Agreement between arterial and end-tidal carbon dioxide in adult patients admitted with serious traumatic brain injury.

Background: Low-normal levels of arterial carbon dioxide (PaCO₂) are recommended in the acute phase of traumatic brain injury (TBI) to optimize oxygen and CO₂ tension, and to maintain cerebral perfusion. End-tidal CO₂ (ETCO₂) may be used as a surrogate for PaCO₂ when arterial sampling is less readily available. ETCO₂ may not be an adequate proxy to guide ventilation and the effects on concomitant injury, time, and the impact of ventilatory strategies on the PaCO₂-ETCO₂ gradient are not well understood. The primary objective of this study was to describe the correlation and agreement between PaCO₂ and ETCO₂ in intubated adult trauma patients with TBI. Methods: This study was a retrospective analysis of prospectively-collected data of intubated adult major trauma patients with serious TBI, admitted to the East of England regional major trauma centre; 2015–2019. Linear regression and Welch's test were performed on each cohort to assess correlation between paired PaCO₂ and ETCO₂ at 24-hour epochs for 120 hours after admission. Bland-Altman plots were constructed at 24-hour epochs to assess the PaCO₂-ETCO₂ agreement. Results: 695 patients were included, with 3812 paired PaCO₂ and ETCO₂ data points. The median PaCO₂-ETCO₂ gradient on admission was 0.8 [0.4–1.4] kPa, Bland Altman Bias of 0.96, upper (+2.93) and lower (-1.00), and correlation R² 0.149. The gradient was significantly greater in patients with TBI plus concomitant injury, compared to those with isolated TBI (0.9 [0.4–1.5] kPa vs. 0.7 [0.3–1.1] kPa, p<0.05). Across all groups the gradient reduced over time. Patients who died within 30 days had a larger gradient on admission compared to those who survived; 1.2 [0.7–1.9] kPa and 0.7 [0.3–1.2] kPa, p<0.005. Conclusions: Amongst adult patients with TBI, the PaCO₂-ETCO₂ gradient was greater than previously reported values, particularly early in the patient journey, and when associated with concomitant chest injury. An increased PaCO₂-ETCO₂ gradient on admission was associated with increased mortality. [ABSTRACT FROM AUTHOR]