ECMO - Extracorporeal Membrane Oxygenation Complications and Troubleshooting Questions and Answers — Questions and Answers
Question 1: A patient on VV ECMO for ARDS secondary to an atypical pneumonia is noted to have persistently low pulse oximetry readings (SpO2 85-88%) despite an excellent post-oxygenator PaO2 of >400 mmHg on blood gas analysis. The blood in the arterial line of the ECMO circuit appears unusually dark or 'chocolate brown.' This discrepancy is most suggestive of which complication?
- Severe hemolysis
- Significant recirculation
- Methemoglobinemia (Correct answer)
- Carbon monoxide poisoning
Correct answer: Methemoglobinemia
Methemoglobinemia occurs when the iron in hemoglobin is oxidized to the ferric (Fe3+) state, rendering it unable to bind oxygen. This causes a 'chocolate brown' blood appearance and a functional anemia. Standard pulse oximetry becomes inaccurate, reading falsely low (tending towards 85%), while the PaO2, which measures dissolved oxygen, remains high. The diagnosis is confirmed with co-oximetry.
Question 2: During management of a patient on VA ECMO with a centrifugal pump, you observe that the measured circuit blood flow has suddenly decreased from 4.5 LPM to 2.5 LPM, while the pump RPM has been increased from 3500 to 4000 in an attempt to compensate. Which of the following is the most likely cause for this change?
- Inadequate venous drainage due to hypovolemia
- An obstruction in the arterial (return) line (Correct answer)
- Sudden improvement in native cardiac function
- Air embolism in the venous drainage line
Correct answer: An obstruction in the arterial (return) line
Centrifugal pumps are afterload-sensitive. A sudden obstruction in the arterial line (e.g., cannula thrombus, kinked tubing, or dissection) increases the resistance (afterload) the pump must work against. This increased resistance directly causes a drop in flow. The combination of a falling flow despite an increasing RPM is a classic sign of a post-pump obstruction.
Question 3: Which of the following clinical signs is the most specific indicator of an impending oxygenator failure due to plasma leak?
- A progressive increase in the pressure gradient across the oxygenator.
- Frothy, pink-tinged fluid appearing in the oxygenator's gas exhaust port. (Correct answer)
- A gradual decrease in the post-oxygenator PaO2.
- A rapid increase in plasma-free hemoglobin levels.
Correct answer: Frothy, pink-tinged fluid appearing in the oxygenator's gas exhaust port.
A plasma leak occurs when the integrity of the microporous membrane is compromised, allowing plasma and proteinaceous fluid to cross from the blood path into the gas path. This fluid mixes with the sweep gas and is expelled from the gas exhaust port, classically appearing as a frothy, pink-tinged exudate. This is a highly specific sign of this mode of oxygenator failure.
Question 4: A patient on VV ECMO requires Continuous Renal Replacement Therapy (CRRT). The CRRT circuit is initiated, drawing blood from a pre-pump access port and returning it post-oxygenator. Shortly after initiation, the ECMO circuit's pre-membrane (access) pressure becomes significantly more negative, and the venous line begins to chatter. What is the most probable cause?
- The CRRT machine is adding excess volume to the circuit.
- A large clot has formed in the ECMO oxygenator.
- The CRRT machine's ultrafiltration rate is too high, causing patient hypovolemia. (Correct answer)
- The sweep gas flow on the ECMO circuit was accidentally increased.
Correct answer: The CRRT machine's ultrafiltration rate is too high, causing patient hypovolemia.
Aggressive fluid removal (ultrafiltration) by the CRRT machine can rapidly deplete the patient's circulating volume. This resulting hypovolemia leads to insufficient venous return to fill the ECMO circuit, causing the drainage cannula to suck against the vessel walls. This is manifested as an increasingly negative pre-membrane pressure and visible 'chatter' in the line.
Question 5: While assessing a patient on VA ECMO, a large bolus of air is accidentally introduced into the venous drainage line just before the pump. What is the most critical and immediate sequence of actions?
- Place the patient in Trendelenburg and administer a fluid bolus.
- Increase pump flow to push the air through the oxygenator quickly.
- Clamp the arterial and venous lines, and take the patient completely off ECMO support. (Correct answer)
- Decrease the sweep gas to zero and observe the circuit.
Correct answer: Clamp the arterial and venous lines, and take the patient completely off ECMO support.
The immediate, life-saving priority is to prevent the massive air bolus from being pumped to the patient, which would cause a catastrophic systemic air embolism. Clamping both lines and stopping the pump (taking the patient off support) isolates the circuit and contains the air. Subsequent steps would involve attempting to aspirate the air before cautiously resuming support.
Question 6: A known complication of ECMO is acquired von Willebrand syndrome (AVWS), which can contribute significantly to bleeding. What is the primary mechanism by which the ECMO circuit leads to this coagulopathy?
- Shear stress from the pump and circuit components causes unfolding and proteolytic cleavage of large von Willebrand factor multimers. (Correct answer)
- Continuous heparin infusion directly inhibits the function of von Willebrand factor.
- Platelets adhere to the circuit surface, depleting the plasma of available von Willebrand factor.
- The oxygenator membrane absorbs von Willebrand factor, removing it from circulation.
Correct answer: Shear stress from the pump and circuit components causes unfolding and proteolytic cleavage of large von Willebrand factor multimers.
The high shear stress generated by the centrifugal pump and passage through the narrow fibers of the oxygenator mechanically unfolds the large, most hemostatically active multimers of von Willebrand factor (vWF). This conformational change exposes them to cleavage by the ADAMTS13 enzyme, leading to a loss of these large multimers and a functional defect in platelet adhesion and aggregation, which manifests as AVWS.
ECMO - Extracorporeal Membrane Oxygenation Complications and Troubleshooting Questions and Answers
A patient on VV ECMO for ARDS secondary to an atypical pneumonia is noted to have persistently low pulse oximetry readings (SpO2 85-88%) despite an excellent post-oxygenator PaO2 of >400 mmHg on blood gas analysis.
The blood in the arterial line of the ECMO circuit appears unusually dark or 'chocolate brown.' This discrepancy is most suggestive of which complication?
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