2.3 Extracorporeal Support (V-V & V-A ECMO)

Extracorporeal Support (V-V & V-A ECMO)

Extracorporeal Membrane Oxygenation (ECMO) is a temporary life-support modality used for patients experiencing refractory respiratory or cardiac failure that is unresponsive to conventional therapies. By routing blood outside the body through an artificial membrane lung, ECMO allows for gas exchange, circulatory support, and a period of 'lung rest' to facilitate tissue recovery.

Basic Principles of the ECMO Circuit

An ECMO circuit consists of a drainage cannula, a centrifugal pump, a membrane oxygenator, and a reinfusion cannula. The drainage cannula draws deoxygenated venous blood, the pump drives the blood, the oxygenator performs gas exchange, and the reinfusion cannula returns the blood.

The clinician controls two primary variables on the ECMO machine:

• Blood Flow Rate (L/min): Primarily determines oxygenation (PaO2). Increasing blood flow increases oxygen delivery.
• Sweep Gas Flow Rate (L/min): The flow of gas (100% O2 or an O2/air mix) passing through the oxygenator fibers. It determines carbon dioxide clearance (PaCO2 and pH). Increasing the sweep gas flow rate increases CO2 clearance and raises the pH.

Venovenous (V-V) vs. Venoarterial (V-A) ECMO

V-V ECMO Indications (ELSO Guidelines)

V-V ECMO is indicated for severe hypoxemic respiratory failure when the risk of mortality is high:

• PaO2/FiO2 ratio < 80 mmHg for >6 hours, or <50 mmHg for >3 hours, despite optimal ventilator management.
• Severe hypercapnic acidosis (pH < 7.20 with PaCO2 > 60 mmHg) for >6 hours.
• Mechanical ventilation should ideally have been initiated for <7 to 10 days to avoid advanced lung fibrosis.

V-A ECMO Indications

Indicated for refractory cardiogenic shock (cardiac index < 2.0 L/min/m2, MAP < 60 mmHg, and metabolic acidosis despite high-dose vasopressors and intra-aortic balloon pump therapy).

Ventilator Management on ECMO: 'Lung Rest'

Once ECMO is initiated, the ventilator settings must be reduced to 'lung rest' parameters. Because the membrane lung handles gas exchange, high airway pressures and large tidal volumes are no longer necessary, which prevents further ventilator-induced lung injury (VILI) and biotrauma.

• Recommended Rest Settings:
  • Ventilation Mode: Pressure Control (PC) or Airway Pressure Release Ventilation (APRV).
  • Tidal Volume (Vt): Ultra-low, 3 to 4 mL/kg PBW (typically 150 to 250 mL).
  • Respiratory Rate (RR): 4 to 10 breaths per minute.
  • PEEP: Moderately high, 10 to 15 cm H2O. This maintains alveolar recruitment and prevents complete lung collapse (atelectasis), which would lead to pulmonary shunting and consolidation.
  • Inspiratory Pressure / Plateau Pressure: Kept strictly < 20 to 25 cm H2O (driving pressure < 10 cm H2O).
  • FiO2: Kept at 0.40 to 0.50 to prevent absorption atelectasis and oxygen toxicity.

Complications and Critical Management

1. Anticoagulation and Bleeding

Continuous anticoagulation (typically with unfractionated heparin) is required to prevent circuit thrombosis.

• Monitoring: Titrated to target an activated clotting time (ACT) of 180-220 seconds, an activated partial thromboplastin time (aPTT) of 60-80 seconds, or an anti-Xa level of 0.3-0.7 IU/mL.
• HIT: If Heparin-Induced Thrombocytopenia is suspected (platelet drop >50%), heparin must be stopped and direct thrombin inhibitors (argatroban or bivalirudin) initiated.

2. Harlequin (North-South) Syndrome in V-A ECMO

This complication occurs in patients receiving peripheral V-A ECMO (femoral vein drainage and femoral artery reinfusion) when their native heart begins to recover, but their lungs remain severely compromised.

• Pathophysiology: The recovering native heart ejects poorly oxygenated blood forward into the aorta. The ECMO circuit pumps highly oxygenated blood retrogradely up the descending aorta. This creates a mixing zone where the upper body receives poorly oxygenated blood from the native heart, while the lower body receives highly oxygenated blood from the ECMO.
• Diagnosis: Compare blood gas samples from the right radial artery (pre-ductal, representing cerebral/coronary perfusion) with samples from the lower extremity. A right radial PaO2 < 55 mmHg confirms Harlequin syndrome.
• Management: Increase ventilator settings (FiO2 and PEEP) to improve gas exchange in the native lungs, convert the system to V-A-V ECMO, or relocate the arterial cannula to the axillary artery.

Weaning Protocols

• V-V ECMO Weaning: Conducted via a sweep gas trial. While keeping blood flow constant, the sweep gas flow rate is turned to 0 L/min. If the patient maintains adequate oxygenation (PaO2 > 60 mmHg) and ventilation (pH > 7.30, PaCO2 < 50 mmHg) on acceptable ventilator settings (Vt 6 mL/kg PBW, PEEP <= 10, FiO2 <= 0.50) for 2 to 4 hours, decannulation is indicated.
• V-A ECMO Weaning: Gradually decrease blood flow to 1.5 to 2.0 L/min while monitoring cardiac contractility via echocardiography and ensuring the patient maintains hemodynamics.

ACCS Exam Traps

• Trap 1: Adjusting the ventilator to correct respiratory acidosis in an ECMO patient. Always adjust the sweep gas flow rate on the ECMO circuit, not the ventilator.
• Trap 2: Setting a low PEEP (e.g., 5 cm H2O) during 'lung rest' ventilation. A PEEP of 10-15 cm H2O is required to prevent complete alveolar collapse.