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.