Carboxyhemoglobin Elevation, a Marker for Hemolysis, Is Common in Veno-venous Extracorporeal Membrane Oxygenation.

Abstract

To the Editor: Giani et al.1 found low levels of carboxyhemoglobin (COHb) in a cohort of patients supported on veno-venous extracorporeal membrane oxygenation (VV ECMO) at their center. It is noteworthy that even on ECMO support, COHb values were below average levels found in general intensive care unit population (1.3–1.9%) without extracorporeal support.2 Moreover, in observational studies of VV ECMO, severe hemolysis occurs between 10% and 23%.3,4 Thus, as a surrogate for hemolysis, one would expect COHb levels to be higher during VV ECMO support. Hence, we speculate if their experience is generalizable. We are in agreement with Giani et al. that excessive negative drainage pressures from high-blood flow rates can lead to elevated COHb. At our institution, an inline compliant bladder on the venous drainage cannula allows noninvasive monitoring and volume displacement to reduce cavitation.5 Additionally, we favor fewer circuit connections to reduce clotting at junctions. However, we would like to add a few additional points that may explain differences between our cohorts. First, 25% of our VV ECMO cohort were supported by single site double lumen cannula. The mobile membrane and smaller effective luminal radius may increase hemolysis at comparable flow rates. In our cohort, this group had the most frequent and highest levels of COHb. Second, our baseline rate of COHb before initiation of ECMO was nearly fourfold higher. Likely, there are other unaccounted factors beyond extracorporeal support that are contributing to elevated COHb in our patients. Finally, hypovolemia, vasodilation, and cannula position also contribute to excessive negative pressures.6 In our retrospective cohort, we were unable to capture the details of these potential causes. Thus, we believe that COHb elevation is a prevalent phenomenon in patients supported on VV ECMO. Causes include but are not limited to excessive negative pressure and high-blood flow.