Troubleshooting: Hypoxemia: V-A ECMO

Remember in V-A ECMO:

  • The circuit is in parallel with the patient's native cardiopulmonary circulation.
  • A certain proportion of the patient's total blood flow needs to be oxygenated by the lungs.
  • Mixing of blood from the native circulation and from the ECMO circuit occurs in the aorta.

The most important organ to oxygenate is the brain. In patients who are on V-A ECMO, the best way to estimate brain oxygenation is by:

  • Placing the pulse oximeter on the right hand or on the forehead.
  • Drawing arterial blood gases from a right radial or brachial arterial line.
  • Blood in the left arm and lower extremities has a differential and usually higher oxygen content because it is mainly composed of well-oxygenated blood from the ECMO circuit.

In V-A ECMO, hypoxemia is caused by:

  • Inadequate oxygenation of blood passing through the patient's lungs; this is by far the most common cause of hypoxemia in V-A ECMO.
  • The fraction of inspired oxygen in the sweep gas isn't high enough to oxygenate the blood in the pump.
  • The oxygenator is malfunctioning and not delivering oxygen to the blood in the circuit.

When someone on V-A ECMO is hypoxemic, a few steps can be taken initially:

  • Adjust the mechanical ventilation settings to optimize gas exchange in the lungs (e.g. adjust the PEEP, driving pressures, and FiO2 if necessary).
  • Increase the flow through the circuit so that more of the total blood flow passes through the oxygenator.
  • If flows are high and the patient is still hypoxemic, increasing the FiO2 of the sweep gas may help.
  • Get a chest-x ray to see if there is any pulmonary edema or other intrapulmonary explanation.
  • If signs and symptoms suggest fluid overload, a trial of diuresis and/or afterload reduction may help.

If a malfunctioning oxygenator is suspected then the steps taken are similar to V-V ECMO:

  • Pre- and post-membrane blood gases should be taken to measure PO2.
  • The pre-membrane gas is analogous to a mixed venous oxygen saturation, and the PO2 is typically 40-60 mmHg.
  • The PaO2 of post-membrane gas should be very high, in the range of 350-500 mmHg, anything lower should raise suspicion that the oxygenator is malfunctioning and needs to be replaced.

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