Association of Carboxyhemoglobin With Severity and Outcomes in Hypothermic Patients: A Retrospective Cohort Study

Introduction
Carboxyhemoglobin (COHb), an endogenous marker of carbon monoxide production mediated by heme oxygenase-1, may reflect physiological stress responses in critically ill patients. However, its clinical relevance in accidental hypothermia remains unclear.

Methods
We conducted a single-center retrospective cohort study of adult patients admitted to the emergency ICU with accidental hypothermia between January 1, 2019, and March 31, 2025. Patients were categorized into low- and high-COHb groups based on median COHb levels upon emergency department arrival. Associations between COHb levels, disease severity (Acute Physiology and Chronic Health Evaluation II (APACHE II) and Sequential Organ Failure Assessment (SOFA) scores), and 28-day mortality were analyzed using regression models adjusted for clinical confounders.

Results
Among the 88 patients, who had a median admission temperature of 28.7°C, 45 were classified into the low-COHb group and 43 into the high-COHb group, based on a median COHb level of 0.3%. Lower COHb levels on admission were significantly associated with higher APACHE II scores (β = −4.20; 95% CI, −7.56 to −0.85), but not with SOFA scores. Admission and minimum COHb levels were not associated with 28-day mortality. However, higher maximum COHb levels within the first 24 hours were independently associated with lower 28-day mortality (adjusted OR, 0.17; 95% CI, 0.023 to 0.93).

Conclusions
Lower COHb levels were associated with greater disease severity, and higher maximum COHb levels were associated with lower 28-day mortality. COHb may reflect systemic stress in accidental hypothermia, but its prognostic value appears limited.