Extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO) is a technique that provides prolonged cardiac and/or respiratory support to patients with potentially reversible cardiac or pulmonary failure unresponsive to conventional treatments. The goal of ECMO is to serve as a bridge for patients to resolve the cardiopulmonary disease process or to long term device placement or transplant. ECMO consists of a blood pump, membrane oxygenator, and vascular access cannula. It is different from cardiopulmonary bypass (CPB) in that it does not contain an open reservoir for direct volume management. ECMO requires anticoagulation, oftentimes with heparin, to achieve goal ACT of around 200 seconds.

Configurations

1. Veno-venous (VV) ECMO
   1. Supports only respiratory function by draining and returning oxygenated blood to venous system
   2. Cannulas
      1. Single dual-lumen cannula in vein
2. Veno-arterial (VA) ECMO
   1. Supports primarily cardiac but also respiratory function by draining and returning oxygenated blood to arterial system
   2. Cannulas
      1. Peripheral VA ECMO
         1. Arterial: femoral artery
         2. Venous: femoral vein or jugular vein
      2. Central VA ECMO
         1. Arterial: aorta
         2. Venous: right atrium or IVC + SVC
3. Veno-arterial-veno (VAV) ECMO
   1. Supports both cardiac and respiratory function by draining and returning oxygenated blood to both venous and arterial system
   2. Often used in patients who develop differential hypoxemia (Harlequin syndrome) during VA ECMO

Settings

• Flow rate ~ cardiac output
  • Typically 3-6 L/min (around 50ml/kg/min)
  • Higher flows are associated with increased risk of hemolysis
  • Lower flows are associated with increased risk of thrombosis
• FiO2 ~ oxygenation
• Sweep gas ~ ventilation
  • Typically 1-9 L/min
  • Adjusted to achieve target partial pressure of carbon dioxide
  • Lower flows are desired if baseline hypercapnia (to avoid dropping too fast) or situations where hypocapnia can be dangerous (e.g. high ICP)
  • Very low flows, however, can start to affect oxygenation
• Revolutions per minute (RPM)
  • Controls speed of ECMO pump, directly affects flow rate
• Other settings
  • Pven = venous pressure
    • Pressure before blood enters pump, or suction needed to drain venous blood
  • Pint = internal pressure
    • Highest pressure in ECMO circuit
  • Part = arterial pressure
    • Pressure inside arterial return tubing after leaving membrane
  • Delta P = transmembrane pressure = Pint - Part

Harlequin Syndrome (North-South Syndrome, Differential Hypoxemia)

Harlequin syndrome is a complication of VA ECMO when there is concomitant severe respiratory failure. In this situation, poorly oxygenated blood from the native lung is ejected by left ventricle into ascending aorta, supplying the upper body (including brain and heart) while well-oxygenated blood from ECMO circuit flows retrograde from the femoral artery, supplying the lower body. Blood can be sampled from a RIGHT radial arterial line to determine the "point" of mixing of well-oxygenated and poorly-oxygenated blood. Treatment includes increasing ECMO flow (to push mixing "point" more proximal to heart) or VAV ECMO (also returning oxygenated blood to right atrium via right internal jugular cannula)

Complications

	Complications
Neurologic	CVA hemorrhage, infarct Seizures
Cardiac	Arrhythmia Tamponade
Pulmonary	Pneumothorax Pulmonary hemorrhage
Renal	Acute kidney injury
Hematologic	Hemolysis Disseminated intravascular coagulation (DIC) Fibrin or coagulation factor consumption Acquired Von Willebrand disease Thrombocytopenia Heparin-induced thrombocytopenia (HIT) Venous thromboembolism Bleeding at cannula site, surgical site, gastrointestinal, pulmonary, retroperitoneal
Infectious	Bacteremia Cannula insertion site infection
Circuit-related	Circuit component clots Oxygenator failure Hemofilter clots Air in circuit Pump failure
Cannula-related	Cannula site bleeding Limb ischemia Compartment syndrome