End-tidal carbon dioxide tension is not a pertinent parameter for monitoring severe head trauma

Purpose: To evaluate the agreement between end-tidal carbon dioxide (PETCO2 ) and arterial CO2 (PaCO2) in patients with traumatic brain injury and to d ocument the course of the (PaCO2 - PETCO2) gradient over time. Methods: Twenty one traumatic brain injury patients (Coma Glasgow Scale les s than or equal to 8) were included in this prospective observational study over a period of sir months. Simultaneous determinations of PoCO2 and PETC O2 (by infrared capnometry) were recorded. Agreement between PaCO2 and PETC O2 was determined by the statistical method described by Blond and Altman, Changes in PETCO2 over time were compared with changes in PaCO2. Factors li kely to explain a gradient superior to +/- 4 mmHg were explored. Results: One hundred and eleven data pairs were obtained. The bios was 5.5 mmHg with a precision of 5.1 mmHg and limits of agreement ranged from -4.5 mmHg to 15.5 mmHg. The latter exceeded the predefined limits of agreement e stablished to determine interchange-ability between methods (+/- 4 mmHg). P ETCO2 and PaCO2 changed in opposite directions in 20% of 90 consecutive mea surements. Only the duration of ventilation was found to be significantly a ssociated with a gradient superior to +/- 4 mmHg. Conclusions In this selected population of patients with severe traumatic b rain injury, measurements of PETCO2 and PaCO2 ore not interchangeable. Furt her, the PaCO2 - PETCO2 gradient is not stable over time and cannot predict variations of PaCO2. The use of PETCO2 instead of PaCO2 could be deleterio us in patients in whom strict control of PaCO2 values is required.