The PaCO-ETCO gradient in pre-hospital intubations of all aetiologies from a single UK helicopter emergency medicine service 2015-2018.

BACKGROUND

Control of the arterial partial pressure of carbon dioxide (PaCO) is important in the ventilated patient. End-tidal carbon dioxide (ETCO) levels are often used as a proxy, but are clinically limited. The difference between the PaCO and ETCO has been suggested to be 0.5-1.0 kPa. However, this has not been consistently reflected in the physiologically unstable pre-hospital patient. This study aims to elucidate the PaCO-ETCO gradient for pre-hospital intubated patients.

METHODS

This was a retrospective, cohort study using data identified from the HEMSbase 2 database (Feb 2015-Nov 2018). Patients were included if they had documented ETCO and arterial PaCO measurements. Arterial PaCO data that could not be linked to within 5 minutes of ETCO were excluded. Bland-Altman plots were calculated to describe agreement.

RESULTS

A total of 73 patients were identified. Aetiology was arranged into three categories: 13 (17.8%) medical, 22 (30.1%) traumatic and 38 (52.1%) out-of-hospital cardiac arrest (OHCA). The median PaCO-ETCO gradient was 2.0 [1.3-3.1] kPa. A PaCO-ETCO gradient of 0-1 kPa was seen for only 11 (15.1%) of total patients. The Bland-Altman agreement for all aetiologies was more than the accepted gradient of 0-1 kPa with the largest bias and widest limits of agreement seen for OHCA (-3.2 [0.3 - -6.8]).

CONCLUSION

The magnitude of the differences between the ETCO and PaCO, levels of variation and inability to predict this suggest that ETCO is not a suitable surrogate upon which to base ventilatory settings in conditions where pH or PaCO require precise control.