icuECMO: Troubleshooting: Hypoxemia: VV-ECMO

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Troubleshooting: Hypoxemia: V-V ECMO

Hypoxemia in a patient on V-V ECMO arises for a few main reasons (these can all occur simultaneously):

Not enough blood is getting to the oxygenator, so a significant amount is bypassing the circuit and going straight to lungs that aren't contributing to gas exchange (circuit shunt).
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.
Blood sent back to the body is being sucked back into the drainage cannula before it is circulated through the patient (recirculation).

Based on these issues, hypoxemia may be treated in the following ways:

This can be addressed by increasing the flow through the pump.
For example, if cardiac output is 5L/min, but the ECMO flow is only 2L/min, then 3L/min of cardiac output is not being oxygenated by the circuit.
Turning up the output to 4L/min will increase the amount of blood oxygenated by the ECMO circuit.
Usually if the ECMO circuit flow is ~60% of cardiac output then circuit shunt should not cause hypoxemia.
In some cases the flow is already very high, but the patient's native cardiac output is so high that a significant amount of blood is still being shunted.

In this case it may be prudent to lower the cardiac output with sedation, analgesia, or even beta blockade.

It is important to make sure ventilator settings are adjusted to optimize oxygenation of blood in the lungs as well, since there will always be an element of shunted blood that rely on the lungs for oxygenation.

This is very easily addressed by increasing the FiO2 of the sweep gas.
It is important to know that the sweep (the rate that sweep gas is delivered to the circuit) does not affect the circuit's ability to oxygenate (as long as it is not zero).
Hypoxemia is very rarely caused by low FiO2 in isolation because the membrane oxygenator is very efficient.
An increase in FiO2 requirements should prompt investigation for other causes of hypoxemia.

If a malfunctioning oxygenator is suspected, then 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.

This phenomenon occurs when the return cannula is sending richly oxygenated blood back to the patient, but the drainage cannula is sucking it all in.
This can be determined by measuring a central venous blood sample away from the ECMO cannulas (e.g. the left internal jugular vein) and comparing it to the pre-membrane blood sample.
If there is a significant difference in the PO2 of these quantities, then recirculation should be suspected.
This is typically a result of very high flows on the ECMO circuit.
Usually this is remedied by decreasing the flow.
Sometimes repositioning of the cannulas is necessary.