This is a single-center, prospective, observational study in adult intensive care unit (ICU) patients. The study will measure exhaled carbon monoxide (eCO) once daily for up to 7 days after ICU admission using a red blood cell lifespan breath analyzer, which simultaneously estimates erythrocyte lifespan. eCO is a gas produced endogenously during heme degradation and may reflect hemolysis, oxidative stress, and inflammation in critically ill patients.
The purpose of this study is to describe how eCO changes over time in ICU patients and to determine whether different eCO patterns are associated with clinical outcomes. The primary outcome is all-cause mortality within 28 days after enrollment. Secondary outcomes include 90-day mortality, acute kidney injury, organ dysfunction assessed by SOFA score, transfusion requirement, changes in anemia-related indices, ICU length of stay, and duration of invasive mechanical ventilation. Participation does not change routine clinical care; study procedures involve breath sampling and collection of clinical data from the medical record.