Service evaluation: Accuracy of using end-tidal CO ₂ to estimate PaCO ₂ values in suspected traumatic brain injury patients requiring mechanical ventilation

This service evaluation assessed the accuracy of estimating arterial partial pressure of carbon dioxide (PaCO 2 ) from end-tidal CO 2 (ETCO 2 ) in prehospital suspected traumatic brain injury (TBI) patients requiring mechanical ventilation. Thirty-one cases with paired ETCO 2 and emergency department (ED) PaCO 2 measurements were analysed. Estimated PaCO 2 (ETCO 2 + 0.5 kPa) significantly underestimated measured PaCO 2 , with a mean bias of 1.17 kPa and wide limits of agreement. Nearly half of patients were outside the normocapnic range despite apparently acceptable estimated values. Findings highlight the risk of inadvertent hypercapnia when relying on ETCO 2 -based estimation alone. Lower ETCO 2 targets or point-of-care PaCO 2 measurement may improve ventilation accuracy and reduce secondary brain injury.