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End-tidal and arterial carbon dioxide gradient in serious traumatic brain injury after prehospital emergency anaesthesia: a retrospective observational study
  1. James Price1,2,
  2. Daniel D Sandbach1,
  3. Ari Ercole1,3,
  4. Alastair Wilson1,4,
  5. Ed Benjamin Graham Barnard1,2,5
  1. 1Department of Research, Audit, Innovation, & Development (RAID), East Anglian Air Ambulance, Norwich, UK
  2. 2Emergency Department, Addenbrooke's Hospital, Cambridge, UK
  3. 3University of Cambridge Division of Anaesthesia, Addenbrooke's Hospital, Cambridge, UK
  4. 4Emergency Department (Retired), Royal London Hospital, London, UK
  5. 5Academic Department of Military Emergency Medicine, Royal Centre for Defence Medicine (Research & Academia), Birmingham, UK
  1. Correspondence to James Price, Department of Research, Audit, Innovation, & Development (RAID), East Anglian Air Ambulance, Norwich, UK; james.price{at}


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 (ETCO2) of 4.0–4.5 kPa (30.0–33.8 mm Hg) to achieve a low-normal arterial partial pressure of CO2 (PaCO2), and reduce secondary brain injury. This recommendation assumes a 0.5 kPa (3.8 mm Hg) 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.

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 (R2). A Bland-Altman plot was used to test for paired ETCO2 and PaCO2 agreement and reported with 95% CI. ETCO2–PaCO2 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 PaCO2 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 ETCO2–PaCO2 gradient was 1.7 (±1.0) kPa (12.8 mm Hg), with moderate correlation (R2=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 (R2=0.13, p=0.10), and this cohort had a larger ETCO2–PaCO2 gradient, 2.0 (±1.1) kPa (15.1 mm Hg), p=0.01. Patients who underwent prehospital arterial blood sampling had an arrival PaCO2 of 4.7 (±0.2) kPa (35.1 mm Hg).

Conclusion There is only moderate correlation of ETCO2 and PaCO2 at hospital arrival in patients with serious TBI. The mean ETCO2–PaCO2 gradient was 1.7 (±1.0) kPa (12.8 mm Hg). Lower ETCO2 targets than previously recommended may be safe and appropriate, and there may be a role for prehospital PaCO2 measurement.

  • prehospital care
  • anaesthesia
  • trauma
  • head

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  • Handling editor Ellen J Weber

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  • Contributors The study was conceived by JP, DDS and EBGB. The study permissions were obtained by EBGB and AW. Data acquisition was undertaken by JP and DDS. DDS, AE and EBGB interpreted the data. The manuscript was prepared by JP and EBGB. Critical revisions were done by AE, AW and EBGB. All authors reviewed and approved the final draft.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting or dissemination plans of this research.

  • Patient consent for publication Not required.

  • Ethics approval This service evaluation was registered with both EAAA and CUH. Local agreement for the use of anonymised data from the EAAA electronic medical record was granted through extant data use protocols. Anonymised, linked data were obtained from the CUH Trauma Office, and PaCO2 values obtained from the CUH electronic medical record. Ethical review was undertaken by the Cambridge University Hospitals NHS Foundation Trust Safety and Quality Support Department (reference: PRN7866).

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement Data are available upon reasonable request.

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