Early hypoxemia and hypercapnia within 24 hours of intensive care unit admission and 30-day mortality in adult ICU admissions with invasive mechanical ventilation during the ICU stay: a multicenter retrospective cohort study in Taiwan.

BACKGROUND: Early arterial blood gas abnormalities may aid risk stratification, but the joint prognostic value of early hypoxemia, hypercapnia, and acid-base status remains uncertain in adult ICU admissions requiring invasive mechanical ventilation during the ICU stay. OBJECTIVES: To evaluate associations of first-day oxygenation and carbon dioxide measures with 30-day mortality. METHODS: We conducted a multicenter retrospective cohort study using a Taiwan critical care registry from 2018 to 2020. Among 11,912 adult ICU admissions with invasive mechanical ventilation during the ICU stay, 4960 had complete first-day arterial blood gas data and interpretable 30-day vital status. Hypoxemia was defined as PaO2/FiO2 <200 mmHg and hypercapnia as PaCO2 >45 mmHg. Multivariable logistic regression adjusted for age, sex, comorbidities, treatment-limitation indicators, and nonrespiratory SOFA score. Additional analyses modeled PaO2/FiO2 and PaCO2 continuously and incorporated pH, severe hypercapnia, palliative care exclusion, and care-unit effects. RESULTS: Overall 30-day mortality was 1141/4960 (23.0%). Compared with the group without a predefined arterial blood gas abnormality, isolated hypoxemia (adjusted odds ratio [aOR], 1.63; 95% CI, 1.33-2.00) and combined hypoxemia-hypercapnia (aOR, 1.67; 95% CI, 1.21-2.32) were associated with higher mortality, whereas isolated hypercapnia was not (aOR, 0.86; 95% CI, 0.54-1.39). As a continuous variable, lower PaO2/FiO2 remained associated with mortality after nonrespiratory SOFA adjustment, whereas PaCO2 did not show a harmful adjusted association. CONCLUSIONS: Early hypoxemia carried the most consistent mortality signal. Isolated mild hypercapnia should be interpreted cautiously with pH, severity threshold, and illness-severity context.