ECMO Principles

This episode explores the clinical application of Extracorporeal Membrane Oxygenation (ECMO), a specialized technology used to support patients facing life-threatening heart or lung failure. It details the two primary configurations, venovenous (VV) for respiratory support and venoarterial (VA) for circulatory assistance, while tracing the historical evolution of the field from early failures to modern success. A significant focus is placed on the necessity of a multidisciplinary medical team and the rigorous criteria required for proper patient selection and cannulation. The authors also address the technical complexities of the ECMO circuit, the management of common complications like bleeding, and the protocols for weaning patients off support. Ultimately, the source highlights that while ECMO is a resource-intensive therapy with persistent controversies, it serves as a vital physiological bridge that allows failing organs the time needed to recover.

 

 

 

DISCLAIMER

The Critical Edge is for educational and informational purposes only and is not intended to diagnose, treat, cure, or prevent any disease, nor does it substitute for professional medical advice, diagnosis, or treatment from a qualified healthcare provider—always seek in-person evaluation and care from your physician or trauma team for any health concerns.

 

 

 

EXTRACORPOREAL MEMBRANE OXYGENATION (ECMO) STUDY GUIDE

TOP TEN TAKEAWAYS

1. Dual Functional Purpose: ECMO (also known as Extracorporeal Life Support or ECLS) serves to replace the oxygenation and ventilation functions of the lungs and, depending on its configuration, the perfusion functions of the heart during acute organ injury.
2. Configuration Distinction: Venovenous (VV-ECMO) is primarily utilized for acute lung injury when cardiac function is adequate, while Venoarterial (VA-ECMO) provides both respiratory and circulatory support for patients with significant cardiac impairment.
3. Historical Evolution: After catastrophic initial trials in the 1970s, ECMO became a viable therapy through the leadership of Dr. Robert Bartlett and the establishment of the Extracorporeal Life Support Organization (ELSO) in the late 1980s.
4. Multidisciplinary Expertise: Successful programs require a highly integrated team including cardiothoracic surgeons, perfusionists, ECMO specialists (RNs or respiratory therapists), and physician champions from various subspecialties like nephrology and neurology.
5. Predictive Scoring: Clinical tools such as the Respiratory ECMO Survival Prediction (RESP) score and the Survival After Veno-Arterial ECMO (SAVE) score help identify optimal candidates and risk-stratify outcomes.
6. Cannulation Safety: Vascular access typically involves large-bore cannulas placed percutaneously via the Seldinger technique. For femoral VA-ECMO, the placement of a distal perfusion catheter is critical to prevent limb ischemia and potential amputation.
7. The ECMO Circuit: The closed-loop system uses a centrifugal pump and a microporous hollow fiber oxygenator. Gas exchange is regulated by "sweep gas" to manage carbon dioxide removal and oxygenation concentration.
8. Management Priorities: The fundamental goal of ECMO is to provide "organ rest," allowing the heart and lungs to recover by minimizing ventilator-induced barotrauma and providing systemic hemodynamic stability.
9. Anticoagulation Necessity: Continuous systemic anticoagulation (typically heparin) is required to prevent circuit thrombosis, though this creates a constant risk-balance challenge regarding clinical bleeding.
10. E-CPR Potential: ECMO-assisted cardiopulmonary resuscitation (E-CPR) is an emerging application that can significantly improve survival and neurologic outcomes for witnessed cardiopulmonary arrests compared to conventional CPR.

STUDY GUIDE

I. Core Mechanics and Physiology

Extracorporeal membrane oxygenation operates by draining deoxygenated venous blood from the body, pumping it through an artificial membrane for gas exchange, and returning it to the patient.

• VV-ECMO: Blood is drained from the venous system and returned near the right atrium. It requires a functioning heart to pump the newly oxygenated blood through the pulmonary and systemic vascular systems.
• VA-ECMO: Blood is returned to the arterial system (typically the aorta), bypassing both the heart and lungs. This configuration supports critical end-organ metabolic needs when native cardiac output is insufficient.

II. Historical Context and Organizations

The history of ECMO is marked by early failure followed by technological and protocol refinement.

• 1970s NIH Trials: Early results were poor, with survival rates below 10%, leading to temporary abandonment of the therapy. These failures were attributed to technological deficiencies and lack of management guidelines.
• The Michigan Influence: Dr. Robert Bartlett’s work in the 1980s and 1990s at the University of Michigan established the foundation for modern ECLS.
• ELSO: The Extracorporeal Life Support Organization maintains an international registry and provides the standards, guidelines, and protocols used by centers of excellence worldwide.

III. Clinical Indicators and Patient Selection

ECMO is a supportive tool, not a primary treatment. It stabilizes the patient while underlying causes (e.g., pneumonia, myocardial infarction) are treated.

• Respiratory Criteria: Indicated when the risk of mortality from respiratory failure exceeds 80%. This is often measured by the Murray Score, which evaluates four variables: Pao2/Fio2 ratio, Positive End-Expiratory Pressure (PEEP), chest x-ray quadrants with infiltrates, and pulmonary compliance.
• Cardiac Criteria: Indicated for cardiogenic shock, such as postcardiotomy shock or acute myocardial infarction, where maximal medical therapies and other mechanical supports (like intra-aortic balloon pumps) have failed.
• Contraindications:
  • Absolute: Terminal malignancies, acute anoxic brain injury, or comorbidities precluding meaningful survival.
  • Relative: Mechanical ventilation for more than seven days on high settings, advanced age, extreme obesity, or severe chronic medical immunosuppression.

IV. Cannulation and Technical Execution

Cannulation is a high-risk technical task requiring proficiency in wire handling and vascular management.

• Access Sites: Common sites include the femoral vein, right internal jugular vein, and femoral artery. Central cannulation (directly into the right atrium or aorta) may be used following cardiac surgery.
• The Distal Perfusion Catheter: In femoral VA-ECMO, a large cannula can occlude the femoral artery. A 6- to 8-French introducer must be placed retrograde to the superficial femoral artery to ensure the lower limb receives adequate blood flow.
• Recirculation: In VV-ECMO, if the drainage and inflow cannulas are too close, oxygenated blood may be immediately drained back into the circuit, reducing the efficiency of systemic oxygenation.
• Dual Lumen Cannulas: Advanced cannulas (e.g., Avalon or Crescent) allow for single-site access (usually the right internal jugular), promoting patient mobility and potential extubation, though they require precise positioning via echocardiography.

V. Maintenance and Circuit Management

• Anticoagulation: Patients must be heparinized before cannulation (Target Activated Clotting Time > 250 seconds). Maintenance PTT levels typically range from 40 to 80 seconds.
• The "Sweep": Gas flow through the oxygenator. Increasing the sweep gas increases the removal of carbon dioxide.
• Ventilator Strategy: The "lung rest" strategy involves minimal Fio2 and low pressure settings to avoid further barotrauma while the patient is on the circuit.
• Sedation: ECMO patients often require higher doses of fentanyl and benzodiazepines than standard ICU patients, though the physiological reason for this remains poorly understood.

VI. Complications and Troubleshooting

• Bleeding: The most common complication due to continuous anticoagulation.
• Thrombocytopenia: Often occurs due to platelet activation and mechanical trauma within the circuit.
• Neurologic Injury: Intracranial bleeding is a significant risk due to the combination of high-level anticoagulation and potential pre-ECMO hypoxia.
• Acute Kidney Injury: Common following the initial ischemic insult; renal replacement therapy can often be integrated directly into the ECMO circuit.

VII. Weaning and Decannulation

• VV-ECMO Weaning: Performed by slowly decreasing the "sweep" gas. When the sweep is off and the patient maintains adequate gas exchange via the lungs alone, they are ready for decannulation.
• VA-ECMO Weaning: A more complex process requiring echocardiography and hemodynamic monitoring. Pump flow is gradually reduced to allow the heart to take over the workload while monitoring for signs of ventricular failure or high filling pressures.
• Decannulation: Venous cannulas can often be removed at the bedside with purse-string sutures and pressure. Arterial cannulas typically require surgical repair in the operating room.

VIII. Glossary of Key Terminology

• AOI (Oxygenation Index): A calculation used to assess the severity of lung injury; higher values indicate more severe impairment.
• E-CPR (ECMO-assisted Cardiopulmonary Resuscitation): The application of VA-ECMO during active cardiac arrest to facilitate stabilization and diagnosis.
• Inflow: The "arterialized" or oxygenated blood being returned to the patient.
• PAPI (Pulmonary Artery Pulsatility Index): A marker for right ventricular function used during weaning from VA-ECMO.
• Recirculation: A phenomenon in VV-ECMO where oxygenated blood is pulled back into the drainage cannula before reaching the systemic circulation.
• Seldinger Technique: A medical procedure to obtain safe access to blood vessels or other hollow organs using a guide wire.
• Sweep Gas: The flow of gas (oxygen/air) across the oxygenator membrane used to clear carbon dioxide from the blood.

REFERENCES

1. Firstenberg MS, Libby M, Abelson J. Extracorporeal membrane oxygenation for acute cardiopulmonary failure. In: Firstenberg MS, ed. Pasted Text. [City, State of publication unknown]: [Publisher unknown]; [Year unknown]:1-25.
2. Murray JF, Matthay MA, Luce JM, Flick MR. An expanded definition of the adult respiratory distress syndrome. Am Rev Respir Dis. 1988;138:720–723.
3. Bartlett RH. Extracorporeal life support: the University of Michigan experience. JAMA. 2000;283(7):904-908.
4. Peek GJ, Mugford M, Tiruvoipati R, et al. Efficacy and economic assessment of conventional ventilatory support versus extracorporeal membrane oxygenation for severe adult respiratory failure (CESAR): a multicentre randomised controlled trial. Lancet. 2009;374(9698):1351-1363.
5. Combes A, Hajage D, Capellier G, et al. Extracorporeal membrane oxygenation for severe acute respiratory distress syndrome. N Engl J Med. 2018;378(20):1905-1915.
6. Chen YS, Lin JW, Yu HY, et al. Cardio-pulmonary resuscitation with assisted extracorporeal life-support versus conventional cardio-pulmonary resuscitation in adults with in-hospital cardiac arrest: an observational study and propensity analysis. Lancet. 2008;372(9638):554-561.
7. Yannopoulos D, Bartos J, Raveendran G, et al. Advanced reperfusion strategies for patients with out-of-hospital cardiac arrest and refractory ventricular fibrillation (ARREST): a phase 2, single centre, open-label, randomised controlled trial. Lancet. 2020;396(10265):1807-1816.
8. Papadimos TJ, Henn MC, Baudendistel TE, et al. Ethics of extracorporeal membrane oxygenation: five dilemmas. Chest. 2014;145(5):1157-1164.