Prehospital EtCO2 and mortality in suspected severe traumatic brain injury patients

What is the association between prehospital end-tidal carbon dioxide (EtCO2) levels and mortality in patients with suspected severe traumatic brain injury (TBI)?

• TBI is a leading global cause of mortality and morbidity. Prehospital treatment including endotracheal intubation and ventilation plays a crucial role in patient outcomes, as patients with severe TBI are at significant risk of secondary brain injury even before reaching the hospital.
• As ventilation affects arterial partial pressure of carbon dioxide (PaCO2) levels and therefore cerebral blood flow, Hypercapnia may result in cerebral vasodilation and increased intracranial pressure, whereas hypocapnia might produce cerebral vasoconstriction and decreased cerebral blood flow, potentially aggravating TBI.
• Recommendations for "normoventilation" and evidence supporting this practice are weak, with conflicting research findings on the association between CO2 levels and TBI outcomes.
• As blood gas analyses are not generally available during prehospital care, prehospital providers usually use EtCO2 values to guide ventilation. 
• Targeting EtCO2 values below the normal range, which is typically ≤ 35 mmHg in protocols, may increase the risk of hypocarbia.
• This study aimed to investigate the association between prehospital EtCO2 levels and mortality (Bossers SM, Mansvelder F, Loer SA, et al. Association between prehospital end-tidal carbon dioxide levels and mortality in patients with suspected severe traumatic brain injury. Intensive Care Med. 2023;49(5):491-504. doi:10.1007/s00134-023-07012-z1​), utilizing data from the BRAIN-PROTECT cohort study in the Netherlands.

BRAIN-PROTECT is a multicenter prospective observational study focusing on the prehospital treatment of patients with severe TBI in the Netherlands.

The study included four Dutch physician-staffed helicopter emergency medical services (HEMS) and nine trauma centers. From February 2012 to December 2017, data was collected from patients with suspected severe TBI treated by participating HEMS services.

• Patients included in the study had trauma mechanisms or clinical findings suggestive of severe TBI and presented with a prehospital Glasgow Coma Scale (GCS) score of 8 or lower.
• After prehospital care by the HEMS, patients were transferred to one of the nine participating trauma centers.  
• Data collection included patient and trauma characteristics, injuries, vital parameters, prehospital treatments, and interventions.
• Sample size of the study was based on the ability to detect an absolute 5% reduction in mortality with 80% power.

After advanced airway management, patients were usually mechanically ventilated or manually ventilated with a self-inflating bag, depending on the availability of a mechanical ventilator in the ambulance, the distance to the hospital, and the preference of the treating physician.

• An L-shaped association was observed between EtCO2 levels and 30-day mortality in patients with severe TBI. 
• Lower EtCO2 values (< 35 mmHg) were associated with a significant increase in mortality within 30 days after sustaining severe TBI.
• An association between low EtCO2 values and mortality was observed across all subgroups, including patients with isolated TBI and signs of cerebral herniation.
• Higher EtCO2 values (> 45 mmHg) did not show a significant association with increased mortality. 
• Maintaining EtCO2 levels within a "safe zone" of 35–45 mmHg could be a reasonable target for prehospital ventilation in patients with severe TBI.

At discharge, 40.6% of the patients who survived the initial injury had a functional neurologic outcome categorized as moderate or good recovery (GOS score of 4 or 5).

• A safe zone of 35–45 mmHg for end-tidal CO2 guidance seems reasonable during prehospital care. 
• In particular, EtCO2 of less than 35 mmHg was associated with significantly increased mortality. 

• Mortality significantly increased with EtCO2 values below 35 mmHg (hypocapnia).
• EtCO2 levels within a range of 35–45 mmHg were associated with lower and constant mortality rates.
• These findings challenge current prehospital ventilation practices and emphasize the importance of monitoring and managing EtCO2 levels to improve outcomes in patients with severe TBI.
• The study focused on 30-day mortality as a primary endpoint; however, other clinically relevant outcomes like neurological recovery were not fully addressed due to incomplete data.

Hamilton Medical ventilators offer a range of features to support effective monitoring of end-tidal CO2, such as the ventilation mode INTELLiVENT-ASV and time-based or volumetric capnography. With these tools, healthcare professionals can measure and maintain EtCO2 levels within the recommended range of 35–45 mmHg for patients with severe traumatic brain injury (TBI). 

The incorporation of time-based and volumetric capnography allows for continuous and accurate monitoring of EtCO2 levels, ensuring precise ventilation management. Being able to observe SBCO2 (single breath of CO2) helps identify ventilation/perfusion changes, lung heterogeneity, etc.

In addition, INTELLiVENT-ASV offers an automated solution that optimizes ventilation parameters by continuously adjusting them breath by breath. EtCO2 is thus kept within the target range, minimizing the risk of hypo- or hypercapnia while promoting better patient outcomes as concluded in this paper.