Preliminary Report of a Mathematical Model of Ventilation and Intrathoracic Pressure Applied to Prehospital Patients with Severe Traumatic Brain Injury

Abstract

Background: Inadvertent hyperventilation is associated with poor outcomes from traumatic brain injury (TBI). Hypocapnic 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: Preliminary report on the application of a previously described mathematical model of perfusion and ventilation to prehospital 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 (V_T). The values for VR and estimated V_T were then applied to a previously described mathematical model of perfusion and ventilation. This model allows input of various lung parameters to define a pressure–volume relationship, then derives mean intrathoracic pressure (MITP) for various V_T and VR values. For this analysis, normal lung parameters were utilized. Separate analyses were performed assuming either fixed or variable PaCO₂–PetCO₂ differences. Ground and air medical patients were compared with regard to VR, PetCO₂, estimated V_T, and predicted MITP. 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 versus air patients (21.6 vs. 19.7 breaths/min; p < 0.01). Estimated V_T values were similar for ground and air patients (399 mL vs. 392 mL; p = NS) in the fixed model but not the variable (636 vs. 688 mL, respectively; p < 0.01). Mean PetCO₂ values were lower for ground versus air patients (30.6 vs. 33.8 mmHg; p < 0.01). Predicted MITP values were higher for ground versus air patients, assuming either fixed (9.0 vs. 8.1 mmHg; p < 0.01) or variable (10.9 vs. 9.7 mmHg; p < 0.01) PaCO₂–PetCO₂ differences. Conclusions: Predicted MITP values increased with ventilation rates. Future studies to externally validate this model are warranted.

Key words:

• Prehospital
• ventilation
• Traumatic Brain Injury (TBI)
• mathematical modeling