Abstract 8: Elevated Intrathoracic Pressure Is Common During Prehospital Ventilation of Patients with Traumatic Brain Injury

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Background: Inadvertent hyperventilation is associated with poor outcomes from traumatic brain injury (TBI). Hypocapneic cerebral vasoconstriction is well described and causes an immediate and profound decrease in cerebral perfusion. The hemodynamic effects of positive-pressure ventilation (PPV) remain incompletely understood but may be equally important, particularly in the hypovolemic patient with TBI. Objective: To apply a previously described mathematical model of perfusion and ventilation to prehospital ventilation data to predict intrathoracic pressure. Methods: Ventilation data from 108 TBI patients (76 ground transported, 32 helicopter transported) were used for this analysis. Ventilation rate (VR) and end-tidal carbon dioxide (PetCO2) values were used to estimate tidal volume (VT). The values for VR and estimated VT were then applied to a previously described mathematical model of perfusion and ventilation. This model allows various lung parameters to be defined to create a pressure-volume relationship, then derives mean intrathoracic pressure (MITP) for various VT and VR values. For this analysis, normal lung parameters were assumed. Separate analyses were performed assuming fixed and variable PaCO2-PetCO2 differences. Data regarding VR, PetCO2, estimated VT, and estimated MITP were presented descriptively. Results: A total of 10,647 measurements were included from the 108 TBI patients, representing about 13 minutes of ventilation per patient. Mean VR values were higher for ground patients (21.6 breaths/min) than for air patients (19.7 breaths/min). Estimated VT values were similar for ground and air patients (399 mL vs. 392 mL, respectively). Mean PetCO2 values were lower for ground patients (30.6 mmHg) than for air patients (33.8 mmHg). MITP values were higher for ground versus air patients, assuming either fixed (9.0 vs. 8.1 mmHg) and variable (10.9 vs 9.7 mmHg) PaCO2-PetCO2 differences. Conclusions: Hyperventilation is common in prehospital TBI patients as reflected by both hypocapnia as well as rapid VR values. MITP values are elevated and would likely result in detrimental hemodynamic effects, particularly in hypovolemic patients.