ECMO Physiology and Principles 4 — Questions and Answers
Question 1: In VA-ECMO, how does left ventricular (LV) afterload change compared to a patient without ECMO support?
- LV afterload decreases due to blood volume offloading
- LV afterload increases because retrograde ECMO flow opposes ventricular ejection (Correct answer)
- LV afterload is unchanged since ECMO bypasses the left heart
- LV afterload fluctuates with sweep gas adjustments
Correct answer: LV afterload increases because retrograde ECMO flow opposes ventricular ejection
Retrograde aortic flow from femoral VA-ECMO increases LV afterload, which can worsen LV distension and pulmonary edema in patients with severely impaired LV function.
Question 2: What does the term 'ECMO recirculation' mean, and how is it measured?
- Blood bypassing the oxygenator; measured by pre/post oxygenator SpO2
- Re-aspiration of already-oxygenated blood into the drainage cannula; estimated from SvO2 and SaO2 values (Correct answer)
- Air entrainment into the circuit; measured by bubble detector
- Backflow from the patient to the pump; measured by flow probe
Correct answer: Re-aspiration of already-oxygenated blood into the drainage cannula; estimated from SvO2 and SaO2 values
Recirculation is the fraction of oxygenated blood returning from the ECMO circuit that is immediately re-drained before circulating through the patient, estimated using venous and arterial saturation values.
Question 3: Which of the following best explains why pulsatility is important to maintain during VA-ECMO support?
- Pulsatility prevents oxygenator membrane fouling
- Pulsatile flow preserves microvascular function and organ perfusion pressure waveforms (Correct answer)
- Non-pulsatile flow increases ECMO circuit thrombosis risk
- Pulsatility improves oxygenator gas transfer efficiency
Correct answer: Pulsatile flow preserves microvascular function and organ perfusion pressure waveforms
Pulsatile flow maintains physiologic microvascular tone and endothelial function; prolonged non-pulsatile VA-ECMO flow is associated with end-organ dysfunction.
Question 4: What is the 'pressure-flow' curve concept as it applies to ECMO centrifugal pumps?
- The relationship between patient blood pressure and cardiac output
- The relationship between pump speed and resulting flow, which shifts with changes in preload and afterload (Correct answer)
- The relationship between sweep gas pressure and CO2 removal
- The relationship between circuit resistance and hemolysis rate
Correct answer: The relationship between pump speed and resulting flow, which shifts with changes in preload and afterload
Centrifugal pump pressure-flow curves show that at a given RPM, flow varies with inlet pressure (preload) and outlet resistance (afterload), requiring continuous flow monitoring rather than relying on RPM alone.
Question 5: During ECMO, what is the physiological basis for 'metabolic alkalosis' developing when citrate anticoagulation is used?
- Citrate directly inhibits renal acid excretion
- Citrate is metabolized to bicarbonate in the liver, raising plasma pH (Correct answer)
- Citrate chelates CO2 from the bloodstream
- Citrate inhibits carbonic anhydrase activity
Correct answer: Citrate is metabolized to bicarbonate in the liver, raising plasma pH
Citrate is metabolized in the liver to bicarbonate, and in high doses (or with impaired liver function) can cause significant metabolic alkalosis.
Question 6: How does the dissolved oxygen (Henry's Law) contribute to ECMO oxygenation at very high FiO2 sweep gas settings?
- Dissolved O2 becomes negligible at high FiO2
- Dissolved O2 increases linearly with partial pressure, providing meaningful additional O2 delivery (Correct answer)
- Dissolved O2 exceeds hemoglobin-bound O2 at FiO2 >0.8
- Dissolved O2 contributes only to CO2 removal, not oxygenation
Correct answer: Dissolved O2 increases linearly with partial pressure, providing meaningful additional O2 delivery
At high PaO2 values achievable via ECMO at FiO2 1.0, dissolved oxygen (0.003 mL/mmHg/dL) contributes meaningfully to total oxygen content, especially in severely anemic patients.
Question 7: What physiological phenomenon explains why a patient's SaO2 may improve after reducing ECMO blood flow in VV-ECMO?
- Lower flow reduces turbulence and improves oxygenator efficiency
- Reduced flow decreases recirculation, allowing more fresh venous blood to be oxygenated (Correct answer)
- Lower flow decreases CO2 production by reducing hemolysis
- Reduced flow allows the native lungs to contribute more to oxygenation
Correct answer: Reduced flow decreases recirculation, allowing more fresh venous blood to be oxygenated
Paradoxically, reducing VV-ECMO flow can decrease recirculation, meaning a greater fraction of the blood processed is truly desaturated venous blood rather than already-oxygenated recirculated blood.
ECMO Physiology and Principles 4
In VA-ECMO, how does left ventricular (LV) afterload change compared to a patient without ECMO support?