ECMO Physiology and Principles 2 — Questions and Answers
Question 1: During VV-ECMO, what is the primary determinant of oxygen delivery to the patient's tissues?
- Sweep gas flow rate
- Native cardiac output (Correct answer)
- ECMO blood flow rate
- Membrane lung surface area
Correct answer: Native cardiac output
In VV-ECMO, oxygen delivery to tissues depends on native cardiac output because the oxygenated blood returning from the ECMO circuit must be pumped by the patient's own heart.
Question 2: Which physical property of the oxygenator membrane determines CO2 removal efficiency in ECMO?
- Membrane thickness and pore size
- Sweep gas flow rate (Correct answer)
- Blood flow path length
- Fiber bundle surface area
Correct answer: Sweep gas flow rate
CO2 removal is primarily controlled by sweep gas flow rate, as CO2 diffuses readily across the membrane and is 'washed out' by the gas flow.
Question 3: What is the effect of hypothermia on oxygen-hemoglobin dissociation during ECMO?
- Shifts curve rightward, improving O2 unloading
- Shifts curve leftward, increasing hemoglobin O2 affinity (Correct answer)
- Has no effect on the dissociation curve
- Eliminates the Bohr effect completely
Correct answer: Shifts curve leftward, increasing hemoglobin O2 affinity
Hypothermia shifts the oxyhemoglobin dissociation curve to the left, increasing hemoglobin's affinity for oxygen and impairing tissue O2 delivery.
Question 4: In VA-ECMO, retrograde aortic flow from a femoral arterial cannula creates a 'mixing zone.' What determines where this zone is located?
- Patient's blood pressure only
- Balance between native cardiac output and ECMO pump flow (Correct answer)
- Sweep gas flow rate
- Venous cannula position
Correct answer: Balance between native cardiac output and ECMO pump flow
The mixing zone (where oxygenated ECMO blood meets desaturated native cardiac output) is determined by the balance between anterograde native cardiac flow and retrograde ECMO flow.
Question 5: What is 'Harlequin syndrome' (differential hypoxia) in the context of VA-ECMO?
- Paradoxical hypertension due to increased afterload
- Upper body hypoxia while lower body is well-oxygenated due to competing blood flows (Correct answer)
- Differential limb ischemia from cannula placement
- Hemolysis caused by shear stress in the pump
Correct answer: Upper body hypoxia while lower body is well-oxygenated due to competing blood flows
Harlequin syndrome occurs when the native heart ejects desaturated blood that perfuses the upper body while the ECMO circuit oxygenates lower body blood, creating differential oxygenation.
Question 6: How does increased ECMO blood flow affect the recirculation fraction in VV-ECMO?
- Decreases recirculation by improving drainage
- Increases recirculation as more oxygenated blood is drawn back into the circuit (Correct answer)
- Has no effect on recirculation
- Eliminates recirculation entirely
Correct answer: Increases recirculation as more oxygenated blood is drawn back into the circuit
Higher ECMO flow increases recirculation because a larger volume of already-oxygenated blood returning to the right atrium is re-aspirated by the drainage cannula before reaching the pulmonary circulation.
Question 7: Which factor primarily determines the maximum blood flow achievable through a venous drainage cannula during ECMO?
- Pump speed (RPM)
- Cannula inner diameter and length (Correct answer)
- Patient's central venous pressure
- Oxygenator fiber bundle resistance
Correct answer: Cannula inner diameter and length
The inner diameter and length of the drainage cannula create the primary resistance to flow; larger bore and shorter cannulas permit higher flow rates per Hagen-Poiseuille law.
ECMO Physiology and Principles 2
During VV-ECMO, what is the primary determinant of oxygen delivery to the patient's tissues?