Implications of Lower Prehospital ETCO2 Levels on 30-Day Mortality in TBI
Published on: December 15, 2023
Guidelines recommend targeting normal PaCO2 values in the care of TBI patients, and End-tidal CO2 (ETCO2) is frequently used in a prehospital setting to assess ventilation. Given that there may be a difference between PaCO2 and ETCO2, some protocols recommend lower targets (between ETCO2 30-35 mmHg) to ensure normocapnia. The purpose of this study is to define the correlation between prehospital ETCO2 and mortality in severe TBI patients.
This prospective observational study included the prehospital management of severe TBI patients across multiple centers in the Netherlands. Inclusion criteria included suspected severe TBI patients based on mechanism of trauma or clinic exam findings. Treatment was provided by physicians in the Helicopter Emergency Medical Services. Exclusion criteria were as follows: transfer to a hospital not participating in study, cardiopulmonary resuscitation performed and no advanced airway placed in field. Vitals with ETCO2 were recorded on the initial arrival of providers, after airway management, and prior to arrival at the hospital. The initial vitals were not included for analysis because these were commonly prior to placement of an advanced airway.
2589 patients were enrolled in the database and 1776 were deemed eligible for further analysis after exclusion criteria were applied. The sample had a median age of 45 years old, and 70% were male. The median GCS was 4. Mortality and ETCO2 data were available for 1342 patients, and a multivariable logistic regression adjusted for confounders showed an increased 30-day mortality with hypocapnia. An L-shaped curve demonstrated the association of mortality and ETCO2, with a sharp increase in mortality below ETCO2 of 35 mmHg and a flattening of the curve above 35 mmHg. Data stratified based on ETCO2 categories, which encompassed hypocapnia, normocapnia, and hypercapnia, revealed that hypocapnia was associated with a statistically significant rise in mortality (OR 1.89, 95% CI 1.53-2.34, p<0.001). There was no significant correlation between hypercapnia and morality.
The study's findings suggest a predicted "safe zone" of prehospital ETCO2 levels ranging from 35 to 45 mmHg. Its strength lies in its pragmatic design, which involves analyzing cases of suspected TBI even without a confirmed diagnoses in the field. Therein lie some of its drawbacks as well, since the study may have included patients with a low GCS who did not actually have a TBI, such as those with spontaneous ICH or seizure disorders. Nonetheless, the researchers also conducted an analysis specifically focused on confirmed TBI patients. This analysis revealed a consistent connection between low ETCO2 levels and higher mortality rates. Remarkably, this connection was observed across various subgroup analyses, even among patients displaying signs of cerebral herniation. Another potential confounding factor is that low ETCO2 measurements may reflect injury (shock, pneumothorax, etc.) rather than strictly reflect ventilation efforts, which in turn may increase mortality. However, the investigators did attempt to adjust for such effects in the multivariate regression. Importantly, the study found a similar link between prehospital hypocapnia and 30-day mortality in patients with isolated TBI. While more research is needed to establish a causal relationship, this outcome highlights the potential risks of routine hyperventilation in prehospital settings and underscores the need for greater awareness.
Bossers, S.M., Mansvelder, F., Loer, S.A. et al. Association between prehospital end-tidal carbon dioxide levels and mortality in patients with suspected severe traumatic brain injury. Intensive Care Med 49, 491–504 (2023). https://doi.org/10.1007/s00134-023-07012-z