Extracorporeal Membrane Oxygenation (ECMO): VV and VA Configurations, Cannulation, and Management

Quick Answer

Extracorporeal Membrane Oxygenation (ECMO) provides temporary cardiopulmonary support by draining venous blood, removing carbon dioxide and adding oxygen through a membrane oxygenator, and returning oxygenated blood to the circulation. VV-ECMO (venovenous) provides respiratory support only: blood drains from the venous system (typically femoral and/or internal jugular veins) and returns to the right atrium, requiring native cardiac function for systemic perfusion. VA-ECMO (venoarterial) provides both respiratory and circulatory support: venous drainage is returned to the arterial system (typically femoral or central cannulation), bypassing the heart and lungs entirely. Cannulation configurations include single-site dual-lumen (Avalon, Origen) for VV-ECMO; femoral-femoral VA-ECMO; and central VA-ECMO (right atrium to ascending aorta) for postcardiotomy support. Anticoagulation is mandatory with unfractionated heparin targeting ACT 180-220 seconds or aPTT 40-60 seconds; direct thrombin inhibitors (argatroban, bivalirudin) are alternatives in heparin-induced thrombocytopenia. Key complications include bleeding (30-50%), thrombosis (circuit clotting, stroke), hemolysis, infection (catheter-related), limb ischemia (VA-ECMO), and neurological injury. Weaning involves assessment of native cardiopulmonary function through flow reduction trials; successful decannulation requires hemodynamic stability, adequate oxygenation and ventilation, and correction of underlying pathology. Survival varies by indication: VV-ECMO for ARDS 55-65% (EOLIA trial), VA-ECMO for cardiogenic shock 35-50%, ECPR 15-30%.

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Clinical Overview

Extracorporeal membrane oxygenation represents the most advanced form of temporary mechanical cardiopulmonary support, bridging patients with severe respiratory or cardiac failure to recovery, definitive therapy, or long-term support devices. Understanding the physiological principles, cannulation strategies, circuit management, and complications of ECMO is essential for anaesthetists and intensivists working in tertiary cardiac and critical care centres.

Definition and Scope

ECMO is a modified form of cardiopulmonary bypass that provides prolonged (days to weeks) extracorporeal gas exchange and circulatory support. Unlike conventional cardiopulmonary bypass used for cardiac surgery, ECMO circuits are designed for longer duration with biocompatible surfaces, centrifugal pumps, and membrane oxygenators that minimize hemolysis and inflammatory activation. [1,2]

Indications and Patient Selection

EOLIA Trial Criteria for VV-ECMO [3]:

• PaO₂/FiO₂ <50 mmHg for >3 hours, or
• PaO₂/FiO₂ <80 mmHg for >6 hours, or
• Arterial pH <7.25 with PaCO₂ >60 mmHg for >6 hours (permissive hypercapnia failure)

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ECMO Configurations

VV-ECMO: Physiology and Indications

Physiological Principles:

Venovenous ECMO provides isolated respiratory support without direct hemodynamic assistance. The native heart remains the sole source of systemic perfusion; therefore, VV-ECMO is contraindicated in patients with significant cardiac dysfunction.

Blood Flow Pathway:

1. Drainage: Deoxygenated blood removed from venous system (typically femoral vein, internal jugular vein, or both)
2. Oxygenation: Blood passes through membrane oxygenator where CO₂ is removed and O₂ added
3. Return: Oxygenated blood returned to venous system (right atrium via IJV or femoral)
4. Recirculation: Some returned blood is immediately redrained (recirculation fraction), reducing efficiency

Key Physiological Limitations: [4]

Oxygen Delivery Calculation in VV-ECMO:

Arterial oxygen content represents a mixture of:

1. Oxygenated blood from ECMO circuit (flow × post-oxygenator saturation)
2. Native venous return bypassing the ECMO circuit (native cardiac output minus ECMO flow)

CaO_2 = \frac{(\text{ECMO flow} \times S_{post}O_2) + [(\text{CO} - \text{ECMO flow}) \times S_vO_2]}{\text{CO}} \times [Hb] \times 1.34 + (PaO_2 \times 0.003)

Where maximum oxygen delivery is limited by the patient's native cardiac output. [5]

VA-ECMO: Physiology and Indications

Physiological Principles:

Venoarterial ECMO provides both respiratory support and circulatory assistance, effectively bypassing the heart and lungs entirely. Blood is drained from the venous system, oxygenated, and returned to the arterial circulation, creating a parallel circulation.

Blood Flow Pathway:

1. Drainage: Deoxygenated blood from venous system (femoral vein, right atrium)
2. Oxygenation: Gas exchange in membrane oxygenator
3. Return: Oxygenated blood to arterial system (femoral artery, ascending aorta)
4. Parallel circulation: Native cardiac output competes with ECMO flow, creating mixing zones

Dual Circulation Phenomenon: [6]

In peripheral VA-ECMO (femoral access), two distinct circulatory flows exist:

Harlequin Syndrome (North-South Syndrome): When native lung function is severely impaired and VA-ECMO returns blood via femoral artery, the coronary and cerebral circulations may receive deoxygenated blood ejected by the native heart, while the lower body receives fully oxygenated blood from the ECMO circuit. This creates differential cyanosis:

• Upper body: Cyanotic, hypoxemic
• Lower body: Pink, well-oxygenated

Recognition: Right radial artery (pre-ductal) saturation lower than femoral or bilateral differential pulse oximetry.

Management:

1. Convert to central VA-ECMO (right atrium to ascending aorta)
2. Add VV-ECMO for respiratory support alongside VA-ECMO (V-VA configuration)
3. Improve native lung oxygenation (recruitment, prone positioning)

Hemodynamic Support:

VA-ECMO provides both pressure and flow support:

• Flow rates: Typically 3.0-5.0 L/min (adult), up to 6-7 L/min maximum
• Mean arterial pressure: Target 65-75 mmHg (avoid excessive hypertension)
• LV afterload: Increased (retrograde arterial flow increases LV wall tension)
• LV decompression: May require inotropes, IABP, or direct venting (LA drain)

Comparison: VV vs VA ECMO

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Cannulation Strategies

VV-ECMO Cannulation

Single-Site Dual-Lumen Cannula:

Positioning Requirements (Avalon/Protek cannulae): [7]

• Drainage ports: Superior and inferior vena cavae
• Return port: Directed toward tricuspid valve
• Imaging: TEE or fluoroscopy essential for optimal positioning
• Blood flow: Up to 4-5 L/min with properly positioned 27-31 Fr cannula

Two-Site Cannulation:

• Drainage: Femoral vein (21-25 Fr) advanced to IVC/RA junction
• Return: Internal jugular vein (19-21 Fr) to right atrium
• Advantage: Easier positioning, higher flows possible
• Disadvantage: Two access sites, patient immobility

VA-ECMO Cannulation

Peripheral (Femoral) VA-ECMO: [8]

Distal Limb Perfusion: Critical to prevent lower extremity ischemia:

• Antegrade perfusion catheter: 6-8 Fr sheath into superficial femoral artery
• Flow: Continuous flow of oxygenated blood to distal limb
• Monitoring: Palpation of dorsalis pedis/posterior tibial pulses, Doppler assessment
• Threshold for intervention: Loss of Doppler signals, increasing lactate, compartment syndrome signs

Central VA-ECMO:

Used for postcardiotomy failure when sternum remains open:

• Drainage: Right atrium via direct cannulation
• Return: Ascending aorta via direct cannulation
• Advantages: No Harlequin syndrome, controlled decompression, higher flows
• Disadvantages: Surgical insertion, bleeding risk, immobility

Alternative Arterial Access: [9]

ECPR (Extracorporeal Cardiopulmonary Resuscitation)

Rapid deployment of VA-ECMO during refractory cardiac arrest: [10]

Indications:

• Witnessed arrest with immediate bystander CPR
• Refractory to conventional CPR (>10-20 minutes)
• Potentially reversible cause
• No significant comorbidities limiting survival

Outcomes:

• Survival to discharge: 15-30% (highly variable by centre and selection)
• Favorable neurological outcome: 10-25%
• Best outcomes with rapid deployment (<60 minutes from arrest)

Protocol Elements:

1. High-quality CPR maintained during cannulation
2. Rapid percutaneous or surgical femoral access
3. Target flow 3-4 L/min within 30 minutes of arrest
4. Temperature management post-resuscitation
5. Early coronary angiography if ischemic cause suspected

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The ECMO Circuit

Circuit Components

Modern ECMO Circuit: [11]

Membrane Oxygenator Physics

Gas Exchange Principles: [12]

The membrane oxygenator functions through diffusion across a semipermeable membrane:

Oxygen Transfer:

• Gradient-driven: High FiO₂ (sweep gas) to low PaO₂ (venous blood)
• Membrane material: Polymethylpentene (PMP) microporous fibers
• Diffusion limitation: Primarily limited by membrane surface area and blood flow
• Maximum O₂ transfer: Approximately 300-400 mL/min per square meter of membrane

Carbon Dioxide Removal:

• Highly efficient: CO₂ is approximately 20× more diffusible than O₂
• Sweep gas dependent: CO₂ removal directly proportional to sweep gas flow rate
• Can achieve near-complete CO₂ removal even at low blood flows
• "Ultra-low tidal volume" strategy: Reduce ventilator settings to rest lungs

Sweep Gas Principles:

Clinical Pearl: For hypercapnic respiratory failure, increase sweep gas flow to augment CO₂ removal. For hypoxemic failure, optimize blood flow rate and FiO₂. In pure CO₂ retention (e.g., status asthmaticus), ECMO can achieve adequate support at relatively low blood flows (2-3 L/min) because CO₂ removal is highly efficient.

Centrifugal Pump Mechanics

Principle of Operation: [13]

Modern ECMO centrifugal pumps use a rapidly spinning impeller (3,000-5,000 RPM) to create a pressure gradient:

Magnetic Levitation:

• Impeller floats in magnetic field (no bearings)
• Reduced hemolysis and thrombosis
• Improved durability for long-term support
• Examples: CentriMag, Rotaflow

Flow Dynamics:

• Flow ∝ RPM × preload × afterload⁻¹
• Preload dependent: Venous drainage limits maximum flow
• Afterload sensitive: Increased arterial resistance reduces flow
• Non-occlusive: No forward flow if pump stopped (risk of regurgitation)

Safety Features:

• Flow probe (ultrasonic Doppler)
• Pressure monitoring (pre- and post-pump)
• Bubble detector
• Temperature monitoring
• Backup battery and hand-crank

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Anticoagulation Management

Anticoagulation Targets

Unfractionated Heparin: [14]

Heparin-Induced Thrombocytopenia (HIT):

• Incidence: 1-5% with prolonged ECMO
• Diagnosis: 4T score, anti-PF4 antibody testing, serotonin release assay
• Management: Discontinue all heparin, initiate direct thrombin inhibitor

Alternative Anticoagulants: [15]

Anticoagulation Monitoring

Standard Protocol:

1. Baseline: Full coagulation profile including platelet count
2. Continuous heparin infusion: 10-20 U/kg/h (variable by centre)
3. ACT monitoring: Hourly while titrating, then 4-6 hourly when stable
4. Platelet count: Daily (monitor for HIT, consumption)
5. Fibrinogen: Daily, maintain >1.5 g/L
6. Anti-Xa or aPTT: Daily

Bleeding Risk Management: [16]

Thrombosis Prevention

Circuit-Related Thrombosis:

• Risk factors: Inadequate anticoagulation, low flow states, prolonged circuit duration
• Manifestations: Rising pump pressures, hemolysis, oxygenator failure
• Prevention: Adequate anticoagulation, maintaining adequate flow (>2.5 L/min), circuit changes if necessary

Patient-Related Thrombosis:

• Stroke (ischemic or hemorrhagic): 5-15%
• Limb ischemia (VA-ECMO): 5-10%
• Cannula thrombosis: Monitoring for reduced flows, rising pressures

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Complications and Management

Bleeding Complications

Incidence and Risk Factors: [17]

Management Strategies:

1. Prevention: Minimize anticoagulation targets, use heparin-coated circuits
2. Early recognition: Regular inspection of access sites, hemoglobin monitoring
3. Surgical control: Direct pressure, surgical repair, embolization
4. Blood products: Maintain Hb >70-80 g/L, fibrinogen >1.5 g/L, platelets >50 × 10⁹/L
5. Antifibrinolytics: Tranexamic acid for surgical bleeding (balance thrombosis risk)

Hemolysis

Mechanism and Monitoring: [18]

Mechanical destruction of red blood cells due to shear stress from the pump and oxygenator:

Causes of Hemolysis:

• Excessive negative pressure on drainage (venous cavitation)
• Thrombus in circuit causing turbulent flow
• Oxygenator failure
• Pump malfunction
• Small cannula relative to flow rate

Management:

1. Optimize cannula positioning and flow
2. Check for kinks or thrombus in circuit
3. Consider circuit change if severe
4. Supportive care: Alkalinize urine, maintain diuresis, dialysis if renal failure

Infection

Nosocomial Infection in ECMO: [19]

Prevention:

• Aseptic insertion technique
• Sterile dressing changes
• Daily review of line necessity
• Minimize immunosuppression
• Early mobilization if feasible

Diagnosis Challenges:

• Inflammatory markers unreliable (systemic inflammation from ECMO)
• Blood cultures may be difficult to interpret
• Clinical suspicion remains key

Limb Ischemia (VA-ECMO)

Pathophysiology: Large-bore arterial cannula occupies femoral artery, potentially compromising distal perfusion:

Prevention:

• Distal perfusion catheter (antegrade femoral access)
• Adequate size match (don't oversize arterial cannula)
• Regular limb assessment

Monitoring:

• Clinical: Pallor, paresthesia, paralysis, pain, pulselessness, poikilothermia
• Doppler: Dorsalis pedis and posterior tibial signals
• Compartment pressure: If concern for compartment syndrome (>30 mmHg)

Management:

• Optimize distal perfusion catheter flow
• Consider alternative cannulation site
• Fasciotomy if compartment syndrome
• Extremity amputation in refractory cases (rare)

Neurological Complications

Spectrum of Injury: [20]

Risk Factors:

• Pre-ECMO cardiac arrest (ECPR)
• Prolonged ECMO duration
• Excessive anticoagulation
• Hypertension or hypotension
• Cannulation complications

Prevention:

• Maintain MAP 70-80 mmHg
• Avoid excessive anticoagulation
• Monitor for Harlequin syndrome (cerebral hypoxia)
• Early rehabilitation assessment

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Indigenous Health Considerations

Cardiovascular Disease Disparities

Aboriginal and Torres Strait Islander peoples experience cardiovascular disease at rates 2-3 times higher than non-Indigenous Australians, with higher rates of acute coronary syndromes, heart failure, and risk factors including diabetes, hypertension, and smoking. This translates to higher potential need for advanced cardiovascular support including ECMO. [21]

Key Health Disparities:

• Rheumatic heart disease prevalence 55× higher (younger age of onset)
• Cardiovascular mortality 1.5× higher
• Acute MI hospitalisations 1.7× higher
• Heart failure hospitalisations 2× higher
• Reduced access to interventional cardiology and cardiac surgery

Access to ECMO Services

Geographic Barriers: The concentration of ECMO-capable centres in major metropolitan areas creates significant access barriers for Indigenous Australians living in rural and remote communities:

1. Retrieval Requirements: Patients in remote communities with refractory cardiac or respiratory failure require aeromedical retrieval to ECMO centres, often over thousands of kilometres
2. Time-Sensitive Nature: ECPR outcomes deteriorate rapidly with time to cannulation; geographic distance may preclude ECMO candidacy
3. RFDS and Retrieval Services: Coordination with Royal Flying Doctor Service and state-based retrieval services is essential for timely transfer
4. Telemedicine: Real-time consultation with ECMO centres enables early identification of candidates and treatment optimization during transfer

Cultural Considerations in ECMO Care

Family and Community Involvement:

• Extended family involvement in decision-making is culturally important
• Allow time for family consultation before initiating ECMO
• Aboriginal Liaison Officers facilitate communication and cultural safety
• Consider family transportation to metropolitan centres for prolonged ECMO support

Communication Challenges:

• Health literacy barriers may complicate understanding of ECMO technology
• Use interpreters and Aboriginal Health Workers for complex discussions
• Visual aids and demonstrations help explain the technology
• Repeated discussions may be necessary to ensure comprehension

Cultural Safety in Critical Care:

• Welcome family presence at the bedside (may be many visitors)
• Respect cultural protocols around death and dying
• Involve Aboriginal Hospital Liaison Officers in care planning
• Consider cultural beliefs regarding mechanical support and body integrity

Remote Practice Considerations

For anaesthetists and intensivists in rural/regional centres:

• Early recognition of ECMO candidates through telemedicine consultation
• Optimization of patients during retrieval (ventilation, hemodynamics, organ perfusion)
• Clear communication with ECMO centres regarding prognosis and appropriateness
• Support for families who must relocate to be with critically ill patients
• Consideration of post-ECMO rehabilitation access in rural areas

Māori Health Considerations (Aotearoa New Zealand)

Māori and Pacific peoples in New Zealand experience similar disparities in cardiovascular disease and access to advanced cardiac care:

• Cardiovascular mortality 1.5-2× higher than European New Zealanders
• Higher prevalence of diabetes and obesity
• Geographic barriers to Auckland and Christchurch ECMO centres

Cultural Considerations:

• Whānau involvement: Extended family decision-making integral to consent processes
• Māori Health Workers: Essential for culturally appropriate communication
• Karakia: Spiritual practices may accompany medical interventions
• Te Reo Māori: Use interpreters for complex technical discussions
• Manaakitanga: Holistic care encompassing physical, spiritual, and whānau wellbeing

Healthcare providers should ensure Māori patients and whānau have equitable access to ECMO services while receiving care that respects cultural values and supports participation in decision-making.

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Weaning and Decannulation

VV-ECMO Weaning

Assessment of Lung Recovery: [22]

Weaning Protocol:

1. Daily assessment: Spontaneous breathing trial on ventilator if stable
2. Sweep gas reduction: Reduce sweep gradually (monitor for CO₂ retention)
3. Flow reduction trial: Reduce ECMO flow to 1-2 L/min for 1-2 hours
4. Ventilator optimization: Progressive increase in ventilator support
5. Decannulation: If stable off ECMO support for 2-4 hours

Decannulation Technique:

• Remove cannulae with continuous manual pressure
• Achieve hemostasis (30+ minutes of pressure for large cannulae)
• Surgical repair of access sites if necessary
• Monitor for bleeding, pseudoaneurysm, AV fistula

VA-ECMO Weaning

Assessment of Cardiac Recovery: [23]

Weaning Protocol:

1. Flow reduction trials: Gradual reduction in ECMO flow (4 L/min → 3 → 2 → 1.5 L/min)
2. Hemodynamic monitoring: MAP, pulse pressure, perfusion markers at each step
3. Echocardiography: Assess LV function, aortic valve opening, chamber dimensions
4. Duration: 30-60 minutes at each flow level if stable
5. Tolerance criteria: MAP >65, adequate perfusion, stable echo

Failed Weaning:

• Return to full ECMO support
• Consider higher level of support (Impella, durable VAD)
• Palliative care consultation if not a transplant/VAD candidate

Decannulation Considerations:

• Venous cannula: Can often be removed percutaneously with pressure
• Arterial cannula (femoral): May require surgical repair due to large arteriotomy
• Central cannulation: Surgical closure of sternotomy
• Distal perfusion catheter: Remove simultaneously with arterial cannula

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ANZCA Exam Focus

High-Yield Topics

1. VV vs VA ECMO physiology: Understand the fundamental differences in respiratory vs cardiopulmonary support
2. Harlequin syndrome: Recognition and management of differential hypoxemia in VA-ECMO
3. Cannulation strategies: Advantages and disadvantages of different configurations
4. Anticoagulation: Targets, monitoring, and management of bleeding/thrombosis
5. Complications: Hemolysis, bleeding, stroke, limb ischemia

Common Viva Questions

"Explain the difference between VV and VA ECMO."

• VV: Respiratory support only; requires native cardiac function; blood returns to venous system
• VA: Cardiac and respiratory support; bypasses heart and lungs; blood returns to arterial system
• Clinical scenarios: When to choose each mode

"What is Harlequin syndrome and how do you manage it?"

• Definition: Differential hypoxemia in VA-ECMO with native lung failure
• Pathophysiology: Deoxygenated blood from LV reaches coronary/cerebral circulation
• Recognition: Right radial saturation lower than femoral; differential cyanosis
• Management: Central VA-ECMO, V-VA conversion, improve native lung function

"How do you anticoagulate a patient on ECMO?"

• Heparin infusion with monitoring (ACT 180-220, aPTT 1.5-2.5×)
• Alternatives in HIT (argatroban, bivalirudin)
• Balance between thrombosis and bleeding

"What are the complications of ECMO?"

• Bleeding (access site, GI, ICH)
• Thrombosis (circuit, stroke, limb ischemia)
• Hemolysis (mechanical destruction)
• Infection (catheter-related, VAP)

Key Equations

Oxygen Delivery (DO₂):

DO_2 = \text{Cardiac Output} \times CaO_2 \times 10

CaO_2 = (Hb \times 1.34 \times SaO_2) + (0.003 \times PaO_2)

Oxygen Consumption (VO₂):

VO_2 = \text{CO} \times (CaO_2 - CvO_2) \times 10

Oxygen Transfer Across Membrane:

O_2 \text{ transfer} = \text{Blood flow} \times (Hb \times 1.34 \times \Delta Sat)

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Assessment Content

SAQ Practice Question (20 marks)

Question:

A 34-year-old woman with severe community-acquired pneumonia develops refractory hypoxemia despite prone positioning, neuromuscular blockade, and maximum ventilatory support. Her PaO₂/FiO₂ ratio is 58 mmHg. She is commenced on VV-ECMO via femoral drainage and internal jugular return.

(a) Explain the physiological principles of gas exchange in VV-ECMO, including the factors that determine arterial oxygen saturation. (8 marks)

(b) Describe the circuit components and their functions. (5 marks)

(c) After 72 hours, her plasma-free hemoglobin rises to 650 mg/L. Discuss the causes of hemolysis in ECMO and your management approach. (7 marks)

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Model Answer:

(a) VV-ECMO Gas Exchange Physiology (8 marks)

Basic Principles (3 marks): VV-ECMO provides respiratory support by draining deoxygenated venous blood, removing CO₂ and adding O₂ through a membrane oxygenator, and returning oxygenated blood to the venous system. Unlike VA-ECMO, the native heart provides all systemic perfusion; ECMO only treats the respiratory failure.

Oxygenation Determinants (3 marks): Arterial oxygen saturation in VV-ECMO is determined by the mixture of:

1. Oxygenated blood from ECMO: Flow rate × post-oxygenator saturation (typically 95-100%)
2. Native venous return bypassing ECMO: (Native cardiac output − ECMO flow) × mixed venous saturation

The equation: CaO₂ = [(ECMO flow × SpostO₂) + ((CO − ECMO flow) × SvO₂)] / CO × [Hb] × 1.34

Key factors: ECMO blood flow rate (target 60-80% of cardiac output), oxygenator efficiency (FiO₂, membrane function), recirculation (minimize with proper positioning), and native cardiac output.

Carbon Dioxide Removal (2 marks): CO₂ removal is highly efficient due to 20× greater diffusibility than O₂. CO₂ removal is primarily determined by sweep gas flow rate rather than blood flow. Increasing sweep gas flow increases CO₂ removal, allowing ultra-protective lung ventilation.

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(b) Circuit Components (5 marks)

Cannulae (1 mark): Large-bore vascular access. VV-ECMO typically uses femoral drainage (21-25 Fr) and IJV return (19-21 Fr), or single-site dual-lumen cannula. Heparin-coated, wire-reinforced.

Centrifugal Pump (1 mark): Magnetic levitation pump spins at 3,000-5,000 RPM creating pressure gradient. Flow probe monitors output. Non-occlusive design requires continuous operation to prevent regurgitation.

Membrane Oxygenator (1 mark): Hollow fiber membrane (polymethylpentene) provides gas exchange. Oxygen transfer limited by surface area and blood flow; CO₂ removal determined by sweep gas flow. Integrated heat exchanger maintains temperature.

Tubing and Monitoring (1 mark): Heparin-coated PVC or silicone tubing. Bubble detector, pressure monitors (pre- and post-pump), temperature sensors, and access ports for sampling.

Console (1 mark): Control unit managing pump speed, sweep gas flow and FiO₂, monitoring displays, and safety alarms. Battery backup for transport.

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(c) Hemolysis Management (7 marks)

Pathophysiology and Causes (3 marks): Hemolysis results from mechanical destruction of RBCs due to shear stress:

1. Excessive negative pressure: Venous cavitation from inadequate drainage (kinked cannula, poor positioning)
2. Thrombus in circuit: Turbulent flow around clot increases shear
3. Oxygenator failure: Deteriorating membrane function
4. Pump malfunction: Bearing thrombosis or mechanical issues
5. Cannula mismatch: Excessive flow through undersized cannula

Clinical Significance (1 mark): Plasma-free hemoglobin >500 mg/L indicates significant hemolysis. Free hemoglobin causes renal injury (direct tubular toxicity), vasoconstriction, and thrombosis risk. Haptoglobin becomes depleted; LDH rises; schistocytes visible on smear.

Management Approach (3 marks):

1. Investigation: Check pre-oxygenator and post-oxygenator pressures; inspect circuit for thrombus; review pump parameters (RPM vs flow relationship); check cannula positioning
2. Optimization: Adjust cannula position under imaging; ensure no circuit kinks; reduce flow if excessive for cannula size; maintain adequate patient volume status
3. Circuit change: If hemolysis severe (>1000 mg/L) or progressive, change oxygenator or entire circuit
4. Supportive care: Maintain urine output (diuretics if needed); urine alkalinization reduces cast formation; dialysis if renal failure develops

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Viva Scenario (15 marks)

Examiner: "A patient on femoral VA-ECMO for cardiogenic shock has a right radial arterial saturation of 82% and a femoral arterial saturation of 98%. What is your diagnosis?"

Candidate: "This differential in oxygen saturations between the upper and lower body represents Harlequin syndrome, also known as North-South syndrome. It occurs in peripheral VA-ECMO when there is significant native lung dysfunction."

Examiner: "Explain the physiology."

Candidate: "In peripheral VA-ECMO, the femoral arterial cannula returns fully oxygenated blood to the distal aorta. This flows retrograde up the aorta. Meanwhile, the native left ventricle continues to eject whatever blood passes through the pulmonary circulation. If the lungs are severely impaired, this ejected blood is poorly oxygenated.

The two circulatory streams meet at a mixing point, typically in the aortic arch or proximal descending aorta. Above this mixing point—the coronary arteries, cerebral circulation, and upper extremities supplied by the brachiocephalic vessels—receive deoxygenated blood from the native heart. Below the mixing point, the lower body receives oxygenated blood from the ECMO circuit."

Examiner: "Why is the right radial saturation particularly concerning?"

Candidate: "The right radial artery is pre-ductal—it receives blood from the brachiocephalic trunk before any mixing with ECMO blood occurs. Therefore, it reflects the oxygen saturation of blood ejected by the native left ventricle. A saturation of 82% indicates severe hypoxemia of the coronary and cerebral circulations.

This is particularly dangerous because the myocardium, which is already failing and on mechanical support, is perfused with hypoxemic blood. This can worsen myocardial dysfunction and create a vicious cycle. Similarly, cerebral hypoxemia can cause neurological injury."

Examiner: "How would you manage this situation?"

Candidate: "I would use a stepwise approach:

First, I'd confirm the findings with bilateral upper and lower extremity pulse oximetry and arterial blood gases from different sites to characterize the mixing point location.

Second, I'd attempt to improve native lung function—increase PEEP, consider recruitment maneuvers or prone positioning if feasible, optimize fluid balance, and treat any reversible pulmonary pathology.

Third, if the native lung function cannot be improved quickly, I need to change the ECMO configuration. Options include converting to central VA-ECMO with the return cannula in the ascending aorta, which would ensure the coronary and cerebral circulations receive oxygenated blood. Alternatively, I could add a VV component to the existing VA-ECMO—creating V-VA ECMO—where an additional drainage and return cannula in the internal jugular vein would oxygenate the blood reaching the right heart before it's ejected by the left ventricle."

Examiner: "What are the relative contraindications to VV-ECMO?"

Candidate: "The absolute contraindication to VV-ECMO is the need for cardiac support—VV-ECMO provides no hemodynamic assistance. Relative contraindications include:

• Severe pulmonary hypertension with right ventricular failure, as the increased afterload from returning blood to the right heart may worsen RV function
• Severe coagulopathy or active bleeding that cannot be controlled, given the need for systemic anticoagulation
• Irreversible underlying lung disease where recovery is not anticipated, unless ECMO is being used as a bridge to lung transplantation and the patient is a suitable candidate
• Advanced age and multiple comorbidities that would preclude meaningful recovery
• Prolonged mechanical ventilation with high pressures and FiO₂ prior to ECMO, which may indicate irreversible lung injury
• Severe neurological injury that would preclude meaningful survival regardless of respiratory recovery"

Examiner: "How do you anticoagulate an ECMO patient?"

Candidate: "Systemic anticoagulation is mandatory while on ECMO to prevent circuit thrombosis. The standard approach uses unfractionated heparin:

• Loading dose: 50-100 units/kg at cannulation
• Maintenance: 10-20 units/kg/hour continuous infusion
• Monitoring: ACT target 180-220 seconds, or aPTT 1.5-2.5 times baseline, or anti-Xa 0.3-0.7 IU/mL

The specific target varies by centre and patient factors. Patients with bleeding complications may be managed with lower targets or even temporary cessation of heparin, accepting some circuit thrombosis risk.

In heparin-induced thrombocytopenia, I would discontinue all heparin and initiate a direct thrombin inhibitor such as argatroban or bivalirudin, monitored by aPTT.

Anticoagulation must be balanced against bleeding risk, which is significant in ECMO patients due to surgical access sites, coagulopathy from circuit inflammation, and frequent need for invasive procedures."

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Circuit Management and Monitoring

Daily Circuit Assessment

Visual Inspection:

• Circuit integrity: No leaks, cracks, or disconnections
• Circuit color: Darkening suggests thrombus formation
• Oxygenator function: No plasma leakage (clear separation between blood and gas phases)
• Cannula sites: No bleeding, hematoma, or infection

Pressure Monitoring:

Flow Dynamics:

Chatter Assessment:

• Venous chatter: Pressure waveform oscillations indicate inadequate venous drainage
• Causes: Hypovolemia, cannula malposition, kinked catheter, excessive flow demand
• Management: Volume administration, reposition cannula, reduce flow temporarily

Flow-Pressure Relationship: Normal: RPM increased → flow increases proportionally Abnormal: RPM increased → minimal flow increase ("stall" suggests thrombus or inadequate drainage)

Anticoagulation Protocols

Standard Heparin Protocol: [53]

Initiation:

• Bolus: 50-100 units/kg at cannulation
• Infusion: Start at 10-20 units/kg/hour
• Target: ACT 180-220 seconds or aPTT 40-60 seconds

Monitoring Schedule:

• ACT: Hourly initially, then every 4-6 hours when stable
• aPTT: Daily
• Anti-Xa: Daily if available
• Platelet count: Every 6-12 hours initially, then daily
• Fibrinogen: Daily

Troubleshooting Anticoagulation:

Direct Thrombin Inhibitors: [54]

Gas Exchange Optimization

Sweep Gas Management:

CO₂ Removal:

• Direct relationship: Sweep gas flow ∝ CO₂ removal
• Clinical target: EtCO₂ or PaCO₂ based on lung-protective strategy
• Typical sweep gas flows: 3-10 L/min (blood flow dependent)

Oxygenation:

• Post-oxygenator saturation target: >95%
• If low: Increase FiO₂, check for oxygenator failure, increase blood flow
• Arterial saturation (VV-ECMO): Depends on native lung function and recirculation

Ventilator Management on ECMO:

Ultra-Lung Protective Strategy:

• Tidal volume: 1-3 mL/kg predicted body weight
• Plateau pressure: <25 cm H₂O
• PEEP: 10-15 cm H₂O (maintain alveolar recruitment)
• Respiratory rate: 4-10 breaths/min
• FiO₂: Minimum to maintain adequate saturation

"Lung Rest" Concept:

• Minimize mechanical stress on injured lungs
• Allow healing while ECMO provides gas exchange
• Periodic recruitment maneuvers to prevent atelectasis

Nutrition and Metabolic Considerations

Nutritional Support: [55]

Caloric Requirements:

• ECMO increases metabolic demand: 1.2-1.5× basal
• Target: 25-30 kcal/kg/day
• Early enteral nutrition preferred if hemodynamically stable

Special Considerations:

• Fat emulsion: Compatible with ECMO; monitor triglycerides
• Propofol: Lipid carrier provides calories; limit to <4 mg/kg/hr
• Blood glucose: Target 6-10 mmol/L; avoid hypoglycemia

Pharmacokinetics on ECMO: [56]

Drug Sequestration:

• Lipophilic drugs: Sequestered in circuit tubing and oxygenator
• Examples: Propofol, midazolam, fentanyl, antibiotics (vancomycin)
• Effect: Higher dosing requirements; decreased drug levels

Recommendations:

• Use hydrophilic alternatives when possible (e.g., hydromorphone instead of fentanyl)
• Monitor drug levels when available (vancomycin, gentamicin)
• Consider increased dosing for lipophilic medications
• Recalculate pharmacokinetics post-decannulation (drug release from circuit)

Rehabilitation and Physiotherapy

Early Mobilization on ECMO: [57]

Feasibility:

• VV-ECMO: Mobilization increasingly practiced (sitting, standing, walking)
• VA-ECMO: Limited due to hemodynamic support requirements; bedside exercises

Safety Considerations:

• Ensure circuit stability before mobilization
• Adequate staff (minimum 2-3 people)
• Emergency equipment available
• Avoid excessive strain on cannulation sites
• Monitor for cannula dislodgement or bleeding

Benefits:

• Prevention of ICU-acquired weakness
• Improved ventilation-perfusion matching
• Psychological benefit for patient
• Shorter rehabilitation post-decannulation

Outcome Prediction and Scoring Systems

RESP Score (VV-ECMO): [58]

The Respiratory ECMO Survival Prediction score predicts survival for VV-ECMO in respiratory failure:

Score Interpretation:

• Category I (>6 points): Survival 92%
• Category II (-1 to 6 points): Survival 76%
• Category III (-5 to -2 points): Survival 57%
• Category IV (≤-5 points): Survival 18%

SAVE Score (VA-ECMO): [59]

The Survival After Veno-Arterial ECMO score predicts outcomes for VA-ECMO:

Score Interpretation:

• Score >4: 75% survival
• Score -1 to 4: 55% survival
• Score ≤-2: 27% survival

Ethical Considerations and End-of-Life Care

Decision-Making on ECMO: [60]

Time-Limited Trials:

• ECMO should be initiated as a time-limited trial with clear goals
• Regular reassessment of goals of care (daily or every 48 hours)
• Clear criteria for continuation vs. withdrawal

Withdrawal of ECMO:

• Ethically equivalent to withdrawal of other life support
• Requires consent/surrogate decision-maker agreement
• Palliative care consultation appropriate
• Focus on patient comfort during withdrawal process

Contraindications to ECMO Initiation:

Absolute Contraindications:

• Irreversible cardiac or pulmonary failure without transplant/VAD option
• Severe neurological injury (established brain death or devastating injury)
• Uncontrolled bleeding
• Pre-existing severe disability inconsistent with meaningful recovery

Relative Contraindications:

• Prolonged CPR (>60 minutes) without ROSC
• Age >75 years (case-by-case basis)
• Severe peripheral vascular disease (for femoral cannulation)
• Limited vascular access

Futility Considerations:

• Prolonged ECMO (>2-3 weeks) without organ recovery
• Multiple complications without improvement trajectory
• Poor neurological status despite hemodynamic recovery
• Multi-organ failure progression despite support

Future Directions in ECMO

Emerging Technologies: [61]

Portable and Implantable Systems:

• Ambulatory ECMO systems enabling discharge home
• Fully implantable artificial lungs (under development)
• Transcutaneous power and gas exchange eliminating external connections

Advanced Oxygenators:

• Membrane oxygenators with improved durability (>30 days)
• Bioactive surfaces reducing thrombogenicity
• Integrated sensors for real-time monitoring

Art Intelligence and Automation:

• Automated weaning protocols
• Predictive algorithms for circuit failure
• Machine learning for outcome prediction

Hybrid Configurations:

• V-VV (veno-venovenous) for differential support
• Renal replacement therapy integrated with ECMO
• Liver support devices combined with ECMO

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