End-tidal and arterial carbon dioxide gradient in serious traumatic brain injury after prehospital emergency anaesthesia: a retrospective observational study.

OBJECTIVES

In the UK, 20% of patients with severe traumatic brain injury (TBI) receive prehospital emergency anaesthesia (PHEA). Current guidance recommends an end-tidal carbon dioxide (ETCO) of 4.0-4.5 kPa (30.0-33.8 mm Hg) to achieve a low-normal arterial partial pressure of CO (PaCO), and reduce secondary brain injury. This recommendation assumes a 0.5 kPa (3.8 mm Hg) ETCO-PaCO gradient. However, the gradient in the acute phase of TBI is unknown. The primary aim was to report the ETCO-PaCO gradient of TBI patients at hospital arrival.

METHODS

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

RESULTS

107 patients were eligible for inclusion. Sixty-seven patients did not receive a PaCO sample within 30 min of hospital arrival and were therefore excluded. Forty patients had complete data and were included in the final analysis; per protocol. The mean ETCO-PaCO gradient was 1.7 (±1.0) kPa (12.8 mm Hg), with moderate correlation (R=0.23, p=0.002). The Bland-Altman bias was 1.7 (95% CI 1.4 to 2.0) kPa with upper and lower limits of agreement of 3.6 (95% CI 3.0 to 4.1) kPa and -0.2 (95% CI -0.8 to 0.3) kPa, respectively. There was no evidence of a larger gradient in more severe TBI (p=0.29). There was no significant gradient correlation in patients with a coexisting serious thoracic injury (R=0.13, p=0.10), and this cohort had a larger ETCO-PaCO gradient, 2.0 (±1.1) kPa (15.1 mm Hg), p=0.01. Patients who underwent prehospital arterial blood sampling had an arrival PaCO of 4.7 (±0.2) kPa (35.1 mm Hg).

CONCLUSION

There is only moderate correlation of ETCO and PaCO at hospital arrival in patients with serious TBI. The mean ETCO-PaCO gradient was 1.7 (±1.0) kPa (12.8 mm Hg). Lower ETCO targets than previously recommended may be safe and appropriate, and there may be a role for prehospital PaCO measurement.