Venoarterial ECMO Hemodynamics

Clinical Significance

Types of Extracorporeal Membrane Oxygenation

ECMO is primarily categorized into 2 types based on the method of circulation—veno-venous (VV) ECMO and venoarterial (VA) ECMO. Each type has a distinct purpose based on the patient's clinical condition (see Image. Types of ECMO Circuits).

Veno-venous extracorporeal membrane oxygenation: This type is indicated for severe but potentially reversible respiratory failure encompassing conditions such as hypoxemia, hypercarbia, or a combination of both). VV ECMO facilitates oxygenation and the elimination of CO₂, avoiding the need for harmful mechanical ventilation. Blood is withdrawn from a large vein, such as the femoral, jugular, or subclavian vein, oxygenated, and returned to another vein. The main indications include acute respiratory distress syndrome, severe pneumonia, aspiration, barotrauma, and interstitial pneumonitis. In some cases, such as end-stage pulmonary disease, ECMO can be utilized as a bridge to lung transplantation. Contraindications for VV ECMO are significant and must be carefully considered, which include central nervous system (CNS) hemorrhage, major CNS injuries or pathologies, systemic bleeding, contraindications to anticoagulation, severe immunosuppression, and prolonged mechanical ventilation under high settings that could have already caused extensive lung damage. In addition, older age is considered a relative contraindication, as the risk of death increases with age, although no specific age threshold is set. Veno-venous ECMO does not provide direct hemodynamic support but aids in gas exchange. Ventilator settings are selected to minimize ventilator-induced lung injury (see Image. Indications and Contraindications of VV ECMO).[3]

Venoarterial extracorporeal membrane oxygenation: VA ECMO is a critical intervention in treating severe cardiac conditions, often coexisting with respiratory failure. The primary role is in extracorporeal blood oxygenation, typically drawn from major veins, such as the femoral or jugular vein, and reinfused into an artery, such as the femoral, axillary or subclavian, or carotid artery. This advanced therapeutic intervention is pivotal in conditions such as cardiogenic shock, biventricular failure, and profound hypoxemia, especially when conventional therapies fail. This intervention is a bridge, sustaining end-organ perfusion and reducing inotrope-induced damage, while clinicians work toward a definitive solution for the underlying cardiac condition. Cannulation methods for venoarterial ECMO encompass both central and peripheral approaches.[4]

• Central cannulation: Central cannulation is often used in post-cardiotomy patients who cannot be weaned off cardiopulmonary bypass. The venous cannula is placed into the right atrium or a central vein, and the arterial cannula into the ascending aorta or another central arterial location. This method is ideal for direct cardiac and pulmonary support during surgeries or complex clinical encounters.

• Peripheral cannulation: Peripheral cannulation is initiated percutaneously or through surgical cut-down and involves inserting the venous cannula into a large peripheral vein, such as the femoral vein, and the arterial cannula into a peripheral artery, such as the femoral artery. Preferred for rapid initiation, peripheral cannulation is a key strategy in emergency settings or when central cannulation is impractical.

Indications and Contraindications

VV ECMO is indicated for a spectrum of severe cardiac conditions, including cardiogenic shock, biventricular heart failure, refractory cardiac arrest, postcardiotomy syndrome, massive pulmonary embolism, profound hypoxemia, and as a bridge to decision-making or device implantation. The role of VV ECMO in extracorporeal cardiopulmonary resuscitation and toxin-induced cardiomyopathies is also significant. However, contraindications such as uncontrollable bleeding, severe peripheral arterial disease, aortic dissection, severe aortic insufficiency, and other irreversible conditions must be carefully weighed against potential benefits in a multidisciplinary setting.

Hemodynamic Management

VV ECMO alters cardiac dynamics by reducing right heart volume and pressure while increasing left ventricular afterload, thus challenging poor contractility. Cannula sizes typically range from 19F to 25F for venous cannulas and 15F to 24F for arterial cannulas.[5] Anticoagulation, typically with unfractionated heparin monitored through anti-Xa assay, is essential to prevent clot formation. The target blood flow for patients with minimal cardiac function is 60 mL/kg/min. Adjustments are made based on the patient's cardiac function and metabolic demands. Maintaining lower flow rates of 3 to 4 L/min is often suitable, especially in patients who are obese with recovering cardiac function. ECMO flow rates must be adjusted based on blood lactate concentration, a marker for end-organ perfusion. Targeting a mean arterial pressure of 65 to 80 mm Hg is recommended for most patients. Higher mean arterial pressure may be needed for some patients, but it increases the risk of thrombosis. The oxygen concentration should not be reduced below 100% to prevent hypoxic blood delivery. Target arterial pH is achieved by adjusting the sweep gas flow rate. The aim should be to normalize the patient’s arterial pH, particularly using samples from the right radial artery.

Complications and Troubleshooting of Venoarterial Extracorporeal Membrane Oxygenation 

• Low circuit blood flow: ECMO circuit tubing and cannulas should be checked for kinks and chattering. Chattering is due to excessive negative suction on the side of the venous cannula. Common causes of chattering include hypovolemia, patient straining, or high-peak airway pressures. For hypovolemia, reducing pump speed and administering a fluid bolus are recommended. Increasing sedation or considering neuromuscular blockade is advised if the patient is straining. Echocardiography might show right atrial involution in patients with hypovolemia. Other medical reasons for reduced venous return include intra-abdominal hypertension and cardiac tamponade.

• Oxygenator thrombosis: Oxygenator thrombosis is indicated by a pressure drop ≥ 100 mm Hg across the oxygenator, which should be replaced, if necessary, with a plan to support the patient's hemodynamics.

• Differential hypoxemia: Differential hypoxemia, also known as Harlequin syndrome or North-South Syndrome, is a unique phenomenon in peripheral VV ECMO, particularly affecting patients with compromised pulmonary function.[6] This condition arises when deoxygenated blood from the heart mixes with oxygenated ECMO blood in the descending thoracic aorta, leading to the upper body receiving deoxygenated blood and the lower body receiving oxygenated blood. This differential in oxygenation can result in complications such as myocardial or cerebral ischemia. Monitoring typically involves assessing arterial blood gases or Sp0₂ from the right upper extremity and utilizing cerebral oximetry. Patients show normal Sp0₂ in the lower extremities but less than 90% in the upper extremities, with cerebral tissue saturation lower compared to that in the leg. Management strategies include enhancing venous drainage, increasing ECMO pump speed, reducing left ventricular ejection, optimizing ventilation parameters, and considering a switch to veno-arteriovenous or central VV ECMO when refractory cases do not respond to medical treatment.

• Left ventricular distention: VV ECMO increases left ventricular end-diastolic pressure and volume, which exacerbates pulmonary edema, raises myocardial oxygen consumption, and elevates atrial pressure. Management strategies include enhancing venous drainage, providing inotropic support, administering diuretic therapy, and utilizing intra-aortic balloon pumps, which improve coronary blood flow but may have variable effects on pulmonary capillary wedge pressure. More direct and controlled left ventricular unloading can be achieved through advanced methods such as left atrial venting, surgical or percutaneous left ventricular venting, and inserting percutaneous transaortic ventricular assist devices such as the Impella. These advanced methods relieve left ventricular pressure, support antegrade flow, and may assist in ECMO weaning. 

• Airway hemorrhage: Patients can present with hemoptysis, which is common in up to 10% of patients. Management strategies include withholding anticoagulation, bronchoscopic interventions, and treating coagulopathy.

• Intracardiac thrombosis: Transesophageal echocardiography can identify intracardiac and valvular thrombosis. To reduce risks, ensuring native cardiac ejection and avoiding hypertension is recommended.

• Limb ischemia: The risk of ischemia is present in the cannulated limb. Treatment strategies include the maintenance of anticoagulation and placement of distal perfusion cannulas.[7][8][9][10][11][12][13]

Weaning From Venoarterial Extracorporeal Membrane Oxygenation Complications

Assessing hepatic, pulmonary, and metabolic status is essential, along with hemodynamic evaluation. Improved cardiac filling pressures, improved arterial pulsatility, and minimal inotrope support are crucial to assess readiness for weaning. Echocardiography is performed to assess the myocardial recovery. Patients should maintain a mean arterial pressure of ≥65 mm Hg with minimal support and adequate ventricular function at low flow.[14]

Alternatives and Emerging Techniques

Recent advancements in ECMO focus on improving cardiac support and patient mobility. Percutaneous catheter-based microaxial transaortic ventricular assist devices for left ventricle venting during VV ECMO is a significant development. These devices help in left ventricle decompression and provide additional antegrade flow to the aortic root. New surgical methods for central VV ECMO, such as dual cannulation with a centrifugal flow pump and ambulatory VV ECMO setups, have also been explored. These innovations are transforming ECMO by enhancing therapeutic effectiveness, patient mobility, and overall quality of life.[15]

Enhancing Healthcare Team Outcomes

VV ECMO is a critical intervention for patients with severe cardiac conditions, requiring meticulous management and continuous assessment of the hemodynamic impacts. Advances in techniques, patient selection, and treatment strategies are expected to improve outcomes in patients requiring this life-saving intervention. Coordination with an interprofessional healthcare team improves patient care.