7.3 Venoarterial ECMO (VA-ECMO) & Differential Hypoxemia

ABIM Critical Care Medicine Study Guide Exam

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1 Chapter 1: Exam Orientation: Format, Blueprint & Study Strategy

1.1 ABIM Critical Care Medicine Exam: Format, Blueprint & Study Strategy

2 Chapter 2: Acute Respiratory Failure & ARDS

2.1 ARDS Definition, Pathophysiology & Phenotypes 2.2 Lung-Protective Mechanical Ventilation 2.3 Refractory ARDS: Prone Positioning, Neuromuscular Blockade & VV-ECMO

3 Chapter 3: Obstructive Airways, Pulmonary Vascular & Pleural Emergencies

3.1 Severe Asthma & COPD Exacerbations in the ICU 3.2 Massive & Submassive Pulmonary Embolism 3.3 Pulmonary Hypertension & Right Ventricular Failure in the ICU 3.4 Massive Hemoptysis, Complex Pleural Effusions & Thoracic Drainage

4 Chapter 4: Airway, Parenchymal & Nonthrombotic Pulmonary Emergencies

4.1 Airway Management, Upper-Airway Obstruction & Artificial-Airway Complications 4.2 Diffuse Alveolar Hemorrhage, Interstitial Lung Disease, Pulmonary Vasculitis & Noncardiogenic Edema 4.3 Severe Community-Acquired & Viral Pneumonia 4.4 Nonthrombotic Embolism (Air, Fat, Amniotic Fluid, Tumor, Septic) & Acute Chest Syndrome

5 Chapter 5: Mechanical Ventilation Modes, Dyssynchrony & Liberation

5.1 Mechanical Ventilator Modes & Advanced Breath Delivery 5.2 Patient-Ventilator Dyssynchrony & Waveform Analysis 5.3 Ventilator Liberation, Spontaneous Breathing Trials & Post-Extubation Support

6 Chapter 6: Shock Syndromes & Hemodynamic Principles

6.1 Shock Classification, Cellular Pathophysiology & Biomarkers 6.2 Hemodynamic Monitoring: Arterial Lines, CVP & Pulmonary Artery Catheter 6.3 Dynamic Assessment of Fluid Responsiveness & Resuscitation Targets

7 Chapter 7: Vasoactive Therapies & Mechanical Circulatory Support

7.1 Vasopressors & Inotropes: Pharmacology & Clinical Titration 7.2 Temporary Mechanical Circulatory Support: IABP, Impella & TandemHeart 7.3 Venoarterial ECMO (VA-ECMO) & Differential Hypoxemia 7.4 Post-Cardiac Arrest Care, Targeted Temperature Management & Neuroprognostication

8 Chapter 8: Acute Kidney Injury & Renal Replacement Therapy

8.1 KDIGO AKI Classification, Etiologies & Diagnostic Evaluation 8.2 Continuous Renal Replacement Therapy (CRRT) vs Intermittent Hemodialysis 8.3 Indications & Timing for RRT in Critical Illness: AKIKI & STARRT-AKI

9 Chapter 9: Electrolyte, Acid-Base & Endocrine Emergencies

9.1 Severe Dysnatremias: Hyponatremia, Hypernatremia & Osmotic Demyelination 9.2 Potassium, Calcium, Magnesium & Phosphate Disorders 9.3 Complex & Mixed Acid-Base Disorders: Boston vs Copenhagen vs Stewart 9.4 Endocrine Crises: DKA, HHS, Thyroid Storm, Myxedema Coma & Adrenal Crisis

10 Chapter 10: Glycemic Control, Hyperammonemia, Pituitary & Adrenal Disorders

10.1 ICU Glycemic Control & Severe Hypoglycemia 10.2 Non-Hepatic Hyperammonemia & Metabolic Encephalopathy 10.3 Pituitary Emergencies, Adrenal Excess & Critical Illness-Related Corticosteroid Insufficiency

11 Chapter 11: Acute Coronary Syndromes & Cardiac Arrhythmias

11.1 Acute Coronary Syndromes & Mechanical Complications of Myocardial Infarction 11.2 Critical Tachyarrhythmias: Atrial Fibrillation with RVR, VT & VF 11.3 Critical Bradyarrhythmias, Conduction Blocks & Temporary Cardiac Pacing

12 Chapter 12: Acute Heart Failure, Valvular & Pericardial Emergencies

12.1 Acute Decompensated Heart Failure & Cardiogenic Pulmonary Edema 12.2 Acute Valvular Regurgitation & Prosthetic Valve Emergencies 12.3 Pericardial Tamponade & Acute Aortic Syndromes

13 Chapter 13: Myocardial Disease, Hypertensive & Cardiac Device Emergencies

13.1 Fulminant Myocarditis, Stress & Peripartum Cardiomyopathy, and Hypertrophic Cardiomyopathy 13.2 Hypertensive Emergencies & Ruptured Abdominal Aortic Aneurysm 13.3 Implanted Cardiac Devices, LVAD Emergencies & Electrical Storm

14 Chapter 14: Sepsis & Systemic Infectious Diseases

14.1 Sepsis-3 Definitions & Surviving Sepsis Campaign 2026 Guidelines 14.2 Refractory Septic Shock & Adjunctive Therapies 14.3 Multidrug-Resistant Pathogens & Antimicrobial Stewardship in the ICU

15 Chapter 15: ICU-Acquired & Severe Focal Infections

15.1 Hospital-Acquired & Ventilator-Associated Pneumonia 15.2 Central Line-Associated Bloodstream Infections & Catheter-Associated UTIs 15.3 Intra-Abdominal Sepsis, Necrotizing Soft Tissue Infections & CNS Infections

16 Chapter 16: Focal, Special-Host & Systemic Infections

16.1 CNS Infections Beyond Bacterial Meningitis: Encephalitis, Brain Abscess & Epidural Abscess 16.2 Infections in the Immunocompromised Host, Tropical & Bioterror Infections 16.3 Infective Endocarditis, Device Infections & Other Focal Cardiovascular Infections 16.4 Antimicrobial Pharmacokinetics, Allergy, Toxicity & Infection Prevention 16.5 Gastrointestinal & Hepatobiliary Infections: Fulminant C. difficile, Typhlitis, CMV Colitis & Abscesses

17 Chapter 17: Neurocritical Care & Coma

17.1 Coma, Altered Mental Status & Acute Delirium in the ICU 17.2 Status Epilepticus & Non-Convulsive Status Epilepticus 17.3 Acute Ischemic Stroke & Intracranial Hemorrhage

18 Chapter 18: Cerebrovascular Syndromes, Encephalopathy & Withdrawal States

18.1 Cerebral Venous Sinus Thrombosis, PRES & Reversible Cerebral Vasoconstriction Syndrome 18.2 Alcohol & Sedative Withdrawal, Wernicke Encephalopathy & Drug-Induced Encephalopathy

19 Chapter 19: Intracranial Hypertension & Neuromuscular Weakness

19.1 Intracranial Pressure Monitoring, Cerebral Perfusion & Tiered ICP Management 19.2 Acute Neuromuscular Respiratory Failure, ICU-Acquired Weakness & Brain Death Determination

20 Chapter 20: Traumatic Brain Injury, Spinal Cord Injury & Toxin-Mediated Paralysis

20.1 Traumatic Brain Injury Management & Paroxysmal Sympathetic Hyperactivity 20.2 Acute Spinal Cord Injury, Neurogenic Shock & Autonomic Dysreflexia 20.3 Toxin-Mediated Paralysis & Neuromuscular Crises: Botulism, Tetanus, Tick Paralysis & Myasthenic Crisis

21 Chapter 21: Gastrointestinal, Hepatic & Abdominal Emergencies

21.1 Acute Upper & Lower Gastrointestinal Bleeding in the ICU 21.2 Acute Liver Failure & Severe Acute Pancreatitis 21.3 Abdominal Compartment Syndrome & Acute Mesenteric Ischemia

22 Chapter 22: Luminal GI Emergencies & Complications of Cirrhosis

22.1 Esophageal, Gastric & Bowel Emergencies: Perforation, Obstruction, Ogilvie Syndrome & Colonic Ischemia 22.2 Complications of Cirrhosis: Alcohol-Associated Hepatitis, Hepatorenal Syndrome & Pulmonary Vascular Syndromes

23 Chapter 23: Trauma, Burns & Perioperative Critical Care

23.1 Polytrauma Resuscitation, Damage Control & Massive Transfusion 23.2 Severe Burn Injury, Inhalation Injury & Environmental Emergencies 23.3 Perioperative Critical Care for High-Risk Surgical Patients

24 Chapter 24: Thoracic Trauma, Environmental Injury & Obstetric Critical Care

24.1 Thoracic Trauma: Flail Chest, Pulmonary Contusion, Great Vessel & Tracheobronchial Injury 24.2 Environmental Injury: Drowning, Diving, Altitude, Heat, Cold, Electrical & Radiation 24.3 Obstetric & Genitourinary Emergencies and the Physiology of Pregnancy

25 Chapter 25: Transplantation, Donor Management & Post-Cardiac Surgery Care

25.1 Solid Organ Transplant Recipients & Deceased Donor Management 25.2 Post-Cardiac Surgery Critical Care: Bleeding, Tamponade, Low Output & Vasoplegia

26 Chapter 26: Hematologic Emergencies, Toxicology, Critical Ultrasound & ICU Liberation

26.1 Hematologic Emergencies in the ICU: HIT, DIC, TMA & Anticoagulant Reversal 26.2 Critical Care Toxicology, Toxidromes & Specific Antidotes 26.3 Critical Care Point-of-Care Ultrasound (POCUS) 26.4 ICU Liberation, Analgosedation, Goals of Care & Critical Care Ethics

27 Chapter 27: Transfusion Medicine, Hematologic & Oncologic Disorders

27.1 Transfusion Medicine: Products, Thresholds, Reactions & Apheresis 27.2 Red and White Cell Disorders, Hemoglobinopathies, Factor Deficiencies & Hypercoagulable States 27.3 Oncologic Emergencies, Hematopoietic Cell Transplant Complications & Cellular Therapy Toxicity

28 Chapter 28: Clinical Pharmacology, Adverse Drug Reactions & Specific Poisonings

28.1 Pharmacokinetic Principles, Dose Adjustment & Drug Interactions in Critical Illness 28.2 Adverse Drug Reactions: Anaphylaxis, Severe Cutaneous Reactions & Hyperthermic Syndromes 28.3 Specific Poisonings: Sodium-Channel Blockers, Lithium, Valproate, Theophylline, Iron, Sulfonylureas & Methemoglobinemia

29 Chapter 29: Ethics, Research Methods, Administration & Abdominal Ultrasound

29.1 Advance Care Planning, Nonbeneficial Treatment, Informed Consent & Confidentiality 29.2 Critical Care Research Methods, Statistics, IRB Oversight & Quality Improvement 29.3 Abdominal, Vascular & Procedural Point-of-Care Ultrasound

7.3 Venoarterial ECMO (VA-ECMO) & Differential Hypoxemia

Key Takeaways Explain This Topic AI

Peripheral venoarterial extracorporeal membrane oxygenation (VA-ECMO) provides biventricular circulatory support and pulmonary gas exchange by draining deoxygenated venous blood from the right atrium and reinfusing fully oxygenated blood retrogradely into the femoral artery.

Retrograde arterial flow elevates left ventricular afterload; in the presence of severe LV systolic dysfunction, this causes failure of the aortic valve to open, progressive LV dilation, acute pulmonary capillary hypertension, pulmonary hemorrhage, and blood stasis leading to intracardiac and aortic root thrombosis.

Proactive left ventricular decompression ('LV venting') is essential in peripheral VA-ECMO when pulsatility is lost or pulmonary edema develops; modalities include microaxial pumps (ECPELLA), intra-aortic balloon pumps, percutaneous balloon atrial septostomy, and surgical vents.

Harlequin syndrome (North-South syndrome or differential hypoxemia) occurs in peripheral VA-ECMO when native LV contractility recovers while pulmonary function remains severely compromised: deoxygenated native cardiac output perfuses the coronary arteries and cerebral circulation, while oxygenated ECMO blood perfuses the lower body.

Differential hypoxemia must be diagnosed using a right radial arterial line and right upper extremity pulse oximetry; definitive management requires ventilator optimization, ECMO flow adjustment, or conversion to a hybrid veno-arterio-venous (VAV) ECMO circuit.

Last updated: October 2026

Venoarterial extracorporeal membrane oxygenation (VA-ECMO) is the ultimate mechanical rescue modality for refractory biventricular cardiogenic shock and extracorporeal cardiopulmonary resuscitation (E-CPR). By bypassing both the failing heart and the injured lungs, VA-ECMO delivers fully oxygenated, decarboxylated blood under high pressure into the arterial system. However, the fluid mechanics of peripheral VA-ECMO present complex hemodynamic trade-offs. Retrograde aortic flow collides with native left ventricular ejection, increasing left ventricular afterload, promoting intra-alveolar pulmonary hemorrhage, and precipitating differential hypoxemia (Harlequin syndrome).

1. Circuit Architecture: Peripheral vs. Central VA-ECMO Peripheral Venoarterial ECMO (VA-ECMO) Circuit Architecture:

[ Right Atrium / Vena Cava ]
│
▼ Multi-stage Venous Drainage Cannula (21–25 Fr)
┌───────────────────────┐
│ Centrifugal Pump │ ──> Negative Drainage Pressure (-30 to -80 mmHg)
└───────────────────────┘
│
▼
┌───────────────────────┐
│ Membrane Oxygenator │ <── Sweep Gas (CO2 Clearance) + Sweep FiO2
│ & Heat Exchanger │
└───────────────────────┘
│
▼ Oxygenated Arterial Return (15–19 Fr)
┌───────────────────────┴───────────────────────┐
▼ ▼
[ Femoral Artery ] [ Distal Perfusion Catheter ]
(Retrograde Flow to Aorta) (Antegrade Flow to Superficial Femoral Artery
Prevents Critical Limb Ischemia)
The Peripheral Cannulation Configuration

Venous Drainage: A large multi-stage cannula ( 21 – 25 Fr 21\text{--}25\text{ Fr} 21 – 25 Fr ) is introduced via the femoral vein, with its tip positioned at the cavoatrial junction or within the right atrium under echocardiographic guidance. Blood is drained from the systemic venous circulation under negative pressure generated by the centrifugal pump.

Centrifugal Pump & Oxygenator: The pump propels blood through a polymethylpentene (PMP) hollow-fiber membrane oxygenator. Oxygenation is controlled by adjusting the fraction of delivered oxygen ( FdO 2 \text{FdO}_2 FdO 2 or Sweep FiO 2 \text{FiO}_2 FiO 2 , typically set at 1.0 1.0 1.0 ), while carbon dioxide clearance is governed by the sweep gas flow rate (liters/min).

Arterial Reinfusion: Oxygenated blood is returned into the systemic circulation via a femoral arterial cannula ( 15 – 19 Fr 15\text{--}19\text{ Fr} 15 – 19 Fr ), reinfusing blood retrogradely up the external iliac, common iliac, and abdominal aorta toward the aortic arch.

Distal Perfusion Catheter (DPC): Insertion of a large-bore cannula into the common femoral artery significantly impedes distal antegrade blood flow to the lower extremity. Placement of a dedicated 5 – 7 Fr 5\text{--}7\text{ Fr} 5 – 7 Fr antegrade distal perfusion cannula into the superficial femoral artery is mandatory to prevent acute lower limb compartment syndrome, rhabdomyolysis, and limb loss.

Central Cannulation Configuration

In post-cardiotomy patients or those with severe peripheral arterial disease, central cannulation is performed via median sternotomy. The venous drainage cannula is placed in the right atrium, and the arterial return cannula is sewn directly into the ascending aorta . While central cannulation eliminates retrograde flow conflicts and prevents differential hypoxemia, it carries substantial risks of surgical bleeding, mediastinitis, and necessitates re-sternotomy for decannulation.

2. Retrograde Flow Mechanics & Left Ventricular Overdistension

In peripheral femoral VA-ECMO, the mechanical pump propels blood upward against the normal direction of aortic blood flow. This creates profound alterations in left ventricular loading conditions: The Left Ventricular Distension Spiral in Peripheral VA-ECMO:

High Retrograde Arterial Flow ──> Massive Increase in LV Afterload
│
▼
Failing LV Cannot Overcome Afterload ──> Aortic Valve Remains Closed Throughout Systole
│
▼
Bronchial & Thebesian Venous Drainage ──> Blood Trapped in Non-Ejecting LV Cavity
│
▼
Extreme Elevation in LVEDP & PCWP (> 25–35 mmHg) ──> Massive Hydrostatic Pulmonary Edema
│
▼
Stasis in LV Cavity & Aortic Root ──> Thrombus Formation ("Smoke" / Echocardiographic Sludge)
The Pathophysiology of LV Distension

Aortic Valve Competence & Stasis: When native myocardial contractility is severely depressed, the LV cannot generate sufficient peak systolic pressure to exceed the high diastolic pressure maintained in the aortic root by the ECMO circuit. The aortic valve fails to open (demonstrated by a flat, non-pulsatile arterial line waveform with pulse pressure < 10 mmHg < 10\text{ mmHg} < 10 mmHg ).

Volume Accumulation: Even if the ECMO circuit drains venous return, approximately 5 % to 10 % 5\%\text{ to }10\% 5% to 10% of cardiac output still enters the left heart via bronchial arterial circulation (draining into pulmonary veins) and thebesian myocardial veins (draining directly into the LV cavity). With the aortic valve permanently closed and the mitral valve incompetent from annular dilation, blood becomes trapped in the left ventricle.

Hydrostatic Pulmonary Edema: LVEDP and PCWP soar to 30 – 40 mmHg 30\text{--}40\text{ mmHg} 30 – 40 mmHg , overwhelming pulmonary capillary oncotic pressure. Patients develop fulminant alveolar flooding, manifesting as copious pink frothy secretions from the endotracheal tube and complete radiographic "white-out" of both lungs.

Intracardiac Thrombosis: Severe blood stasis within the non-ejecting LV cavity and the closed aortic root promotes rapid stagnation. Spontaneous echocardiographic contrast ("smoke") evolves into massive intraventricular or aortic root thrombi that can embolize or prevent ventricular recovery.

3. Left Ventricular Venting Modalities

Active mechanical unloading ("LV venting") is essential whenever a patient on VA-ECMO exhibits loss of arterial pulsatility, elevated PCWP ( > 20 – 25 mmHg > 20\text{--}25\text{ mmHg} > 20 – 25 mmHg ), worsening pulmonary edema, or echocardiographic evidence of LV dilation.

Decompression Modalities Comparison

Modality Mechanism of Venting Advantages Disadvantages & Risks

ECPELLA (Impella + VA-ECMO) Direct transvalvular continuous aspiration of blood from LV cavity into ascending aorta Most potent unloading modality ; directly reduces LVEDP and volume; accelerates recovery; promotes aortic valve opening Requires second large-bore arterial access (14 Fr); increased hemolysis; increased vascular access bleeding

Intra-Aortic Balloon Pump (IABP) Diastolic counterpulsation reduces presystolic afterload and induces pulsatility Readily available; preserves intermittent aortic valve opening; augments coronary perfusion; low access profile (8 Fr) Modest venting capacity; ineffective if the LV is completely akinetic and unable to generate any ejection

Percutaneous Balloon Atrial Septostomy Creation of an interatrial communication permitting left-to-right decompression into RA Avoids additional arterial puncture; decompresses left atrium directly into the ECMO venous drainage line Risk of cardiac perforation; unpredictable defect size; leaves persistent residual ASD requiring closure

Surgical Left Heart Vent Direct cannula placed in LV apex or right superior pulmonary vein channeled to venous limb Highly effective complete decompression; standard in central post-cardiotomy ECMO Requires surgical thoracotomy or sternotomy; invasive; risk of bleeding and air embolism

Pharmacologic / Ventilator Venting Inotropic support (dobutamine/milrinone), systemic vasodilators, and elevated PEEP Non-invasive; inotropes restore aortic valve opening; high PEEP (10–15 cmH2O) reduces LV transmural pressure Inotropes increase M VO 2 M\text{VO}_2 M VO 2 ; vasodilators require adequate MAP; inadequate for profound mechanical standstill

Tip

The combination of VA-ECMO and an Impella microaxial pump is termed ECPELLA . In propensity-matched registry analyses, LV unloading with an Impella during VA-ECMO was associated with lower mortality, and some analyses suggest earlier unloading is better; randomized confirmation is lacking, and the strategy adds bleeding and hemolysis.

Evidence Base: ECLS-SHOCK and the ECPR Trials

ECLS-SHOCK (Thiele et al., NEJM 2023): 420 patients with acute MI-related cardiogenic shock and planned early revascularization were randomized to early VA-ECMO plus usual care or usual care alone. 30-day mortality was 47.8% vs. 49.0% (relative risk 0.98, 95% CI 0.80-1.19), while moderate or severe bleeding (23.4% vs. 9.6%) and peripheral vascular complications needing intervention (11.0% vs. 3.8%) were more frequent with ECMO. Routine early VA-ECMO in AMI-CS is therefore not supported; it remains a rescue option for selected refractory shock.

ARREST (Yannopoulos et al., Lancet 2020): In 30 patients with refractory out-of-hospital VF arrest, early ECMO-facilitated resuscitation raised survival to discharge from 7% to 43%; the single-center trial stopped early for benefit.

Prague OHCA (Belohlavek et al., JAMA 2022): In 264 patients with refractory OHCA, an invasive strategy (mechanical CPR, intra-arrest transport, ECPR) did not significantly improve 180-day survival with good neurologic outcome (31.5% vs. 22.0%, p = 0.09 p = 0.09 p = 0.09 ).

INCEPTION (Suverein et al., NEJM 2023): In a pragmatic multicenter trial of 134 patients with refractory VF/VT arrest, ECPR did not significantly improve 30-day survival with favorable neurologic outcome (20% vs. 16%).

The common thread is that ECPR helps only in highly selected patients (witnessed arrest, shockable rhythm, short low-flow time) treated in systems that can achieve rapid cannulation; current AHA guidance considers ECPR reasonable for such patients within an appropriately trained and equipped system of care.

Loading diagram... Harlequin Syndrome (Differential Hypoxemia) Hemodynamic Collision Zone

4. Harlequin Syndrome (Differential Hypoxemia / North-South Syndrome)

Harlequin syndrome—also designated North-South syndrome or differential hypoxemia —is a unique, life-threatening complication seen exclusively in peripheral femoral VA-ECMO .

Pathophysiologic Mechanism

Differential hypoxemia develops when two distinct clinical phenomena occur simultaneously:

Recovery of Native Left Ventricular Function: The myocardium begins to recover (e.g., following revascularization for myocardial infarction, resolution of stun, or myocarditis), regaining the ability to eject forward stroke volume.

Persistent, Severe Pulmonary Parenchymal Dysfunction: The patient's lungs remain severely compromised by acute respiratory distress syndrome (ARDS), dense pulmonary contusions, diffuse aspiration, or massive hydrostatic pulmonary edema.

When deoxygenated blood returning to the left heart fails to undergo gas exchange in the damaged lungs, the recovering left ventricle ejects hypoxemic, deoxygenated blood into the ascending aorta.

Simultaneously, the peripheral ECMO circuit returns hyperoxygenated blood retrogradely up the descending thoracic aorta. These two opposing streams of blood collide within the aorta at an interface known as the mixing point or watershed zone : Harlequin Syndrome Anatomical Vascular Perfusion:

[ Deoxygenated Ejection from Native LV ]
│
▼
┌────────────────────┴────────────────────┐
▼ ▼
Coronary Arteries Brachiocephalic Trunk
(Myocardial Hypoxia) │
┌────────┴────────┐
▼ ▼
Right Carotid Right Subclavian
(Brain Hypoxia) (Right Arm Cyanosis)

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ [ WATERSHED MIXING POINT ] ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

▲
│
[ Hyperoxygenated Retrograde Flow from Femoral VA-ECMO ]
│
┌────────────────────┴────────────────────┐
▼ ▼
Celiac / Mesenteric Arteries Iliac & Lower Extremities
(Normal / High Saturation) (Warm, Pink, SaO2 100%)
Clinical Presentation & The Diagnostic Imperative

The Anatomical Trap: Because the coronary ostia and the brachiocephalic trunk take off proximal to the watershed in the ascending aorta and proximal arch, they receive pure, deoxygenated native cardiac output. Consequently, the heart and brain endure profound hypoxia , while the kidneys, viscera, and lower extremities receive hyperoxygenated ECMO blood.

Monitoring Rule: Pulse oximetry placed on the lower extremity or even the left hand (which is distal to the left subclavian take-off and may receive retrograde ECMO flow) will show misleadingly normal saturations ( 98 % to 100 % 98\%\text{ to }100\% 98% to 100% ).

Mandatory Right Radial Arterial Line: Continuous arterial blood gas monitoring and pulse oximetry must be placed on the right upper extremity (right radial artery) . The right radial artery branches from the brachiocephalic trunk; therefore, its blood reflects the exact oxygen tension delivered to the brain and coronary circulation.

Warning

In peripheral VA-ECMO, an arterial blood gas drawn from the femoral line or left foot may show a PaO 2 > 300 mmHg \text{PaO}_2 > 300\text{ mmHg} PaO 2 > 300 mmHg , while the right radial artery reveals a lethal PaO 2 \text{PaO}_2 PaO 2 of 35 mmHg 35\text{ mmHg} 35 mmHg . Relying on lower extremity blood gases results in undetected, fatal cerebral and myocardial anoxia.

Management & Circuit Escalation Strategies

When Harlequin syndrome is recognized (right radial PaO 2 < 60 mmHg \text{PaO}_2 < 60\text{ mmHg} PaO 2 < 60 mmHg or SpO 2 < 88 % \text{SpO}_2 < 88\% SpO 2 < 88% ):

Ventilator Optimization: Increase mechanical ventilator FiO 2 \text{FiO}_2 FiO 2 to 1.0 1.0 1.0 , optimize PEEP to recruit collapsed alveoli, and perform gentle recruitment maneuvers. If the lungs can partially oxygenate blood, the native LV will eject oxygenated blood, abolishing differential hypoxemia.

Increase ECMO Pump Flow: Increasing pump revolutions per minute (RPM) elevates retrograde flow velocity, forcing the watershed collision zone further proximally toward the aortic valve. However, this raises LV afterload and can worsen LV dilation.

Conversion to Veno-Arterio-Venous (VAV) ECMO:

The Definitive Configuration: A Y-connector is spliced into the arterial reinfusion limb. A portion of the hyperoxygenated ECMO blood is diverted into a second venous cannula placed into the right internal jugular vein , with its tip in the right atrium or superior vena cava.

Mechanism: Oxygenated blood is delivered directly into the right heart, passing through the pulmonary circulation into the left heart. The native LV now ejects oxygenated blood into the aortic root, curing coronary and cerebral hypoxia while preserving systemic circulatory support.

Conversion to Central VA-ECMO: Re-cannulating the arterial return cannula directly into the ascending aorta via sternotomy ensures that all blood delivered to the aortic arch and coronary ostia is fully oxygenated.