One of the daily checks of the ECMO circuit that should be done is referred to as the “flashlight test”. For this you take a flashlight and inspect both the upstream and downstream surface of the oxygenator. What you are looking for is the accumulation of clot within the oxygenator.


When clot accumulates on the oxygenator, the pressure gradient across the membrane increases and the pump needs to generate higher pressures to maintain the same blood flows through the circuit. Clot in the circuit can come from patient hypercoagulability, inadequate anticoagulation, areas of stasis, or slower blood flows in the circuit.

The clots can usually be identified by black or dark spots on the surface of the oxygen meter membranes. The clots can can be found on both sides of the membrane, but tend to accumulate more on the upstream surface because this is serves as a catch for the entire venous ECMO circuit. On the downstream side, the clot can break from the oxygenator and travel to the patient. The clots tend to form in the corners of the oxygenator, which are areas of slower flow. If you look closely you can often see blood flow being diverted around the clots.

The steps for this check are:

  1. A darkened or shaded room helps.
  2. Used a standard powered flashlight. I have a very powerful penlight, and it actually provides too much light.
  3. Hold the flashlight at an oblique angle to plastic surface of the oxygenator. This helps eliminate glare and allows the clots and blood flow to cast shadows.
  4. Inspect the entire surface of the membrane paying particular attention to the corner.
  5. Check both sides of the oxygenator.
  6. Note the location and approximate size of the clot on the ECMO flowsheet.

The video below shows an oxygenator with small areas of clot accumulation.

As more and more clot accumulates the pressure gradient across action