ECMO Circuit Components and Management 3 — Questions and Answers
Question 1: What is the function of the bladder (reservoir) in a gravity-siphon ECMO circuit?
- To buffer venous return and trigger servo-regulation if flow drops (Correct answer)
- To filter microemboli before blood reaches the oxygenator
- To regulate FiO2 delivered to the membrane lung
- To store anticoagulant medication for continuous infusion
Correct answer: To buffer venous return and trigger servo-regulation if flow drops
The venous bladder acts as a reservoir that collapses when venous return is inadequate, triggering servo-regulation to reduce pump speed and prevent suction injury.
Question 2: Polymethylpentene (PMP) hollow fiber oxygenators have largely replaced silicone membrane oxygenators in ECMO because PMP offers:
- Lower resistance to blood flow and reduced plasma leakage (Correct answer)
- Higher heparin binding capacity
- Greater CO2 permeability at lower sweep flows
- Lower cost of manufacture
Correct answer: Lower resistance to blood flow and reduced plasma leakage
PMP fibers are true diffusion membranes that minimize plasma leakage (weeping) while providing low resistance and efficient gas exchange.
Question 3: During ECMO, a sudden rise in the pre-membrane pressure with a normal post-membrane pressure indicates:
- Clot formation within the oxygenator fiber bundle (Correct answer)
- Arterial cannula obstruction
- Pump head thrombosis
- Venous drainage line kink
Correct answer: Clot formation within the oxygenator fiber bundle
A rising gradient between pre- and post-membrane pressures signals increasing resistance within the oxygenator, most commonly due to clot accumulation in the fiber bundle.
Question 4: What is the typical target activated clotting time (ACT) range during standard ECMO anticoagulation?
- 180–220 seconds (Correct answer)
- 90–120 seconds
- 300–400 seconds
- 60–80 seconds
Correct answer: 180–220 seconds
Most ECMO centers target an ACT of 180–220 seconds to balance thrombosis prevention in the circuit against bleeding risk in the patient.
Question 5: The 'north-south syndrome' in VA-ECMO via femoral cannulation describes:
- Differential hypoxia where upper body receives poorly oxygenated native cardiac output while lower body receives well-oxygenated ECMO flow (Correct answer)
- Pressure gradient between upper and lower body cannulas
- Migration of the arterial cannula toward the aortic arch
- Bilateral renal ischemia from reduced pulsatile flow
Correct answer: Differential hypoxia where upper body receives poorly oxygenated native cardiac output while lower body receives well-oxygenated ECMO flow
When the recovering heart ejects poorly oxygenated blood, a mixing zone forms in the aorta creating a differential: hypoxic upper body (brain, coronaries) versus oxygenated lower body from ECMO.
Question 6: Which intervention is most appropriate for managing harlequin syndrome in VA-ECMO?
- Add a second venous drainage cannula to convert to VAV configuration or place the return cannula in the right subclavian/axillary artery (Correct answer)
- Increase ECMO flow rate to maximum
- Administer high-dose vasopressors
- Reduce sweep gas FiO2 to match native cardiac output oxygen content
Correct answer: Add a second venous drainage cannula to convert to VAV configuration or place the return cannula in the right subclavian/axillary artery
Converting to a VAV configuration or using an upper-body arterial return ensures oxygenated blood reaches the coronary and cerebral circulations despite poor native cardiac function.
Question 7: Membrane lung 'failure' is best identified by which combination of findings?
- Elevated transmembrane pressure gradient AND deteriorating post-membrane blood gas PO2 (Correct answer)
- Rising sweep gas flow AND falling pump RPM
- Decreasing ACT AND increasing fibrinogen levels
- Falling circuit temperature AND rising venous saturation
Correct answer: Elevated transmembrane pressure gradient AND deteriorating post-membrane blood gas PO2
Oxygenator failure manifests as increasing resistance (elevated pressure gradient) alongside reduced gas exchange efficiency detected by post-membrane blood gas analysis.
ECMO Circuit Components and Management 3
What is the function of the bladder (reservoir) in a gravity-siphon ECMO circuit?