ECMO Physiology and Principles 5 — Questions and Answers
Question 1: What is the significance of the 'venous inlet pressure' (P-in) alarm during ECMO, and what does it indicate?
- High P-in indicates circuit obstruction distal to the pump
- Highly negative P-in indicates inadequate venous return, risking cavitation and hemolysis (Correct answer)
- P-in reflects oxygenator transmembrane pressure gradient
- P-in measures arterial afterload on the pump
Correct answer: Highly negative P-in indicates inadequate venous return, risking cavitation and hemolysis
A very negative inlet (drainage) pressure indicates hypovolemia, cannula malposition, or kinking, and can cause circuit cavitation, hemolysis, and 'chatter' (intermittent collapse of the venous line).
Question 2: In the context of ECMO physiology, what is 'sweep gas' and what is its primary purpose?
- Pressurized saline used to prime the circuit
- A blend of O2 and air passed through the oxygenator to facilitate gas exchange across the membrane (Correct answer)
- Nitrogen gas used to prevent air embolism
- CO2-enriched gas used to calibrate blood gas analyzers
Correct answer: A blend of O2 and air passed through the oxygenator to facilitate gas exchange across the membrane
Sweep gas (typically blended O2 and air) flows through the gas side of the membrane oxygenator, creating the concentration gradient needed for O2 loading and CO2 removal from the blood.
Question 3: Why is systemic anticoagulation required during ECMO, and what is the most common agent used?
- To prevent arterial vasospasm at cannulation sites; warfarin is standard
- To prevent circuit thrombosis from blood-surface contact activation; unfractionated heparin is most common (Correct answer)
- To maintain laminar blood flow through the oxygenator; bivalirudin is standard
- To prevent air embolism by lowering blood viscosity; aspirin is routinely used
Correct answer: To prevent circuit thrombosis from blood-surface contact activation; unfractionated heparin is most common
Blood contact with the artificial circuit surfaces activates the coagulation cascade; unfractionated heparin is the most widely used anticoagulant to prevent life-threatening circuit thrombosis.
Question 4: What is the 'oxygen delivery index' (DO2I) target during ECMO, and why is it important?
- >600 mL/min/m² to ensure adequate supraphysiologic oxygen reserves
- Approximately 520-600 mL/min/m² to match normal physiologic oxygen demand (Correct answer)
- >1000 mL/min/m² to compensate for ECMO-related hemolysis losses
- 300-400 mL/min/m² since ECMO patients have reduced metabolic needs
Correct answer: Approximately 520-600 mL/min/m² to match normal physiologic oxygen demand
Targeting physiologic DO2I (approximately 520-600 mL/min/m²) ensures adequate tissue oxygenation while avoiding excessive flow that could increase hemolysis and recirculation.
Question 5: How does the position of the return (reinfusion) cannula relative to the drainage cannula affect recirculation in single-access dual-lumen VV-ECMO?
- Position has no effect since blood flow is controlled by the pump
- The return jet must be directed toward the tricuspid valve to minimize recirculation (Correct answer)
- The return jet should face the drainage holes to maximize mixing
- The cannula must be placed in the SVC to prevent recirculation
Correct answer: The return jet must be directed toward the tricuspid valve to minimize recirculation
In dual-lumen cannulas (e.g., Avalon), proper positioning directs the return jet toward the tricuspid valve, propelling oxygenated blood into the right ventricle rather than back toward the drainage holes.
Question 6: What does 'circuit compliance' refer to in ECMO, and how does it affect blood pressure waveform dampening?
- The ability of circuit tubing to expand and store energy, which attenuates pulse pressure (Correct answer)
- The rigidity of the oxygenator membrane under high blood flow
- The elasticity of the pump head under varying RPM loads
- The capacity of the heat exchanger to buffer temperature changes
Correct answer: The ability of circuit tubing to expand and store energy, which attenuates pulse pressure
Compliant (flexible) ECMO tubing absorbs kinetic energy during each pump stroke, damping the arterial pulse waveform and contributing to reduced pulsatility during VA-ECMO.
Question 7: What is the physiological consequence of severe 'LV distension' that can occur during VA-ECMO, and how does it manifest clinically?
- Decreased myocardial oxygen demand due to reduced wall tension
- Increased LV wall stress, subendocardial ischemia, pulmonary edema, and thrombus formation in the stagnant LV (Correct answer)
- Improved coronary perfusion from elevated aortic root pressure
- Reduced risk of arrhythmias due to decreased catecholamine release
Correct answer: Increased LV wall stress, subendocardial ischemia, pulmonary edema, and thrombus formation in the stagnant LV
LV distension from afterload increase and blood accumulation raises wall stress and myocardial oxygen demand, worsens pulmonary edema, and creates stagnant LV blood prone to thrombus formation.
ECMO Physiology and Principles 5
What is the significance of the 'venous inlet pressure' (P-in) alarm during ECMO, and what does it indicate?