Case of the Month #11

Louise Hartley

A 40 year old male was admitted to intensive care unit requiring intubation and ventilation for community acquired pneumonia. He developed acute respiratory distress syndrome (ARDS) and on day 2 commenced an atracurium infusion for worsening hypoxaemia.

Despite two sessions of prone ventilation he failed to improve and required high ventilatory pressures. On day 5 he was placed on venovenous extracorporeal membrane oxygenation. (VV ECMO).

Bifemoral cannulae (25F access cannula and 23F return cannula) were inserted. Standard heparin anticoagulation was commenced. After an initial period of stability he became hypoxaemic (arterial oxygen saturations 82%) despite 5.2L extracorporeal blood flow and 6L sweep gas flow.

What are the indications for VV ECMO?

How does VV ECMO work?

How do you troubleshoot hypoxaemia?

Indications for VV ECMO

In patients with life threatening hypoaexmia or hypercapnia despite optimal mechanical ventilation extracorporeal support should be considered and discussed early. In the UK the provision of VV ECMO is delivered through a national severe respiratory failure network.

The general indications, as per the CESAR trial, are as follows:

  1. Lung Injury Score>3 (includes PF ratio, consolidation on CXR, PEEP and compliance)
  2. Uncompensated hypercapnia with pH<7.20
  3. Reversible pathology
  4. No underlying life limiting co-morbidity leading to dependency on ECMO

Patients must have failed optimal conventional management, including trials of prone ventilation, when advised by the accepting centre.

During the coronavirus pandemic, in the absence of previous disease specific data, referral criteria were more explicit to standardise referral and triage. The criteria were revised to include clinical frailty score and RESP score (a VV ECMO survival prediction score).

Principles of VV ECMO

VV ECMO provides oxygenation and decarboxylation at a point distinct from the lungs; facilitating lung rest allowing a bridge to recovery. The circuit consists of four key components: a venous cannula for blood drainage, blood pump, membrane oxygenator and venous cannula for return of blood.

Components

The membrane oxygenator is a gas exchange interface analogous to the native lung whereby oxygen is provided and carbon dioxide is removed through a capillary network of tubules. Diffusion occurs down a concentration gradient with blood and gas (known as sweep gas flow) in counter-current flow. A heat exchanger can be coupled for systemic temperature control.

Control of oxygenation

Blood entering the oxygenator will be fully saturated with oxygen. The contribution this makes to systemic oxygenation is dependent on extracorporeal blood flow relative to the patient’s cardiac output. The fully saturated blood from the oxygenator mixes with deoxygenated venous return (which has not passed through the oxygenator) to return to the right atrium. The closer the extracorporeal blood flow to cardiac output (and therefore venous return), the greater the proportion of blood entering the pulmonary artery which has been oxygenated, and

the higher the oxygen content of the mixed blood.

The native lungs initially provide minimal contribution to gas exchange, thus blood entering the pulmonary artery will have same oxygen content as the left sided circulation.

Oxygenation is primarily controlled by manipulating extracorporeal blood flow but is affected by recirculation haemoglobin concentration and membrane efficiency. Recirculation is where reinfused oxygenated blood is withdrawn through the drainage cannula without passing through the systemic circulation.

Control of decarboxylation

Carbon dioxide removal is directly related to sweep gas flow rate: as the flow rate is increased more CO2 removal will occur. Contribution from the native lungs to removal of CO2 will be additive.

The control of CO2 allows not only maintenance of safe acid base status but also reduction in tidal volume and alveolar pressure avoiding the injurious effects of mechanical ventilation.

Hypoxaemia in VV ECMO

Troubleshooting the hypoxaemic patient involves reviewing the patient, the circuit and their interaction.

Circuit

  1. Maximise circuit blood flow: increasing extracorporeal blood flow will improve oxygenation (relative to cannula size) but may increase recirculation. High blood flow causes excess negative pressure in the vessel and increases risk of haemolysis. An additional drainage cannula can be considered if flow inadequate.
  2. Reduction or loss of circuit flow: ensure intravascular volume is maintained and exclude any kink in circuit or obstruction from high intra-abdominal or intra-thoracic pressure (ie pneumothorax). Cannula position may need to be manipulated to increase flow.
  3. Minimise recirculation: identified by minimal colour change between access and return lines and high pre-oxygenator saturations. Cannula manipulation may be required.
  4. Failing oxygenator: rising transmembrane pressure or reduced gas exchange (falling post oxygenator saturations) can indicate thrombus within the circuit. A circuit change may be required.
  5. Sweep gas flow: ensure oxygen supply is connected and set at FiO2 1.0

Patient

  1. Reducing oxygen consumption: temperature control to 36°C using heat exchanger, increasing sedation and use of neuromuscular blockade.
  2. Reducing cardiac output: beta blockade (ie esmolol) can  reduce the proportion of venous return that does not enter the membrane lung and subsequently improve arterial oxygenation. High cardiac output is often seen in septic patients.
  3. Increasing oxygen delivery: blood transfusion to increase haemoglobin content.
  4. New or worsening lung problem: pulmonary haemorrhage, pneumothorax and progressive consolidation can all occur. Intervention may be required but is associated with a significant risk of bleeding therefore a conservative approach is taken.
  5. Optimising mechanical ventilation: ventilator settings or FiO2 may need to be increased to improve arterial oxygenation.

Key take home messages

  • Discuss potential patients for ECMO early with a severe respiratory failure centre
  • Venovenous extracorporeal membrane oxygenation provides oxygenation and decarboxylation facilitating lung rest.
  • A systematic approach is required in troubleshooting both the circuit and patient to identify the cause of hypoxaemia in VV ECMO.
  • Get help early especially if cannula manipulation required or circuit dysfunction.

Further Reading