27, 37, 29
Thirty-three consecutive ECMO patients at one ICU, October 2009 to February 2015, with 110 paired measurements of plasma free haemoglobin and carboxyhaemoglobin. Haemolysis defined as plasma free haemoglobin above 50 mg/dL per the ELSO reference.
- All carboxyhaemoglobin values were above 3% when haemolysis was present. All but two were below 3% when it was absent; both exceptions (4.4% and 7.1%) came from a single patient.
- A 3% cut-off discriminated haemolysis, Fisher's exact test p = 0.006.
- In the two patients with clinical haemolysis, carboxyhaemoglobin rose proportionally with plasma free haemoglobin, peaking at 9.1%.
- Background: carboxyhaemoglobin is typically 0.9–1.9% in critically ill patients; ELSO-reported haemolysis rates are 5.8% on venovenous and 5.6% on venoarterial support.
Mechanism: free haemoglobin is degraded by haem oxygenase to free iron, biliverdin and carbon monoxide — a stoichiometric product of haem catabolism. Haem oxygenase-1 is inducible by haem, oxidative stress and inhaled nitric oxide, so the process is self-perpetuating. Carboxyhaemoglobin then impairs oxygen delivery twice over (fewer binding sites, leftward curve shift) and corrupts pulse oximetry.
Certainty: very low. Thirty-three patients, single centre, retrospective, only two with haemolysis, no multivariable adjustment; the authors call it hypothesis-generating.
NUMERICAL PROBLEM FLAGGED AND NOT CORRECTED: the retrieved text mixes units for plasma free haemoglobin — a threshold given as 500 mg/L equal to 50 mg/dL, alongside individual patient values of '1600 mg/dL' and 'greater than 2000 mg/dL', which would be 16 and 20 g/dL. Almost certainly mg/L mislabelled; both forms appear in the source and neither is asserted. The conclusion used does not depend on the disputed magnitudes.
Read as indexed full-text passages. DOI not independently resolved.