Reply to Letter: Early ventilation in traumatic brain injury

The PaCO2-ETCO2 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 (PaCO2) is important in the ventilated patient. End-tidal carbon dioxide (ETCO2) levels are often used as a proxy, but are clinically limited. The difference between the PaCO2 and ETCO2 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 PaCO2-ETCO2 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 ETCO2 and arterial PaCO2 measurements. Arterial PaCO2 data that could not be linked to within 5 minutes of ETCO2 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 PaCO2-ETCO2 gradient was 2.0 [1.3-3.1] kPa. A PaCO2-ETCO2 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 ETCO2 and PaCO2, levels of variation and inability to predict this suggest that ETCO2 is not a suitable surrogate upon which to base ventilatory settings in conditions where pH or PaCO2 require precise control.

The utility of early end-tidal capnography in monitoring ventilation status after severe injury

BACKGROUND

An arterial CO2 (Paco2) of 30 mm Hg to 39 mm Hg has been shown to be the ideal target range for early ventilation in trauma patients; however, this requires serial arterial blood gases. The use of end-tidal capnography (EtCO2) has been recommended as a surrogate measure of ventilation in the prehospital arena. This is based on the observation of close EtCO2 Pa(CO2) correlation in healthy patients, yet trauma patients frequently suffer from impaired pulmonary ventilation/perfusion. Thus, we hypothesize that EtCO2 will demonstrate a poor reflection of actual ventilation status after severe injury.

METHODS

Prospective observational study on consecutive intubated trauma patients treated in our emergency department (ED) during 9 months. Arterial blood gas values and concomitant EtCO2 levels were recorded. Regression was used to determine the strength of correlation among all trauma patients and subgroups based on injury severity (Abbreviated Injury Score and Injury Severity Score) and physiologic markers of perfusion status (lactate, shock index, and arterial base deficit).

RESULTS

During 9 months, 180 patients were evaluated. The EtCO2 Paco2 correlation was poor at R2 = 0.277. Patients ventilated in the recommended EtCO2 (range, 35 to 40) were likely to be under ventilated (Pa(CO2) > 40 mm Hg) 80% of the time, and severely under ventilated (Pa(CO2) > 50 mm Hg) 30% of the time. Correlation was best for patients with isolated traumatic brain injury and worst for those with evidence of poor tissue perfusion.

CONCLUSION

EtCO2 has low correlation with Pa(CO2), and therefore should not be used to guide ventilation in intubated trauma patients in the ED. Better strategies for guiding prehospital and ED ventilation are needed.