CES ECMO Physiology & Gas Exchange 2 — Questions and Answers
Question 1: The 'Harlequin' (North-South) syndrome in femoral VA-ECMO occurs when:
- Pump speed is too high, causing hemolysis in the circuit
- The drainage cannula becomes acutely obstructed
- The upper body receives desaturated native cardiac output while the lower body receives oxygenated ECMO blood (Correct answer)
- Air embolism enters the arterial return cannula
Correct answer: The upper body receives desaturated native cardiac output while the lower body receives oxygenated ECMO blood
When the native heart partially recovers and ejects desaturated blood, the coronary arteries and brain may receive poorly oxygenated blood while the ECMO return site (femoral artery) perfuses the lower body with well-oxygenated blood.
Question 2: In VV-ECMO, which factor most directly determines the patient's cardiac output?
- ECMO pump flow rate
- Sweep gas FiO2 setting
- The patient's own intrinsic cardiac function and vascular tone (Correct answer)
- Oxygenator membrane surface area
Correct answer: The patient's own intrinsic cardiac function and vascular tone
VV-ECMO does not provide direct cardiac support; the patient's native cardiac output depends entirely on their own myocardial function and loading conditions.
Question 3: A progressively decreasing pre-membrane to post-membrane PO2 gradient in the oxygenator over several days suggests:
- Improved native lung function reducing ECMO dependence
- Progressive membrane lung failure or clotting reducing gas transfer (Correct answer)
- A decrease in ECMO flow requirements
- Resolution of the patient's underlying respiratory disease
Correct answer: Progressive membrane lung failure or clotting reducing gas transfer
A falling transmembrane PO2 gradient indicates the oxygenator is becoming less efficient, often due to fibrin/thrombus deposition reducing the effective gas-exchange surface.
Question 4: In ECMO, carbon dioxide clearance is most directly controlled by adjusting:
- ECMO blood flow rate (pump speed)
- FDO2 (fraction of delivered oxygen in sweep gas)
- Sweep gas flow rate (Correct answer)
- Membrane oxygenator surface area alone
Correct answer: Sweep gas flow rate
CO2 removal is highly sensitive to sweep gas flow rate; increasing sweep gas increases the CO2 gradient across the membrane, enhancing clearance.
Question 5: Transmembrane pressure (TMP) across the ECMO oxygenator is monitored primarily to detect:
- Changes in the patient's cardiac output
- Clot formation or membrane failure increasing resistance within the oxygenator (Correct answer)
- Changes in sweep gas composition
- Patient fluid balance status
Correct answer: Clot formation or membrane failure increasing resistance within the oxygenator
A rising TMP indicates increased resistance across the oxygenator, often due to thrombus formation, signaling the need for oxygenator replacement.
Question 6: In VA-ECMO, the presence of a pulsatile arterial waveform on the arterial line monitoring indicates:
- Adequate oxygenator function
- Residual or recovering native cardiac contractility (Correct answer)
- Adequate circuit anticoagulation
- Circuit recirculation
Correct answer: Residual or recovering native cardiac contractility
Pulsatility on the arterial waveform reflects the native heart ejecting blood; loss of pulsatility at high ECMO flows indicates full cardiac bypass.
Question 7: The oxygen transfer capacity of an ECMO membrane oxygenator is most directly influenced by:
- Patient core body temperature alone
- Blood flow rate through the oxygenator (Correct answer)
- The crystalloid volume used for circuit priming
- The patient's anticoagulation level
Correct answer: Blood flow rate through the oxygenator
Higher blood flow through the oxygenator increases oxygen transfer up to the membrane's rated capacity; inadequate flow limits contact time and gas exchange.
CES ECMO Physiology & Gas Exchange 2
The 'Harlequin' (North-South) syndrome in femoral VA-ECMO occurs when: