End-tidal carbon dioxide and arterial to end-tidal carbon dioxide gradient are associated with mortality in patients with neurological injuries

Pre-hospital end-tidal carbon dioxide (E_tCO₂) monitoring and arterial to end-tidal carbon dioxide gradient (P_a-EtCO₂) have been associated with mortality in patients with traumatic brain injury. Our study aimed to analyze the association between alveolar E_tCO₂ or P_a-EtCO₂ and mortality in patients admitted in intensive care unit (ICU) with neurological injuries. In our retrospective analysis from using large de-identified ICU databases (MIMIC-III and -IV and eICU databases), we included 2872 ICU patients with neurological injuries, identified according to the International Classification of Diseases (ICD-9 and -10), who underwent E_tCO₂ monitoring. We performed logistic regression and extended Cox regression to assess the association between mortality and candidate covariates, including E_tCO₂ and P_a-EtCO₂. In-hospital mortality was 26% (n = 747). In univariate analysis, both the P_a-EtCO₂ gradient and E_tCO₂ levels during the first 24 h were significantly associated with mortality (for a 1 mmHg increase: OR = 1.03 [CI₉₅ 1.016-1.035] and OR = 0.94 [CI₉₅ 0.923-0.953]; p < 0.001). The association remained significant in multivariate analysis. The time-varying evolution of EtCO₂ was independently associated with mortality (for a 1 mmHg increase: HR = 0.976 [CI₉₅ 0.966-0.985]; p < 0.001). The time-varying P_a-EtCO₂ gradient was associated with mortality only in univariate analysis. In neurocritical patients, lower E_tCO₂ levels at admission and throughout the ICU stay were independently associated with mortality and should be avoided.