title: 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 (VT). The values for VR and estimated VT 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 VT 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 VT, 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 VT 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.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Davis
authors.forename: Steve A
authors.initials: SA
authors.lastname: Aguilar
authors.forename: Kimberly
authors.initials: K
authors.lastname: Smith
authors.forename: Ruchika D
authors.initials: RD
authors.lastname: Husa
authors.forename: Anushirvan
authors.initials: A
authors.lastname: Minokadeh
authors.forename: Gary
authors.initials: G
authors.lastname: Vilke
authors.forename: Rebecca
authors.initials: R
authors.lastname: Sell
authors.forename: Roger
authors.initials: R
authors.lastname: Fisher
authors.forename: Criss
authors.initials: C
authors.lastname: Brainard
authors.forename: James V
authors.initials: JV
authors.lastname: Dunford
mesh_terms: D000328:Adult; D001930:Brain Injuries / Q000628:therapy*; D006801:Humans; D008168:Lung / Q000503:physiopathology*; D008962:Models, Theoretical; D011175:Positive-Pressure Respiration / Q000379:methods*
keywords: Prehospital; Traumatic Brain Injury (TBI); mathematical modeling; ventilation