ECMO Physiology and Principles 3 — Questions and Answers
Question 1: What is the primary mechanism by which ECMO causes hemolysis?
- Osmotic stress from blood-gas interface
- Shear stress from the centrifugal or roller pump mechanism (Correct answer)
- Contact activation of the complement cascade
- Turbulent flow in the oxygenator fibers
Correct answer: Shear stress from the centrifugal or roller pump mechanism
Mechanical shear stress generated by the ECMO pump (particularly at high flows or with small tubing) is the primary cause of red blood cell destruction during ECMO.
Question 2: During ECMO, why is the SvO2 (mixed venous oxygen saturation) measured in the drainage limb clinically important?
- It reflects oxygenator membrane integrity
- It indicates adequacy of oxygen delivery relative to consumption (Correct answer)
- It measures recirculation fraction directly
- It reflects sweep gas flow adequacy
Correct answer: It indicates adequacy of oxygen delivery relative to consumption
Low drainage-limb SvO2 indicates that tissues are extracting more oxygen than is being delivered, signaling inadequate ECMO support or increased metabolic demand.
Question 3: What happens to PaCO2 when sweep gas flow is doubled while ECMO blood flow remains constant?
- PaCO2 increases proportionally
- PaCO2 decreases, potentially causing hypocapnia (Correct answer)
- PaCO2 remains unchanged as it depends on blood flow
- PaCO2 increases due to back-diffusion
Correct answer: PaCO2 decreases, potentially causing hypocapnia
Doubling sweep gas flow dramatically increases CO2 removal, often causing significant hypocapnia, so sweep adjustments require careful monitoring of blood gas values.
Question 4: In a centrifugal ECMO pump, what is the effect of an acute increase in afterload (e.g., arterial hypertension) on pump flow?
- Flow increases due to increased driving pressure
- Flow decreases as the pump is sensitive to pressure changes (Correct answer)
- Flow remains constant as centrifugal pumps are flow-controlled
- Flow oscillates due to pressure-flow coupling
Correct answer: Flow decreases as the pump is sensitive to pressure changes
Unlike roller pumps, centrifugal pumps are not occlusive and their flow decreases when afterload increases, making continuous flow monitoring essential.
Question 5: What is the physiological rationale for targeting a lower 'lung-protective' ventilator strategy (ultra-protective ventilation) during VV-ECMO?
- ECMO eliminates the need for any ventilation
- ECMO assumes gas exchange, allowing the lungs to rest and potentially recover from VILI (Correct answer)
- Lower ventilation reduces recirculation
- Reduced ventilation decreases sweep gas requirements
Correct answer: ECMO assumes gas exchange, allowing the lungs to rest and potentially recover from VILI
With ECMO handling gas exchange, tidal volumes and pressures can be minimized to prevent ventilator-induced lung injury (VILI) and promote lung recovery.
Question 6: How does the 'oxygen transfer capacity' of an ECMO oxygenator change over time during prolonged use?
- It increases as the fibers become conditioned
- It decreases due to plasma leak, protein deposition, and thrombus formation (Correct answer)
- It remains stable for the device's rated lifespan
- It fluctuates based on blood flow rate only
Correct answer: It decreases due to plasma leak, protein deposition, and thrombus formation
Over time, plasma leakage through membrane pores, protein deposition, and thrombus formation on fibers progressively reduce the oxygenator's gas transfer efficiency.
Question 7: What is the clinical significance of a sudden decrease in the pressure gradient across the ECMO oxygenator?
- Indicates improved patient lung function
- May signal oxygenator thrombosis or clot formation within the device (Correct answer)
- Indicates a circuit air embolism
- Reflects improved pump efficiency
Correct answer: May signal oxygenator thrombosis or clot formation within the device
An increasing or suddenly changing transmembrane pressure gradient across the oxygenator may indicate thrombus formation, prompting urgent evaluation and possible oxygenator exchange.
ECMO Physiology and Principles 3
What is the primary mechanism by which ECMO causes hemolysis?