Neonatal Extracorporeal Membrane Oxygenation (ECMO): Key Considerations
Extracorporeal Membrane Oxygenation (ECMO), also known as Extracorporeal Life Support (ECLS), is an advanced technology that provides temporary respiratory and/or cardiac support for critically ill neonates when conventional treatments are insufficient. It acts as a bridge to recovery, transplantation, or other supportive systems, but it is not a cure. ECMO is an invasive, time-limited procedure associated with significant risks, including hemorrhagic, thrombotic, infectious, and neurological complications.
Phases of ECMO Support: ECMO support typically follows a structured pathway:
Transition to ECMO: This initial phase involves rapid assessment, establishing vascular access, managing hemostasis, and preparing the circuit.
Stabilization: The first 24 hours focus on using ECMO to reperfuse, reoxygenate, and reventilate the patient, while also assessing for any damage incurred prior to or during cannulation.
ECMO Support ('Main run'): This is the main period of support, where continuous monitoring, complication management, and preparation for recovery or alternative pathways are key.
Transition Off ECMO: Weaning the patient from ECMO support as their native organ function recovers.
"Elevator" Evaluation for ECMO Candidacy:
When considering ECMO, a quick "elevator" evaluation assesses several critical factors:
High Risk of Mortality: ECMO is typically reserved for patients with a high risk of mortality (around 50% in most scenarios) despite maximal conventional therapy.
Reversible/Recoverable Condition or Reasonable Chance of Transplantation: The underlying condition should ideally be reversible, or there should be a viable path to recovery or organ transplantation.
Conventional Treatment Optimized: All conventional treatments should be optimized before initiating ECMO. Simple issues, like an endotracheal tube leak, can sometimes be resolved to avoid ECMO entirely.
Contraindications to ECMO:
Contraindications may include chromosomal disorders (e.g., Trisomy 13, 18, but not 21), irreversible brain damage, and uncontrolled bleeding or Grade III/IV intraventricular hemorrhage (IVH).
Relative Contraindications include prematurity (less than 34 weeks post-menstrual age or weight less than 2 kg/2.5 kg at some centres), mechanical ventilation for more than 10-14 days (especially with extremely damaging ventilation), and irreversible organ damage (unless transplant is planned).
Indications for Neonatal ECMO ECMO is indicated for severe respiratory or cardiac failure.
Cannulation Strategies The choice of cannulation (VA or VV) depends on the patient's condition, the surgeon's preference, and center experience.
VA ECMO (Veno-Arterial): Provides both respiratory and cardiac support by diverting venous blood, oxygenating it, and returning it to the arterial system, bypassing the heart and lungs.
Central Cannulation: Direct access to the heart and aorta, requiring an open chest (sternotomy). Often used for postcardiac surgery patients.
Peripheral Cannulation: Typically involves the internal jugular vein and carotid artery in the neck for neonates, or femoral vessels for older children. For VA ECMO, the arterial cannula is often connected to a graft to avoid obstruction of the aorta.
Deoxygenated Coronary Perfusion: A key consideration in VA ECMO is that the coronary arteries will receive deoxygenated blood from the native heart, which is pumping against the ECMO flow.
VV ECMO (Veno-Venous): Provides only respiratory support (oxygenation and CO2 removal) by taking venous blood, oxygenating it, and returning it to the venous system (right atrium). The native heart maintains circulatory function.
Single Lumen Catheters: Commonly used in neonates, with specialized designs to pull deoxygenated blood and return oxygenated blood through the same catheter, aimed towards the tricuspid valve.
Advantages of VV ECMO: Avoids arterial (carotid) ligation, maintains pulsatile flow, and provides oxygenated blood to the pulmonary artery (oxygen is a pulmonary vasodilator) and coronary arteries.
Recirculation: A common complication where oxygenated blood from the return cannula is immediately re-aspirated by the drainage cannula, reducing efficiency. Good cannula placement and sufficient ventricular function are crucial to prevent this.
The ECMO Circuit
The ECMO circuit is a complex system that mimics the functions of the lungs and/or heart.
Components: Includes drainage cannulae, a pump, an oxygenator (artificial lung), a heat exchanger, and return cannulae. A bridge connecting the venous and arterial lines is common, allowing recirculation of blood within the circuit without stopping flow to the patient, useful during wean trials.
Pumps:
Roller Pumps: Mechanically squeeze tubing to propel blood. They offer precise flow control but have a risk of circuit rupture if distal pressure is too high.
Centrifugal Pumps: Use a spinning magnet to create flow. They avoid blowouts but are more sensitive to afterload (patient's peripheral vascular resistance) and can cause more hemolysis, especially at low flow rates.
Pressure Dynamics: The circuit operates with negative pressure before the pump (sucking blood in) and positive pressure after the pump (pushing blood out). Medications and blood products are typically administered on the positive pressure side of the circuit.
Blood Prime: All neonatal ECMO circuits require a blood prime due to the large dead space volume relative to a neonate's total blood volume.
Oxygenator (Artificial Lung): Responsible for gas exchange (oxygenation and CO2 removal) and temperature regulation.
FiO2 (Fraction of Delivered Oxygen): Controls oxygenation.
Sweep Gas Flow: Controls CO2 removal. This is the primary means of adjusting CO2 on ECMO.
Membrane Lung Performance: Oxygenators have a limited lifespan (typically 5-7 days for membrane oxygenators) due to fibrin accumulation, which can impair gas exchange. Monitoring pressure gradients across the oxygenator helps detect this.
Routine Management and Monitoring on ECMO Daily surveillance is crucial and involves both patient and circuit assessment.
Patient Assessment:
Respiratory: Ventilator settings (often "rest settings" for lung protection), lung compliance, chest X-ray, blood gases.
Hemodynamic: Conventional support (vasopressors/dilators), arterial blood pressure (ABP), central venous pressure (CVP), mixed venous saturation (SvO2), lactate, perfusion, urine output.
Left Atrial Decompression: Critical for cardiac recovery in VA ECMO if the left ventricle is unable to eject blood. Signs of left heart distention include pulmonary edema, dilated cardiac cavities, very negative venous pressures, and high left atrial pressure (goal <10 mmHg). Decompression can be achieved via a surgical left atrial vent, a septostomy (Rashkind procedure in the cath lab), or cautious use of low-dose inotropes.
In cases of severe left ventricular failure or myocardial stunning on VA ECMO, the left side of the heart can become severely distended and congested, impairing coronary perfusion and hindering cardiac recovery. Left atrial (LA) decompression aims to relieve this tension. Methods include: Surgical placement of a left atrial vent if the chest is open. Rashkind septostomy in the cath lab if the chest is closed. Careful use of low-dose inotropes to promote enough myocardial contraction to prevent stasis and distension without increasing oxygen consumption significantly. Monitoring for inadequate LA decompression includes elevated LA pressure (goal <10 mm Hg), pulmonary edema on X-ray despite negative fluid balance, very negative venous pressures, and elevated proBNP.
Hematology: Balancing bleeding and thrombosis risks, checking for clots or bleeding, daily labs (ACT, aPTT, Anti-Xa, TEG/ROTEM, CBC, ATIII, Fibrinogen, Platelets).
Infection: Temperature is regulated by the ECMO circuit, so fever is not a reliable sign. Monitor CRP and look for unexplained variations in anticoagulation.
Neurology: Daily head ultrasounds (HUS) for the first 3-5 days are common to monitor for intracranial hemorrhage (ICH), which is a serious complication.
Circuit Assessment: ECMO flow, venous and arterial pressures, membrane lung performance (pressure gradient, sweep gas output), and visual inspection for fibrin/clot buildup.
Anticoagulation
Maintaining appropriate anticoagulation is critical to prevent circuit thrombosis while minimizing bleeding. This is particularly challenging in neonates due to their immature hemostatic system and lack of reserve capacity.
Unfractionated Heparin (UFH): The most commonly used anticoagulant. It has a short half-life and is reversible with protamine. However, its efficacy can be limited by low antithrombin (ATIII) levels in neonates, sometimes requiring ATIII replacement (e.g., with FFP).
Direct Thrombin Inhibitors (DTIs): Such as bivalirudin, are gaining use. They do not depend on ATIII and can inhibit clot-bound thrombin. Their short half-life is an advantage, but they lack reversal agents and require careful management, especially during weaning or in areas of blood stasis. Clot may formed if there is stasis (example: left atrial stasis).
Monitoring: A combination of tests is often used, as no single test is ideal. These include Activated Clotting Time (ACT), activated Partial Thromboplastin Time (aPTT), Anti-Xa assay, and viscoelastic tests like Thromboelastography (TEG) or Rotational Thromboelastometry (ROTEM). These tests can be affected by factors like hemolysis and hyperbilirubinemia.
Weaning from ECMO
Weaning is a gradual process that involves reducing ECMO support as the patient's native organ function recovers.
Timing and Duration: Average ECMO run times vary by diagnosis (e.g., Meconium Aspiration Syndrome: 5-6 days; Congenital Diaphragmatic Hernia: median 2-3 weeks, but can be longer). Longer runs (beyond 4-6 weeks for CDH) may have limited benefit, but universally accepted limits are not established.
Signs of Recovery:
Respiratory: Improved lung compliance, clearing chest X-ray, good lung expansion on low ventilator settings (e.g., PEEP 10, Delta 10, RR 10 for "rest settings"), and reduced oxygen requirements.
Cardiac: Development of a pulsatile arterial trace (if previously flat), signs of intrinsic cardiac output, improving ventricular function on echocardiography, decreasing left atrial pressures, and minimal or no need for inotropes/vasopressors.
Weaning Strategies:
Gradual Flow Reduction: ECMO flows are progressively decreased (e.g., from 90-100 mL/kg/min down to 50 mL/kg/min or less) while conventional support is increased.
Clamp Trial: The classic method involves clamping the cannulae proximal to the patient and opening a bridge to recirculate blood within the circuit. This allows complete separation of the patient from the circuit for a trial period (15 minutes to 2 hours) to assess their tolerance without ECMO support. Adequate anticoagulation must be maintained in the circuit during the clamp trial to prevent thrombosis.
Pump-Controlled Retrograde Trial-Off: A newer technique that uses the patient's native cardiac output to maintain circuit integrity, acting as a "stress test" without clamping the circuit.
Post-Weaning Considerations: If successful, decannulation follows. If unsuccessful, the patient can return to full ECMO support, and further investigations for the cause of weaning failure (e.g., residual cardiac lesions, pulmonary issues) are pursued.
The "clamping of the bridge test" is a classic approach used to assess a patient's readiness for weaning off support. It is often referred to as a clamp trial.
Outcomes and Prognosis
Overall survival to hospital discharge for neonates on ECMO for respiratory disease is around 73%. For cardiac indications, survival is typically lower, around 40%. Survival rates vary significantly by specific diagnosis (e.g., Meconium Aspiration Syndrome: 92%; Congenital Diaphragmatic Hernia: ~50%; Cardiac Diagnoses: 35-45%). Longer ECMO runs are associated with increased risk of complications and generally lower survival. Survivors of neonatal ECMO, particularly those with CDH, are at increased risk for long-term morbidities, including chronic lung disease and neurodevelopmental delay. For more in-depth information and specific guidelines, the Extracorporeal Life Support Organization (ELSO) website (www.elso.org) is an invaluable resource, offering comprehensive practice guidelines and registry data.
Flow rates
The usual flow rates for Extracorporeal Membrane Oxygenation (ECMO) vary depending on the type of support (respiratory or cardiac), the patient's size, and underlying physiological factors.
General Considerations
For pediatric patients, ECMO circuits are typically modified from adult circuits, aiming for less tubing and dead space while including infusion ports for medications.
The goal is to achieve the desired flow rate without excessive venous pressure. Using the biggest and shortest cannula possible is crucial, similar to managing an airway in ECMO. To allow for the least amount of hemolysis.
In VA ECMO, systemic oxygen extraction is continuously monitored via the drainage cannula (SvO2). The goal is to maintain oxygen delivery (DO2) at least three times oxygen consumption (VO2), with an SvO2 greater than 66%.
The desired blood flow rate for an oxygenator should be over 500 mL/min.
Neonatal Respiratory ECMO
For respiratory ECMO in neonates, the typical aim is about 100 mL/kg/min.
Oxygenation is affected by blood flow, hemoglobin, and oxygen saturation. Support is usually initiated with a sweep gas of 100% FiO2.
In VV ECMO, pump flow increases might not directly result in higher patient saturation due to recirculation, which is influenced by cannula position, patient volume status, native cardiac function, and flow rates.
Neonatal Cardiac ECMO
For cardiac ECMO in neonates, the usual target flow is about 150 mL/kg/min.
If a baby with cardiac ECMO has a BT shunt, the flow rate might need to be higher, around 180 mL/kg/min.
Pump flow is gradually increased until adequate flow is achieved, then decreased to the lowest level that supports cellular metabolic demands.
Ideally, an arterial pulse pressure of at least 10 mm Hg should be maintained, indicating systemic ventricular ejection and reducing the risk of thrombosis. If the heart is not contracting at all or very little, the arterial trace might appear as a flat line because the ECMO is supplying all the cardiac output.
If systemic perfusion is inadequate (e.g., low urine output, poor perfusion), pressure can be increased by increasing pump flow, transfusing blood products, or titrating vasopressor infusions.
Larger Children (Pediatric Cardiac ECMO)
Monitoring and Adjustment
During ECMO support, daily assessment includes checking if support is adequate, monitoring for complications, evaluating the circuit, and checking thromboprophylaxis.
Oxygenation can be increased by increasing the FiO2 to the oxygenator or by increasing the blood flow. Carbon dioxide (CO2) clearance is controlled by the sweep gas flow rate, which can be adjusted to maintain the patient’s PaCO2 within target ranges (e.g., 40-45 mm Hg for neonates).
If a centrifugal pump is used (which is common), it is sensitive to afterload. High blood pressure, pain, or waking up can increase peripheral vascular resistance and impede flow.
In the case of troubleshooting, a decrease in venous pressure (ECMO preload) might indicate hypovolemia, cardiac compression, or a displaced/obstructed/kinked cannula. Actions include giving volume or temporarily decreasing flow. An increase in pre-membrane pressure could indicate an issue with the oxygenator or an obstruction.
Weaning
When planning to wean a patient off ECMO, flows are progressively decreased. For respiratory support, flows might be around 90-100 mL/kg/min when evaluating for weaning.
A minimum flow must be maintained through the centrifugal pump to prevent retrograde flow within the circuit. During weaning trials, the bridge line can be opened to allow recirculation within the circuit, maintaining flow without stopping it entirely.
In cardiac ECMO, weaning begins by gradually decreasing ECMO flow once signs of myocardial recovery are observed, such as increasing pulse pressure, rising systolic pressure, and improved ventricular function on echocardiography. The goal is to reach minimal ECMO support, typically around 50 mL/kg/min, before a trial-off.
ECMO criteria:
CDH (ELSO): The decision to initiate ECMO for CDH patients involves considering several physiological parameters that indicate a failure of conventional therapies. While there are no uniformly accepted and rigidly followed criteria for ECMO initiation in CDH, ELSO consensus indications based on expert opinion include:
