Prospective Observational Study of Exhaled Carbon MonoxideTrajectories and Outcomes in ICU Patients

Prospective Observational Study of Exhaled Carbon Monoxide Trajectories and Outcomes in ICU Patients

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.

Study Overview

• Status: Not yet recruiting
• Conditions: Critical Illness； Intensive Care Unit Patients

Detailed Description

Background: Endogenous carbon monoxide is generated in the human body and may increase during critical illness due to hemolysis, oxidative stress, and inflammation. Whether longitudinal patterns of exhaled carbon monoxide (eCO) provide prognostic information in critically ill patients remains unclear.

Design and setting: This is a single-center, prospective, observational cohort study conducted in an adult intensive care unit (ICU). The study is designed to characterize longitudinal eCO patterns and evaluate their clinical significance in critically ill patients without altering routine clinical management.

Study procedures and data collection: eCO will be measured once daily from Day 0 (baseline, within 24 hours of ICU admission/enrollment) through Day 7 when feasible, using a standardized breath analysis device. For patients receiving mechanical ventilation, breath sampling will be performed according to a standardized protocol for ventilated patients. In addition to serial eCO measurements, demographic characteristics, admission diagnosis, comorbidities, severity-of-illness scores, key treatments, and laboratory variables relevant to hemolysis, inflammation, organ function, anemia, and iron metabolism will be collected from the electronic medical record and protocol-defined assessments, as applicable.

Analytic approach: Group-based trajectory modeling (GBTM) will be used to identify distinct eCO trajectory groups from repeated measurements. Associations between eCO trajectories and prespecified clinical outcomes will be evaluated using Kaplan-Meier methods and multivariable Cox proportional hazards models with adjustment for clinically relevant covariates. Additional exploratory analyses will assess relationships between eCO trajectories and laboratory biomarker patterns, including anemia-related, iron metabolism-related, and inflammatory indices. Machine learning methods (e.g., XGBoost with SHAP-based interpretation) may also be used in exploratory analyses to develop risk prediction models incorporating eCO and other clinical variables.

Study conduct: No intervention is assigned as part of the study, and all treatment decisions remain at the discretion of the treating clinicians.

• Study Type: Observational
• Enrollment (Estimated): 220

Contacts and Locations

• Study Contact: Name: Fei Leng, MD.; Phone Number: 021-64041990; Email: leng.fei@zs-hospital.sh.cn
• Study Contact Backup: Name: Hongyu He; Phone Number: 021-64041990; Email: he.hongyu@zs-hospital.sh.cn

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult; Older Adult
• Accepts Healthy Volunteers: No

• Sampling Method: Non-Probability Sample

Study Population

Adult intensive care unit (ICU) patients (≥18 years) admitted to the study site will be screened consecutively. Eligible participants are those expected to remain in the ICU for at least 48 hours and able to complete daily exhaled carbon monoxide (eCO) measurements (spontaneously breathing or mechanically ventilated). Written informed consent will be obtained from the patient or a legally authorized representative. Patients meeting any exclusion criteria (e.g., carbon monoxide poisoning, active smoking within 24 hours, inability to perform breath sampling, pregnancy/breastfeeding, hereditary hemoglobinopathies) will not be enrolled. Participants may be withdrawn if informed consent is revoked, if continued data collection is judged clinically inappropriate, if the patient is transferred and lost to follow-up, or if eCO data are missing for more than 50% of the 7-day collection period (fewer than 4 valid measurements).

Description

Inclusion Criteria:

1. Age ≥ 18 years
2. Admitted to the intensive care unit (ICU) at the study site
3. Expected ICU length of stay ≥ 48 hours
4. Able to complete exhaled carbon monoxide (eCO) measurement (spontaneously breathing or mechanically ventilated)
5. Written informed consent provided by the patient or a legally authorized representative

Exclusion Criteria:

1. Carbon monoxide poisoning
2. Active smoking within 24 hours prior to ICU admission
3. Unable to perform breath sampling (e.g., massive airway bleeding, severe airway obstruction, extracorporeal membrane oxygenation)
4. Ineffective sampling due to poor mask seal or special ventilation modes
5. Pregnant or breastfeeding
6. Known hereditary hemoglobinopathies (e.g., sickle cell disease, thalassemia)
7. Patient or family refusal
8. Other conditions deemed unsuitable by investigators

Study Plan

How is the study designed?

Number of groups / cohorts

1

Cohorts and Interventions

Group / Cohort
Adult ICU Patients; Consecutive adult intensive care unit (ICU) patients enrolled in a single-center, prospective observational cohort. Exhaled carbon monoxide (eCO) is measured once daily for up to 7 days after ICU admission. Clinical data and outcomes are collected from the medical record and follow-up. No treatment is assigned as part of the study.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Time Frame
28-day all-cause mortality	From enrollment (Day 0) up to Day 28

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
90-day all-cause mortality		From enrollment up to Day 90
Acute kidney injury (AKI) (KDIGO)	Incidence and staging of acute kidney injury defined by KDIGO 2012 criteria (serum creatinine increase ≥0.3 mg/dL within 48 hours, or ≥1.5 times baseline within 7 days, or urine output <0.5 mL/kg/h for ≥6 hours) during ICU hospitalization.	From enrollment through ICU discharge (up to 28 days)
Organ dysfunction assessed by SOFA score	Daily Sequential Organ Failure Assessment (SOFA) score, a composite organ dysfunction scale ranging from 0 to 24, encompassing respiratory, coagulation, liver, cardiovascular, neurological, and renal subsystems. Each of the 6 organ system components is scored from 0 to 4, with higher scores indicating worse organ dysfunction and greater illness severity. Scores will be assessed daily from Day 0 (baseline) through Day 7.	Daily from Day 0 (baseline) through Day 7
Red blood cell transfusion requirement	Whether red blood cell transfusion was administered (yes/no), number of transfusion episodes, and total units transfused within 28 days after ICU admission.	Up to 28 days after ICU admission/enrollment
ICU length of stay	Total duration of invasive mechanical ventilation during the ICU stay.	From ICU admission to ICU discharge (up to 90 days)
Duration of invasive mechanical ventilation	Total duration of invasive mechanical ventilation during the ICU stay.	From enrollment to liberation from invasive mechanical ventilation or ICU discharge (up to 28 days)
Complete blood count-related panel	Longitudinal changes in complete blood count-related indices, including hemoglobin, reticulocyte count, and red cell distribution width (RDW), and their associations with eCO trajectories.	Up to 28 days after enrollment
Changes in iron metabolism related indices	Longitudinal changes in iron metabolism- and erythropoietin-related indices, including serum iron, ferritin, transferrin saturation, soluble transferrin receptor, and erythropoietin (EPO), and their associations with eCO trajectories.	Baseline and up to 28 days after enrollment

Other Outcome Measures

Outcome Measure	Measure Description	Time Frame
Exploratory mortality risk prediction model	Machine learning-based mortality risk prediction model (XGBoost) incorporating eCO trajectories and clinical variables, with variable importance assessed by SHAP values. This is an exploratory analysis evaluated by 5-fold cross-validation.	From enrollment up to Day 28

Collaborators and Investigators

• Sponsor: Shanghai Zhongshan Hospital
• Investigators: Principal Investigator: Ming Zhong, Fudan University

Study record dates

Study Major Dates

• Study Start (Estimated): March 15, 2026
• Primary Completion (Estimated): August 30, 2027
• Study Completion (Estimated): November 30, 2027

Study Registration Dates

• First Submitted: February 10, 2026
• First Submitted That Met QC Criteria: March 5, 2026
• First Posted (Actual): March 6, 2026

Study Record Updates

• Last Update Posted (Actual): March 6, 2026
• Last Update Submitted That Met QC Criteria: March 5, 2026
• Last Verified: March 1, 2026

More Information

Terms related to this study

• Keywords: Inflammation; Prognosis; ICU outcomes; Exhaled carbon monoxide；; Group-based trajectory modeling
• Additional Relevant MeSH Terms: Pathologic Processes; Pathological Conditions, Signs and Symptoms; Inflammation
• Other Study ID Numbers: B2026-018

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO
• IPD Plan Description: Individual participant data will not be shared because this is a single-center observational study with a limited sample size. De-identified summary-level data may be made available upon reasonable request to the corresponding investigator.

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No