Aims/Objectives/Background In the United Kingdom (UK), 20% of patients with severe traumatic brain injury (TBI) receive pre-hospital emergency anaesthesia (PHEA). Current guidance recommends an end-tidal carbon dioxide (ETCO₂) of 4.0–4.5kPa to achieve a low-normal arterial partial pressure of CO₂ (PaCO₂), and reduce secondary brain injury. This recommendation assumes a 0.5kPa ETCO₂-PaCO₂ gradient. However, the gradient in the acute phase of TBI is unknown. Our primary aim was to report the ETCO₂-PaCO₂ gradient of TBI patients at hospital arrival.

Methods/Design A retrospective cohort study of adult patients with serious TBI, who received a PHEA by a pre-hospital critical care team in the East of England between 1st April 2015 to 31st December 2017. Linear regression was performed to test for correlation and reported as R-squared (R²). A Bland-Altman plot was used to test for paired ETCO₂ and PaCO₂ agreement and reported with 95% confidence intervals (95%CI). ETCO₂-PaCO₂ gradient data were compared with a two-tailed, unpaired, t-test.

Results/Conclusions 107 patients were eligible for inclusion. Sixty-seven patients did not receive a PaCO₂ sample within 30 minutes of hospital arrival and were therefore excluded. Forty patients had complete data and were included in the final analysis; per protocol.

The mean ETCO₂-PaCO₂ gradient was 1.7 (±1.0) kPa, with only moderate correlation of ETCO₂ and PaCO₂ at hospital arrival (R²=0.23, p=0.002). The Bland-Altman bias was 1.7 (95%CI 1.4–2.0) kPa with upper and lower limits of agreement of 3.6 (95%CI 3.0–4.1) kPa and -0.2 (95%CI -0.8–0.3) kPa respectively. There was no significant gradient correlation in patients with a co-existing serious thoracic injury (R²=0.13, p=0.10), and this cohort had a larger ETCO₂-PaCO₂ gradient, 2.0 (±1.1) kPa, p=0.01. Patients who underwent pre-hospital arterial blood sampling had an arrival PaCO₂ of 4.7 (±0.2) kPa.

Lower ETCO₂ targets than previously recommended may be safe and appropriate. The use of pre-hospital PaCO₂ measurement is advocated.