OBJECTIVE : Oxidative stress conditions may be responsible for an up-regulation of the expression of heme oxygenase (HO), the enzyme synthesizing carbon monoxide (CO) in cells. Elevated levels of arterial carboxyhemoglogin (aCOHb) have been found in critically ill patients, including those suffering from acute lung injury. We aimed to investigate the changes of aCOHb levels in COVID-19 critically ill patients. DESIGN : Single center retrospective cohort study between March 1 and December 31, 2020. SETTING: University teaching hospital with 900 beds including 48 ICU beds. PATIENTS: Consecutive adults requiring ICU admission for severe COVID-19 infection. MEASUREMENTS AND MAIN RESULTS : After exclusion of 23 patients who required extracorporeal membranous oxygenation (ECMO), a total of 135 patients was considered, from whom 76 required mechanical ventilation (MV) that was prolonged for more than 20 days in 28 patients. Demographics, laboratory values, treatment and outcomes were recorded. The evolution of aCOHb over time was investigated among survivors and non-survivors and among patients with or without MV. The admission and peak aCOHb values did not differ among survivors or non-survivors. In the patients with MV, there was a significant increase of aCOHb with time, with a biphasic evolution for the patients ventilated for more than 20 days: a parallel increase of aCOHb and PaCO2 followed by a trend to a plateau of aCOHb during PaCO2 recovery. CONCLUSIONS : Progressive increase in aCOHb from endogenous source is observed in COVID-19 patients requiring prolonged mechanical ventilation. Decrease of aCOHb appears slower than PaCO2 recovery.
