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 pre-hospital emergency anesthesia (PHEA). Current guidance recommends an end-tidal carbon dioxide (ETCO2) of 4.0-4.5kPa (30.0-33.8mmHg) to achieve a low-normal arterial partial pressure of CO2 (PaCO2), and reduce secondary brain injury. This recommendation assumes a 0.5kPa (3.8mmHg) ETCO2-PaCO2 gradient. However, the gradient in the acute phase of TBI is unknown. The primary aim was to report the ETCO2-PaCO2 gradient of TBI patients at hospital arrival. The secondary aims evaluated the relationship between the gradient and i) the severity of TBI, and ii) the presence of co-existing thoracic injury. Methods A consecutive series of adult patients with serious TBI that received a PHEA over a 32-month period were included (1 April 2015 to 31st December 2017). Patients without a PaCO2 sample within 30 minutes of hospital arrival were excluded. ETCO2 and PaCO2 data were compared with a Mann-Whitney U Test. Fisher’s exact test was used to compare proportions. Linear regression was performed to test for correlation and reported as R-squared (R2) with gradient of the slope (m). Results Forty patients had complete data. The median gradient was 1.6 [0.9-2.2] kPa (12.0mmHg), with moderate correlation (R2=0.23, p=0.002). There was no evidence of a larger gradient in more severe TBI (p=0.46). 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 gradient, 2.1 [1.2-5.6] kPa (15.8mmHg), p=0.01. Patients who underwent prehospital arterial blood sampling had an arrival PaCO2 of 4.6 [4.6-4.8] kPa (34.5mmHg). Conclusion There is only moderate correlation of ETCO2 and PaCO2 at hospital arrival in patients with serious TBI. The median ETCO2-PaCO2 gradient was 1.6 [0.9-2.2] kPa (12.0mmHg), greater than previously reported. Lower ETCO2 targets than previously recommended may be safe and appropriate, particularly in the presence of thoracic injury. There may be a role for pre-hospital PaCO2 measurement