ECMO Physiology and Pathophysiology 2 — Questions and Answers
Question 1: During VV-ECMO, a patient's PaO2 remains low despite adequate circuit flow. What is the most likely cause?
- Membrane lung thrombosis
- High recirculation fraction (Correct answer)
- Inadequate anticoagulation
- Pump head failure
Correct answer: High recirculation fraction
High recirculation occurs when oxygenated blood returning from the circuit is immediately re-entrained by the drainage cannula, reducing effective oxygen delivery to the patient.
Question 2: Which hemodynamic effect distinguishes VA-ECMO from VV-ECMO in cardiogenic shock?
- VA-ECMO increases preload while VV-ECMO decreases it
- VA-ECMO provides systemic perfusion pressure support while VV-ECMO does not (Correct answer)
- VV-ECMO reduces pulmonary vascular resistance more effectively
- VV-ECMO directly augments cardiac output
Correct answer: VA-ECMO provides systemic perfusion pressure support while VV-ECMO does not
VA-ECMO directly bypasses the heart and lungs, providing mechanical circulatory support and augmenting systemic arterial pressure, whereas VV-ECMO only supports gas exchange.
Question 3: What is the physiological consequence of 'North-South syndrome' (Harlequin syndrome) in femoral VA-ECMO?
- Lower body receives poorly oxygenated blood from the ECMO circuit
- Upper body receives poorly oxygenated blood from the native failing heart while lower body receives oxygenated ECMO blood (Correct answer)
- The coronary arteries are preferentially perfused by ECMO flow
- Cerebral autoregulation is abolished by retrograde flow
Correct answer: Upper body receives poorly oxygenated blood from the native failing heart while lower body receives oxygenated ECMO blood
In femoral VA-ECMO, oxygenated retrograde ECMO flow may not reach the upper body if the recovering heart ejects deoxygenated blood antegrade, creating a watershed zone of hypoxemia in the upper extremities and brain.
Question 4: How does increased ECMO pump speed affect left ventricular (LV) wall stress in VA-ECMO?
- Decreases LV wall stress by reducing afterload
- Increases LV wall stress by increasing afterload via elevated aortic pressure (Correct answer)
- Has no effect on LV wall stress
- Reduces LV wall stress by improving coronary perfusion
Correct answer: Increases LV wall stress by increasing afterload via elevated aortic pressure
Higher VA-ECMO flows increase mean arterial pressure retrograde into the aorta, elevating LV afterload and potentially worsening LV distension if the ventricle cannot eject against the increased pressure.
Question 5: In a patient on VV-ECMO, the sweep gas FiO2 is increased from 0.6 to 1.0 with minimal change in patient PaO2. What does this suggest?
- The membrane lung has failed
- Recirculation or inadequate ECMO blood flow relative to cardiac output (Correct answer)
- The patient has developed a tension pneumothorax
- Hemolysis is impairing oxygen carrying capacity
Correct answer: Recirculation or inadequate ECMO blood flow relative to cardiac output
When increasing oxygenator FiO2 has little effect on systemic oxygenation, insufficient ECMO flow relative to native cardiac output means a large fraction of venous blood bypasses the circuit entirely.
Question 6: What is the primary determinant of CO2 removal on ECMO?
- Blood flow (pump speed)
- Sweep gas flow rate (Correct answer)
- Membrane lung surface area alone
- Patient's native minute ventilation
Correct answer: Sweep gas flow rate
CO2 removal is primarily governed by sweep gas flow rate because CO2 diffuses readily across the membrane, and higher sweep flows increase the concentration gradient driving CO2 out of the blood.
Question 7: Which pathophysiological mechanism explains why patients in refractory hypoxemic respiratory failure develop pulmonary hypertension that may necessitate ECMO?
- Systemic vasodilation from inflammatory mediators
- Hypoxic pulmonary vasoconstriction causing increased right ventricular afterload (Correct answer)
- Left ventricular failure reducing pulmonary perfusion pressure
- Microthrombi in the pulmonary veins
Correct answer: Hypoxic pulmonary vasoconstriction causing increased right ventricular afterload
Hypoxic pulmonary vasoconstriction is a protective reflex that diverts blood from poorly ventilated lung regions, but when hypoxemia is diffuse it causes global pulmonary vasoconstriction, dramatically increasing right ventricular afterload.
ECMO Physiology and Pathophysiology 2
During VV-ECMO, a patient's PaO2 remains low despite adequate circuit flow.
What is the most likely cause?