Extracorporeal respiratory support

These techniques have declined in popularity over recent years after several trials failed to demonstrate clear outcome benefit in adults with very severe respiratory failure. Survival rates of 50–60% are reported, but clear superiority over conventional ventilation has not yet been demonstrated in controlled studies. A large, prospective, randomised study (the ‘CESAR’ trial) has recently been completed, but failed to show benefit from extracorporeal support.

Extracorporeal CO2 removal (ECCO2R)

An extracorporeal veno-venous circulation allows CO2 clearance via a gas exchange membrane. Blood flows of 25–33% cardiac output are typically used, allowing only partial oxygenation support. Low frequency (4–5/min) positive pressure ventilation is usually used with ECCO2R, with continuous oxygenation throughout inspiration and expiration. The lungs are ‘held open’ with high PEEP (20–25cmH2O), limited peak airway pressures (35–40cmH2O), and a continuous fresh gas supply. Thus, oxygenation is provided

with lung rest to aid recovery. Anticoagulation of the extracorporeal circuit can be reduced by using heparin-bonded tubing and membranes.

Extracorporeal membrane oxygenation (ECMO)

An extracorporeal veno-arterial circulation with high blood flows (approaching cardiac output) through a gas exchange membrane enables most if not all of the body’s gas exchange requirements to be met. Main disadvantages compared to ECCO2R are the need for large bore arterial puncture with its consequent risks and high extracorporeal blood flows with the potential for cell damage.

Indications

Failure of maximum intensive therapy and ventilatory support to sustain adequate gas exchange as evidenced by the criteria opposite.

Contraindications

• Chronic systemic disease involving any major organ system (e.g. irreversible chronic CNS disease, chronic lung disease with FEV1 <1L, FEV1/FVC <0.3 of predicted, chronic PaCO2 >6.0kPa, emphysema or previous admission for chronic respiratory insufficiency, incurable or rapidly fatal malignancy, chronic failure of heart, kidney or liver, HIVrelated disease).

• Lung failure for >7d (although treatment with extracorporeal respiratory support may persist for longer than 14d).

• Burns (>40% of body surface).

• More than three organ failures in addition to lung failure.

Criteria for ECCO2R/ECMO

• Rapid failure of ventilatory support: immediate use of these techniques should be considered in those meeting the following criteria for a period >2h despite maximum intensive care:

• PaO2 <6.7kPa.

• FIO2 1.0.

• PEEP >5–10cmH2O.

Slow failure of ventilatory support: consider use after 48h maximum intensive care for those meeting the following gas exchange and mechanical pulmonary function criteria for a period >12h:

• PaO2 <6.7kPa.

• PEEP >5–10cmH2O.

• Qs/Qt >30% on FIO2 of 1.0.

• PaO2/FIO2 <11.2kPa.

• TSLC <30mL/cmH2O at 10mL/kg inflation.