Manipulation of extracorporial gas exchange

ECMO is briefly covered in a short summary elsewhere on this site. One of these days, a more detailed exploration of this fascinating modality will become available. In the meantime, an excellent troubleshooting guide is offered by LITFL, and brilliant videos accompany a practical guide based at the venerable merylandccproject.org This page does not touch on the savage madness of troubleshooting the ECMO circuit, because that is another vastly different topic area.

Oxygenation via VV ECMO

• Oxygenation of the VV ECMO patient is determined by
  • ECMO flow rate
  • ECMO output blood oxygen content
  • Patient's cardiac output
• Total blood oxygen content = (patient's cardiac output ) + (ECMO output )
• Even though the blood leaving the oxygenator might have a PaO₂ of around 500mmHg, the patient may still be hypoxic if the ECMO flow rate is low.
• Thus: increasing the VV ECMO flow rate increases the PaO₂, right up until it begins to recirculate. In absence of recirculation and with maximum flow, adjusting the FiO₂is the only option (i.e. increasing the fraction of oxygen in the "sweep" gas)

Recirculation

• The return (atrial) line should NOT be close to the venous access line.
• If it is too close, or the flow rate is too high, the jet of oxygenated returning blood will get sucked up into the venous circuit, and the systemic oxygenation will actually drop as a consequence.
• One needs to carefully down-adjust the blood flow rate to achieve maximal PaO₂. If the PaO₂ is inadequate in spite of blood flow manipulation, only then does one attempt to manipulate the cannula position.
• The minimum flow rate is about 1.0-1.5L/min; anything less results in circuit thrombosis.

Oxygenation via VA ECMO

• Oxygenation of the VA ECMO patient -if their entire cardiac output is being supplemented by ECMO -is determined entirely by the gas exchange properties of the circuit.
• The expected systemic PaO₂ with 100% FiO₂ is therefore around 400-500mmHg, if the oxygenator is working properly.

CO₂ removal

CO₂ is removed by "sweep". This is the countercurrent flow of oxygen-rich gas which circulates over the porous oxygenator membrane. It is measured in L/min, and adjusted by a gas flow regulator.