV) and Veno-Arterial (VA) ECMO?
- VV ECMO provides direct hemodynamic support, while VA ECMO only supports gas exchange.
- VA ECMO bypasses the pulmonary circulation, while VV ECMO does not. (Correct answer)
- VV ECMO requires central cannulation, while VA ECMO can only be done peripherally.
- VA ECMO is used for isolated respiratory failure, while VV ECMO is used for cardiogenic shock.
Correct answer: VA ECMO bypasses the pulmonary circulation, while VV ECMO does not.
A key difference is that VA ECMO drains blood from the venous system and returns it to the arterial system, effectively bypassing the heart and lungs and providing both cardiac and respiratory support. In contrast, VV ECMO drains blood from and returns it to the venous system, allowing the patient's own heart to pump the newly oxygenated blood through the pulmonary circulation and to the rest of the body; it provides only respiratory support.
ECMO - Extracorporeal Membrane Oxygenation ECMO Physiology and Principles Questions and Answers
In a patient on Veno-Venous (VV) ECMO for severe ARDS, the primary determinant of carbon dioxide removal (decarboxylation) is the: